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A new type of asphalt pavement disease, known as arch expansion, has appeared in the saline soil regions of Northwest China. The emergence, as well as the evolution of the disease, is strongly related to the distribution of sulfate. However, the decay of Cement-stabilized gravel (CSG) mechanical properties is closely related to the onset and development of arch expansion. The relationship between CSG mechanical strength and salinity in the arch expansion area is analyzed based on field investigations to systematically study the mechanical property degradation law and mechanism of CSG. The mechanical strength and deterioration patterns of CSG with different coupling conditions were investigated. The mineral composition and micromorphology of the erosion products were analyzed. The results show that the mechanical properties of CSG in the area of the arch deteriorate more than those in the regular section. The mechanical strength of the specimens initially containing salt and partially immersed in salt solution decayed most severely. In terms of the deterioration mechanism of mechanical strength of CSG, the water-heat-salt erosion process is also divided into two parts, including the formation stage of gypsum and ettringite, the stage of C-S-H decomposition and thaumasite formation, respectively. The degradation mechanism of CSG is a combination of physical and chemical erosion. • The law between the mechanical properties of CSG and salt content are revealed. • The mechanical property decay law of the CSG under coupling condition are studied. • The property decay mechanism of CSG under coupling condition are revealed. [ABSTRACT FROM AUTHOR]

Lepidolite slag (LES) is a kind of solid waste produced by extracting lithium from lepidolite, but researches focused on it are still limited. This paper works on understanding the utilization potential of LES in cementitious materials from its composition characteristics, hydration properties, and high value-added cementitious system design. Results show that the reactivity of LES is notably low in both water and alkaline solution. As a supplementary cementitious material (SCM), LES exhibits several drawbacks, including prolonging setting time, reducing compressive strength, and deteriorating pore structure. With addition of 30 % LES, the compressive strength at 28 d decreased by 63.3 %. Considering the high gypsum content within LES, an all-solid waste cementitious system was designed using high-volume LES combined with blast furnace slag (BS) and a small amount of lime. System with 40 % LES presents similar compressive strength with system composed entirely of BS at both 7 d and 28 d. The mechanism is that the excellent compound activation effect of gypsum in LES and lime on BS promoted the transformation of hydrotalcite to ettringite in the system. Even when LES content reached 70 %, compressive strength at 28 d surpassed 15 MPa. This study proposes a novel high value-added utilization way for LES in cementitious materials. ● Composition characteristics, hydration properties, and high value-added cementitious system design of LES were studied. ● The reactivity of LES is notably low in both water and alkaline solution. ● As a SCM, LES exhibits several drawbacks, especially in compressive strength. ● An all-solid waste cementitious system was designed using high-volume LES combined with BS and a small amount of lime. ● System with 40 % LES presents similar compressive strength with system composed entirely of BS. [ABSTRACT FROM AUTHOR]

In order to investigate the impact of different calcium sulfate (CS) sources and forms on the rheology, hydration, and mechanical properties of supersulfated cement (SSC), SSC pastes were produced with anhydrite from gypsum calcined at 650ºC and raw and calcined phosphogypsum (PG) at 150, 350, and 650ºC. Isothermal calorimetry, rheometry using parallel-plate geometry, compressive strength testing, and microstructural analysis through X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG) were performed. Results showed that SSC containing gypsum and PG calcined at 650ºC exhibited similar rheological behavior. Pastes containing PG calcined at 150ºC and raw PG respectively, exhibited the highest and lowest shear stress at a given shear rate. The calcination temperature and CS source did not significantly impact SSC compressive strength. Rietveld XRD analysis revealed that pastes with PG calcined at 350ºC and gypsum calcined at 650ºC had the lowest and highest ettringite development, respectively, from 7 to 28 days of curing. • Calcination temperature impacts SSC rheological properties. • Dihydrate, hemihydrate, anhydrous forms result in low, high, intermediate viscosity. • Hemihydrate form shows fast ettringite development at early age. [ABSTRACT FROM AUTHOR]

The goal of the study was to investigate the reactivity at different Blaine, hydration behavior, and performance of calcium sulfoaluminate-ferrite cement (CSA-F). The raw meal incorporated bauxite residue (38 wt%) fired at 1260 °C and 1300 °C for 30 and 60 minutes. Based on multi-objective optimization, an optimum dosage of citric acid ranges between 0.4

The application of seawater concrete is a good strategy to deal with global water shortage. Its potential issues are durability performances due to high concentrations of ions, but they have not been fully studied and discussed. This paper focuses on the sulfate attack from mixed seawater and external seawater to cement paste. The degradation over 360 days was evaluated by expansion, mass change, strength development, reaction product identifications through TGA and XRD, and element distribution through SEM-EDS. The findings reveal that mixed seawater did not induce internal sulfate attack, as evidenced by low expansion (<0.1 %) and ettringite content. Mixed seawater increased the expansion of specimens subjected to external seawater attack but did not alter the degradation depth and mechanism. Both internal and external seawater attacks induced the highest expansion of cement paste, which was 0.107 % at 6 months, slightly higher than the limit value of the moderately sulfate-resistant mortar sample. However, no obvious debonding phenomenon was observed for all OPC paste under seawater exposure for 360 days. Cement paste with 70 % GGBS replacement experienced more severe damage than OPC paste under seawater exposure due to the combined sodium sulfate and magnesium sulfate attack. The mechanism of sulfate attack induced by seawater on OPC paste was governed by ettringite formation and portlandite leaching, with the degradation layer confined to a depth of 2 mm. GGBS blended paste exhibited a different deterioration pattern, with surface debonding being the predominant degradation mode instead of significant bulk expansion. Its ettringite enrichment layer was thinner, while the Portlandite leaching depth was larger than that of OPC paste. • Mixed seawater did not induce the internal sulfate attack. • Mixed seawater did not alter the deterioration depth of OPC paste exposed to seawater. • Mechanisms of seawater attack are ettringite formation and portlandite leaching. • Degradation depth of cement paste under seawater attack for 360 days was determined. • Deterioration mode of cement paste with 70 % GGBS under seawater attack is different. [ABSTRACT FROM AUTHOR]

Calcium sulfoaluminate (CSA) cement offers a sustainable alternative to ordinary portland cement by using less energy and producing fewer CO 2 emissions. However, the high cost and limited availability of alumina-rich resources like bauxite pose significant challenges for CSA cement production. This study addresses these issues by synthesizing CSA cement through an industrial symbiosis framework, incorporating natural materials (limestone and gypsum) with industrial wastes and by-products (Serox, ladle furnace slag, ceramic waste, and glass waste). This approach aims to minimize these challenges to CSA production while promoting environmental sustainability in the construction industry. Laboratory studies have resulted in the successful synthesis of three distinct CSA cements with at least 40 % recovered material content. Ye'elimite, anhydrite, merwinite, and fluorellestadite were identified as major compounds of the CSA cements. Hydration studies revealed ettringite as the primary hydration product, contributing to rapid strength gains with compressive strengths over 30 MPa within a day. In addition, a tendency to ettringite carbonation was observed over the long term, the adverse effects of which need to be further investigated. These results demonstrate the feasibility of producing environmentally friendly CSA cements through industrial symbiosis and highlight their potential for scalable and sustainable construction applications. • Eco-friendly CSA cements with over 40 % recovered material were synthesized. • Serox, ladle furnace slag, ceramic waste, and glass waste were used in CSA cement production. • Ye'elimite, anhydrite, merwinite, and fluorellestadite were identified as main compounds. • CSA cements achieved rapid strength gains of over 30 MPa in one day due to formation of ettringite. • The ettringite seemed to carbonate in the long-term. [ABSTRACT FROM AUTHOR]

The potential of a calcined Australian bentonite (CB) to produce low embodied carbon limestone-calcined-clay-cement with 50 % clinker replacement was compared against metakaolin (Mk). Recycled plasterboard waste (PW) was also investigated as an alternative to virgin gypsum to enhance cement sustainability. CB-based cement blends exhibited greater pozzolanic reactivity, improved phase development and greater 28-day compressive strength compared to Mk-based blends, as confirmed by X-ray diffraction, thermogravimetry, infrared spectroscopy and electron microscopy. Reactive silica from CB promoted formation of calcium-silicate-hydrates with higher Si/Ca and lower Al/Ca ratios, as well as more alumino-silicate phases (strätlingite), whereas Mk favored formation of ettringite and carbo-aluminates. Additional sulphates enhanced ettringite and carbo-aluminate formation, and PW improved early age pozzolanic reactivity in both blends as confirmed by thermal analysis. This study demonstrated that both the CB and PW show great potential for producing low embodied carbon cement, highlighting that other local clay-mineral resources should also be investigated. • Limestone-calcined-clay-cement (LC3) was produced using two calcined clays. • Calcined bentonite was used as an alternative to metakaolin. • Compressive strength and phase development of LC3 were investigated. • The difference in chemistries of hydration phases was explained. • Calcined bentonite based LC3 performed better than metakaolin based LC3. [ABSTRACT FROM AUTHOR]

The substantial use of fly ash as cement replacement to reduce carbon emissions is identified as the primary goal, 50 % of the cement was substituted with fly ash and in order to avoid the low early stage mechanical properties of the composite, varying doses of sodium sulfate were used as an activator of fly ash. Additionally, sisal fibers were chosen to enhance the flexural strength. The results indicate that sodium sulfate significantly enhances the early stage mechanical performance of the cementitious composites and the matrix exhibits the optimal mechanical performance at 4 wt% sodium sulfate dosage (4SS). The addition of sisal fibers resulted in an enhancement of the flexural performance of the material. The collaboration between sodium sulfate and sisal fibers results in an improvement in the initial flexural performance of the material while preserving its compressive strength. The single-fiber pullout test further confirmed that adding sodium sulfate enhances the fiber-matrix interface and bonding. Microstructural investigations by XRD, TGA, MIP, and SEM-EDS revealed accelerated hydration and early formation of a large amount of ettringite due to the addition of sodium sulfate. Ettringite acts as a gel nucleation seed, promoting the formation of a dense gel structure and effectively improving the early mechanical properties of the composite. Furthermore, with curing age, sodium sulfate also effectively improves the pore structure of the matrix in the late hydration stage. • Sisal fiber-reinforced fly ash cement mortars were activated by sodium sulfate. • Fiber-matrix interface & bonding at varying sodium sulfate dosages were evaluated. • Mechanical and microstructural properties of composites were studied. • Performance enhancement mechanism was studied by XRD, TG, MIP, & SEM-EDS. • Sisal fibers & FA increase materials performance and reduce carbon emissions. [ABSTRACT FROM AUTHOR]

The benefits of limestone waste as a partial replacement of cement have been well established. In this paper, to better understand the effect of limestone waste on the properties of cement-based materials, the hydration process and microstructural properties of cement are evaluated. The results showed that the limestone waste decreased the fluidity of cement mortar, while the mortar having up to 10 % limestone waste provided competitive properties with pure cement mortar. It is concluded that the presence of limestone waste has both negative and positive effects simultaneously on the hydration and microstructural properties of the cement paste. During the hydration process, the net reaction heat of limestone waste in the cement blends ranged from 0.38 J/g to 4.1 J/g. In addition, the thermodynamic modeling and XRD indicated that the monocarbonate and hemicarbonate were stable in the limestone-cement blends, and the limestone waste increased the stability of ettringite. • The effect of limestone waste on the hydration and strength development of cement was investigated. • The net reaction heat of the limestone fines in the cement system was 3.5 J/g∼4.1 J/g. • An appropriate content of the limestone waste improves the chemical shrinkage of cement. [ABSTRACT FROM AUTHOR]

The electrodeposition method (EDM) has been regarded as a new technique for healing concrete microcracks in recent years due to its advantages of excellent remediation in aqueous environments. However, a flowing water environment may influence the effectiveness of the EDM. Therefore, in this paper, the effect of flowing water on the synthesis of ettringite by the EDM to heal concrete cracks was investigated through experiments and molecular dynamics simulations. The experiment results showed that the impermeability coefficient of concrete decreased from 6.41 to 5.36 and 5.18 as the flow rate increased from 0 to 1 m/d and 3 m/d after 7d. The healing products on the surface developed from a cotton-like floc structure to a gel-like morphology. At the microscopic level, with the increase in flow rate, the ettringite changed from the disordered distribution of reticulation to the orderly growth of clusters in the form of long rods. Meanwhile, the phase compositions of healing products transformed from ettringite to gypsum and calcium carbonate. In addition, combined with the molecular dynamics simulations by LAMMPS, the increase in flow rate changed the crystal characteristics of the synthesized ettringite, both the decrease in length-to-diameter ratio and crystallinity, as well as the increase in specific surface area, d-spacing, and crystallite size. • The flowing water environment was introduced into the healing of concrete cracks by electrodeposition method at 0, 1 m/d and 3 m/d. • As the flow rate increased, the impermeability coefficient of concrete decreased from 6.41 to 5.36 and 5.18 respectively. • The content of ettringite decreased from 91.8 % to 67.9 % as the flow rate increased from 0 to 1 m/d after 7d. With a further increase in flow rate to 3 m/d, the phase composition of the product was gypsum completely. • At higher flow rates, the ettringite had larger specific surface areas, crystallite sizes, as well as smaller d-spacing, crystallinity and length-to-diameter ratio. [ABSTRACT FROM AUTHOR]

Generally, high-water-content of dredged sediment (DS) tends to suffer from inferior mechanical properties and obvious shrinkage after solidification, so finding solutions to this issue is helpful for promoting the recovery and recycling of DS. In this paper, in reference to natural gypsum (NG), phosphogypsum (PG) was incorporated into DS solidified with alkali-activated slag (AAS) system. The effect of PG (0 %-20 %) on the hydration process (0–168 h), mechanical properties (3 d, 7 d and 28 d) and autogenous shrinkage (0–7 d) of DS solidified with AAS was investigated. It is found that the addition of PG not only induces the generation of ettringite to compensate for shrinkage, but also accelerates the formation of C-A-S-H by providing active calcium to promote stiffness to resist shrinkage. This results in a reduction of autogenous shrinkage by 74.3 % and an increase of compressive strength by 28.5 % when PG dosage is 15 %. Compared with NG, the difference in 28d-compressive strength of PG group is not more than 7.34 % under equivalent dosages. The dissolved SO 4 2– from PG could be adsorbed on C-A-S-H and preserved in pore solution in the form of Na 2 SO 4. The decrease in S/Si from 0.31 to 0.09 indicates stored SO 4 2– could be released back into system to promote the further generation of ettringite. To obtain superior mechanical properties and volume stability, appropriate PG dosage is 10 %-15 %. Compared with the control group, it increases the content of ettringite and amorphous phase by 2.4 %-4.6 % and 3.3 %-3.7 %, respectively. This research not only provides theoretical support for DS solidified with AAS to realize efficient utilization of solid waste resources (i.e., DS, PG and slag), but also gives a new insight into solidification of other high-water-content system, such as backfill mining, grouting materials and treatment of soft soil foundations. • Different solid wastes were utilized to treat high-water-content system. • PG induces the formation of ettringite in DS solidified with AAS system. • PG accelerates C-A-S-H formation and increases stiffness to resist shrinkage. • Ettringite consolidates massive water and compensates for shrinkage of ASD system. [ABSTRACT FROM AUTHOR]

The preparation of gypsum-based self-leveling mortar (GSLM) from solid waste-derived sulfoaluminate cement-modified hemihydrate gypsum is an effective path to enhance its mechanical properties. However, the phosphorus impurities in phosphogypsum (PG) still pose a challenge to the hemihydrate gypsum & sulfoaluminate cement composite system. To evaluate the impact of phosphorus impurities and develop targeted removal strategies, this study first assessed the effects of different types of phosphorus impurities on the mechanical properties and hydration characteristics of the GSLM. Then, a method of pretreating PG with carbide slag (CS) was proposed, and the removal efficiency and mechanism of phosphorus were examined by ICP-OES and XPS testing. The results indicated that the soluble phosphorus impurities severely inhibit the hydration of both CŜH 0.5 and C 4 A 3 Ŝ, leading to a most notable reduced mechanical performance. The sequence of inhibitory action was as follows: H 3 PO 4 >> Ca(H 2 PO 4) 2 ·H 2 O > CaHPO 4 ·2H 2 O > Ca 3 (PO 4) 2. CS could effectively immobilize the soluble phosphorus in PG. The pretreatment by CS notably enhanced the hydration degree of the GSLM by converting soluble phosphorus into Ca 3 (PO 4) 2 beforehand. In addition, the alkaline environment provided by CS promoted the ettringite formation. When the CS content approaches 3%, a PG-based GSLM with good performances meeting the G20 standard requirements of GSLM can be prepared. • The negative influence degree: H 3 PO 4 >> Ca(H 2 PO 4) 2 ·H 2 O > CaHPO 4 ·2 H 2 O > Ca 3 (PO 4) 2. • PG pretreatment by CS reduces soluble phosphorus and promotes ettringite formation. • GSLM meeting G20 standard was successfully prepared with CS-pretreated PG. [ABSTRACT FROM AUTHOR]

This study investigates the effects of accelerated carbonation on calcium sulfoaluminate (CSA) cement concrete, focusing on mixtures enhanced with 20 % fly ash (FA), 20 % remediated fly ash (RF), 15 % limestone powder (LP), and a combination of 20 % FA with 15 % LP (35 %). The study further evaluates the mechanical properties including compressive strength, splitting tensile strength, elastic modulus, along with drying shrinkage and bulk resistivity. To delve into the microstructural characteristics of moist curing versus carbonation exposure on the CSA cement system, X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were employed, particularly analyzing phase assemblage changes. The results show that the addition of FA reduced the carbonation depth in concrete mixtures over time (105 days). However, LP and the combination of FA and LP presented mixed effects. The microstructural analysis highlighted ettringite as the predominant phase in samples moist cured for 3 days. In contrast, carbonation-cured samples were characterized by different calcium carbonate (CaCO 3) polymorphs alongside aluminum hydroxide (Al(OH) 3) and residual ye'elimite, with the formation of low-pH carbonic acid facilitating the conversion of ettringite into CaCO 3. This study highlights the impact of different SCMs on the durability and microstructural characteristics of CSA cement concrete, underscoring the interplay between curing methods, effects of SCM, and carbonation processes. • Mechanical, durability, and microstructural properties of CSA cement incorporating SCMs were studied. • Mechanical properties are improved with limestone powder or fly ash, but not both. • Appropriate amount of fly ash and limestone in CSA reduce carbonation depth over time. • Carbonation-cured samples show calcium bicarbonate polymorphs as a predominant phase. [ABSTRACT FROM AUTHOR]

A clinker-free cement (CFC) based on portlandite, calcined clay, slag, limestone powder, anhydrite, and C-S-H seeding is reported. The aim was to develop a low-carbon clinker-free cementitious binder using supplementary cementitious materials (SCMs), portlandite, a sulfate source, and accelerators. The impacts of ordinary Portland cement (OPC), portlandite, calcined clay and anhydrite on the strength of the C-S-H-seeded CFC were investigated. Strength, flow, setting and expansion of the mortar were investigated. Hydration kinetics were studied using calorimetry and XRD Rietveld analysis. Results showed that the seeded CFC exhibited similar strength and flow to OPC but produced significantly less heat. The factors contributing to the enhanced strength and improved flow behavior of the CFC were examined. Expansion behavior of CFC and its relationship with anhydrite content and ettringite formation was analyzed. Additionally, the microstructure of the hardened paste was characterized using SEM and mercury intrusion porosimetry (MIP) to evaluate the impact of C-S-H seeding. The combination of a low-carbon binder mix design and C-S-H seeding effectively reduced the CO 2 footprint of cementitious binders, with the seeded-CFC achieving >50 % reduction in CO 2 emissions per MPa compared to OPC. The proposed CFC binder approach was further validated using other types of SCMs. [Display omitted] • A clinker-free cement (CFC) based on portlandite, SCM and C-S-H seeding. • C-S-H seeding accelerates SCM reaction, leading to strength gain and meeting 32.5 grade cement. • CFC advantages: similar mortar flow to OPC, low heat release, and reduced CO 2 footprint. • CFC exhibits enhanced early strength with C-S-H seeding; late strength correlates with seeding dose. • CFC shows better mortar flow than LC3 cement, limited expansion, and dense structure. [ABSTRACT FROM AUTHOR]

The study investigated the effects of calcium aluminate cement (CAC) and calcium sulfoaluminate cement (CSA) on the early hydration kinetics of super-sulfated cement (SSC) derived from arsenic-containing bio-oxidation waste (BW) to enhance its early strength development. The results demonstrated that the minor addition of both CAC and CSA significantly promoted SSC hydration, leading to improved early-age strength, with CAC showing superior performance. The incorporation of 0.5 wt% CAC (CAC10) resulted in a substantial increase in compressive strength at 1 d and 3 d by approximately 37.2 % and 22.5 %, respectively, which was attributed to the in-situ formation of ettringite (AFt) serving as nucleation sites for further precipitation. Nano-indentation analysis indicated that C-S-H gels formed in the presence of CAC demonstrated increased values of elastic modulus and hardness, which contributed to favorable conditions for SSC strength development. However, a higher dosage of CAC and CSA imposed restrictions on the early reaction of SSC, which were linked to reduced alkalinity and limitations in blast furnace slag (BFS) dissolution. Moreover, effective arsenic immobilization in SSC from BW was observed, and the life cycle assessment (LCA) indicated lower environmental impacts compared to ordinary Portland cement (OPC). • The in-situ formation of ettringite serves as nucleation sites for further precipitation. • C-S-H gels formed in the presence of CAC exhibited higher elastic modulus and hardness. • Higher dosage of CAC and CSA imposed restrictions on the early reaction of SSC. • Life cycle assessment indicated lower environmental impacts compared to ordinary Portland cement. [ABSTRACT FROM AUTHOR]

This study explored the utilization of industrial waste Na 2 SO 4 to develop a calcined clay-based sustainable cementitious composite inspired by ancient Roman concrete. Na 2 SO 4 with or without the addition of NaCl was utilized, keeping the total Na 2 O quantity constant, and their performances at the macro and micro scale were compared. A separate set of samples was prepared using seawater. A medium-grade calcined clay, along with lime, was utilized as the binder. The salts were first dissolved in water and then used as the mixing water for sample preparation. A diverse array of characterization methods was utilized, encompassing thermogravimetric analysis, chemical extraction, X-ray diffraction (XRD), and Backscattered Scanning electron microscopy (SEM). It was observed that the salt-containing batches achieved compressive strength faster than the seawater samples. However, the strength remained in a comparable range (∼30 MPa) for all the samples after 28 days of curing. The initial strength of salt-containing samples was attributed to the formation of gypsum, ettringite, and hydrocalumite. Additionally, the geopolymer gel and calcium aluminum silicate hydrate (C-A-S-H) gel phases were present in all samples. Finally, this study also showed that lime-clay mortars can achieve a reduction in carbon footprint of up to 50 % compared to ordinary portland cement (OPC) mortars. • Sodium sulfate acts as an activator of calcined clay-rich binders. • Sodium sulfate activated composites showed similar performance as seawater cured. • SO 4 and Cl+SO 4 -activated samples achieved 50 % of the strength within 24 hours. • Early age strength originated from gypsum, ettringite, and hydrocalumite formation. • Na 2 SO 4 activated lime-calcined clay binder shows 50 % reduction in carbon footprint. [ABSTRACT FROM AUTHOR]

The study systematically examined the hydration and strength development patterns within ordinary Portland cement (OPC) when triethanolamine (TEA) and limestone (LS) powder were introduced, with a particular emphasis on unraveling their synergistic effects. The findings reveal that 0.02 % TEA addition has a beneficial impact on the early strength of OPC while diminishing later strength. However, the co-incorporation of LS powder effectively eliminates the detrimental effect of TEA on later strength while maintaining the growth of strength. In addition, the concurrent addition of TEA and LS powder reduces the overall cement porosity as well as modulates the pore size distribution, increasing the proportion of non-harmful pores. Through monitoring the hydration process of the OPC, a notably positive synergistic effect between TEA and LS powder is revealed. Firstly, the complexation of TEA indirectly enhances the dilution impact of LS powder, leading to a substantial enhancement in early cement hydration. Secondly, the nucleation effect of LS powder effectively counteracts the inhibitory influence of TEA on C 3 S, thus eliminating the associated drawbacks. Most significantly, the synergistic effect between TEA and the chemical effect of LS powder accelerates the hydration of C 3 A and C 4 AF while preserving ettringite, ultimately resulting in reduced porosity of the samples. • The addition of LS powder counters the detrimental effect of TEA on later strength while preserving strength enhancement. • The co-incorporation of LS powder eliminates the induction period delay caused by TEA and significantly enhances the hydration process. • TEA promotes early AFt formation but accelerates its decomposition after gypsum depletion. • TEA and LS powder synergistically promote AFm formation and retain AFt, enhancing mechanical properties. [ABSTRACT FROM AUTHOR]

Low vacuum is one of the most challenging service environments for cementitious materials, which would affect their hydration and microstructure, and thereby impair strength and durability. In this study, the hydration phases and microstructural evolution of pastes based on Portland cement (PC), sulphoaluminate cement (SAC), and tricalcium silicate (C 3 S, the clinker of PC) were investigated under low vacuum conditions. The results show that rapid mass loss could be observed at the initial stage of low vacuum treatment, which was mainly attributed to the dissipation of free water. After that, the mass loss rate was greatly slowed down and maintained at a relatively low level, which was primarily due to the loss of chemical bound water in hydration products. As a result, C 3 S samples, with the slowest early hydration procedure, exhibited the highest mass loss, which was followed by PC and SAC samples. The dissipation effect by low vacuum treatment resulted in the inhabitation of cement hydration, and correspondingly the contents of hydration products remained almost constant, or even decreased especially for ettringite (AFt). Moreover, the examination of the hydration product morphology demonstrates that under low vacuum conditions, the needle-like morphology of the hydration product C-S-H transferred gradually from divergence to convergence or underwent collapse, while the AFt crystals experienced degradation and then transformed into an amorphous state. This consequently led to the sample after low vacuum treatment exhibiting increased total volume of pores less than 100 nm and elevated percentage of pores with sizes from 50 nm to 100 nm. Overall, this research holds great scientific significance in guiding the engineering application of concrete in low vacuum environments. • Hydration and microstructure evolution of PC, SAC and C 3 S pastes were investigated. • Low vacuum inhibits hydration and even affects the chemical bound water in hydration products. • Low vacuum induces pore coarsening and increased porosity. • The crystallization of ettringite gradually degrades under low vacuum. [ABSTRACT FROM AUTHOR]

Limestone of the purity mandated by the standards for use in cement manufacture (as raw material or as performance improver) is only a fraction of the mined material. Sometimes, limestone contains impurities like silica, clay, dolomite etc. beyond the levels specified for use as an additive or for cement production. This limestone, commonly known as low-grade limestone, also may have a potential to be used as a supplementary cementitious material. This paper reports a study conducted to understand the hydration behaviour of ordinary Portland cement in combination with low-grade limestone. Low-grade limestone samples with dolomite, silica and clay impurities were collected from various limestone mines in India and used in cementitious blends at various replacement levels after suitable processing. Reaction kinetics was studied using calorimetry and phase assemblage using techniques like X-ray diffraction and thermogravimetry. Mercury intrusion porosimetry was employed to study the microstructure of cementitious blends. Dilution effect of low-grade limestone was studied by measuring compressive strength of cement mortar cubes for various replacement levels. Shortening of induction period, increased peak heat rate and reduced time to peak were observed in the blends of ordinary Portland cement with low-grade limestone. Carboaluminate formation, ettringite stabilisation and higher bound water content were also seen. The studies showed that low-grade limestone also can be used as a partial replacement for ordinary Portland cement without compromising the strength up to 15% replacement. Higher replacement levels, although showing a reduction in strength due to dilution effect, could still be considered for non-structural applications. • A systematic assessment of the effect of low-grade limestone with different types of impurities on the hydration of ordinary Portland cement is done. • Similarities and differences in the behaviour of OPC blended with cement-grade and low-grade limestone are brought out. • Non-cement grade limestone, or low grade limestone, can efficiently be utilised as SCM with up to 15% replacement of OPC. [ABSTRACT FROM AUTHOR]

To address the resource utilization issue of MSWI fly ash, a method has been proposed to prepare cementitious materials using harmlessly processed MSWI fly ash (HPFA). After washing and thermal treatment, The physicochemical properties of HPFA were characterized using QXRD, ICP, XRF, and GC-MS. Cementitious materials were prepared using HPFA, gypsum and blast furnace slag (BFS) as raw materials, Through XRD, TG, FTIR, TEM, and SEM-EDS methods, the hydration and hardening characteristics were investigated, and the leaching of heavy metals from solidified materials in simulated surface water and landfill environments was evaluated using ICP-MS. The results indicate that the harmless treatment can effectively reduce the chloride content and dioxins in MSWI FA but still contains heavy metals. The addition of HPFA (10%-50%) can enhance the compressive strength of cementitious materials, accelerate the exothermic process of cement hydration, and increase the total heat release of hydration, implying an improvement in the degree of hydration of the system.HPFA could effectively promote the hydration of gypsum and BFS, resulting in the higher formation of ettringite and C-(A)-S-H（C/S=1，Al/Si=0.3）.The formation of C(Mg/Fe)-(A)-S-H suggests that the Fe-Mg solid solution phase in HPFA may have participated in the hydration reaction. The increased stability of highly cross-linked (Q4) silicate tetrahedral structure with time implies that the material may have good durability. The leaching amount of heavy metals met the standards of Class I surface water in GB3838–2002 and GB5085.3–2007 which implies that the material has environmental safety. This study innovatively provides a preparation path for harmless MSWI fly ash and cementitious material ratios, reveals the hydration mechanism of HPFA in cementitious materials, evaluates the solidification ability of heavy metals, and provides important ideas for the resource utilization of MSWI fly ash. [Display omitted] • The chlorine and dioxin content of HPFA was 3.28% and 0.93 ng-TEQ/kg. • The optimal doping concentration for HPFA is 50%. • HPFA promoted the hydration process of gypsum and BFS. • The heavy metals leaching amount of HPFA50 met the China national standards. [ABSTRACT FROM AUTHOR]

Storage of low-grade diatomite can lead to environmental pollution. This study prepares a low-grade diatomite-based cementitious material (LGDCM) and assesses the influence of CaO/SiO 2 molar ratio on LGDCM strength within a low-grade diatomite–lime–desulfurized gypsum system. The carbon emissions from LGDCM were quantified, and the strength-formation mechanism and microstructure evolution were determined using traditional characterization methods. LGDCM strength was optimized at a CaO/SiO 2 ratio of 1.3 and a desulfurized gypsum content of 2 wt %. The compressive and flexural strengths at 28 d reached 26.66 and 5.38 MPa, respectively. LGDCM reduced the CO 2 emissions (kg/t) and energy consumption (MJ/t) by 46 % and 37 %, respectively, compared to traditional cement, indicating substantial low-carbon and environmentally friendly benefits. Incorporation of lime and desulfurized gypsum modified the alkaline environment of LGDCM. Specifically, when the Ca/Si ratio falls between 1.8 and 4.9, and the Al/S ratio ranges from 0.59 to 1.23 within the LGDCM,fostering the formation of C-S-H gel and ettringite and filling the microstructure pores. This transformation led to the evolution of mesopores and gel pores in the hardened LGDCM over time, which contributed to strength enhancement. This study provides a design method and theoretical basis for the high-value use of low-grade diatomite in novel low-carbon cementitious materials. [Display omitted] • Provided novel insights into the utilization of low-grade diatomite. • Developed a new type of low-grade diatomaceous earth-based cementitious material (LGDCM). • Determined the strength formation mechanism and microstructure evolution patterns of LGDCM. • The LGDCM regulates Ca/Si and Al/S ratios to control the formation of C-S-H gel and ettringite. [ABSTRACT FROM AUTHOR]

A low-carbon binder with ettringite as the main binding phase is prepared at ambient temperature using waste gypsum and metallurgical slag as raw materials. The technical limits of slow setting and low early strength are caused by the insufficient ettringite formation. This is tackled by fine tuning the thermodynamic route of ettringite formation process using polyaluminium compound via promoting the nucleation and Ostwald ripening process of ettringite formation. The high positive charge polymeric Al species and numerous Al(OH) 4 - generated by the hydrolysis of polyaluminium compounds increase the oversaturation degree and enhance the driving force of the Ostwald ripening process of ettringite. The volume ratio of ettringite/C-(A)-S-H in hydration products is further increased remarkably, resulting in the reduction of setting time and the enhancement of compressive strength. The carbon emission of the non-calcined ettringite-based binders developed in this study is 71–87 kg CO 2 -e/t. The research provides theoretical support for the development and application of a novel ettringite-based low carbon binders. [Display omitted] • Novel chemical route to promote ettringite formation into dominant binding phase. • Polyaluminum compound enhances driving force of Ostwald ripening process of ettringite. • Volume ratio of ettringite/C-(A)-S-H in hydration products is increased remarkably. • The carbon emission of the non-calcined ettringite-based binders is 71–87 kg CO 2 -e/t. [ABSTRACT FROM AUTHOR]

Gypsum slag cement is a cost-effective and eco-friendly alternative to ordinary Portland cement, but it lacks early strength. To enhance its early compressive strength, γ-Al 2 O 3 was added to the gypsum slag cement. In this study, composites of in-situ grown nano-γ-alumina @ fly ash (INA) were synthesized using the carbonization method. The effects of INA and commercial nano-Al 2 O 3 (CNA) on gypsum slag cement were investigated. The results indicated that the growth amount of γ-Al 2 O 3 was controlled by NaAlO 2 , and the diameter of nanoparticles was affected by CO 2. Due to the excellent dispersion of nanoparticles, the paste containing INA exhibited better fresh performance than that containing CNA. The nucleation of INA accelerated the formation of ettringite (AFt), which was beneficial to the early strength of gypsum slag cement. Moreover, INA optimized the spatial distribution of AFt, preventing the concentrated aggregation of AFt on the surface of slag, and achieving a low expansion rate. Additionally, gypsum slag cement containing INA exhibited more C-S-H and finer AFt, with a more compact microstructure and lower porosity, and improved strength and carbonation resistance. • The growth amount of INA is controlled by the concentration of NaAlO 2. • Gypsum slag cement with INA outperforms CNA in terms of fresh properties. • INA can greatly enhance the early compressive strength of gypsum slag cement. • INA optimized the growth mode and spatial distribution state of AFt. • INA modification increases the hydration products of gypsum slag cement. [ABSTRACT FROM AUTHOR]

This study develops a novel accelerator with dopamine for rapid setting of shotcrete. Based on the oxidative self-polymerization, the dopamine reacts with O 2 to produce polydopamine (PDA) in the agitation process of cement paste. The as-prepared PDA facilitates the generation of ettringite. Additionally, it controls ettringite to nucleate and grow on the surfaces of various clinkers, which decreases the negative effect of surface-bound hydrate on the dissolution of C 3 A. As a result, with 0.10% dosage of dopamine, the initial setting time of cement paste is shortened from 118 min 45 s to 3 min 38 s, and the final setting time is shortened from 212 min 45 s to 11 min 8 s. This dosage is much lower than those achieved by any hitherto-reported materials. Besides, the compressive strength of cement mortar reaches 33.6 Mpa in 28 days, which can meet the requirements of building engineering. [Display omitted] • The polydopamine is prepared with concurrency of the rapid setting of cement pate. • Meeting the requirement of building engineering only needs 0.1% dosage of dopamine. • Polydopamine promotes the production of AFt via increasing local supersaturation. • Polydopamine controls AFt to nucleate and grow on the surface of various clinker. • Polydopamine disturbs the adsorption of Ca- Ŝ ion pair on Al-rich leached layer. [ABSTRACT FROM AUTHOR]

Ferric-rich sulfoaluminate cement (FR-SAC) is a promising low-carbon cement and is usually used to prepare precast products due to its short setting time. However, the fast setting of FR-SAC paste also leads to its poor fluidity and workability. Polycarboxylate superplasticizer (PCE) and hydroxypropyl methylcellulose (HPMC) can improve the fluidity and viscosity but may affect other properties of FR-SAC products. To get a suitable performance of FR-SAC by adding PCE or HPMC, the properties of FR-SAC paste with PCE and HPMC added and their influence mechanism were explored. The results indicate that PCE adsorbs on the surface of positively charged ettringite in the slurry under the electrostatic attraction, which makes the negatively charged or uncharged FR-SAC particles uniformly dispersed in the slurry, thus regulating the rheology and other properties of FR-SAC paste. The compressive strength of FR-SAC paste increases with the increase of PCE addition. PCE slightly retard the early hydration rate of FR-SAC but does not affect the formation of later hydration products. Adding HPMC to FR-SAC paste can decrease the fluidity and increase the viscosity of FR-SAC paste but has little effect on its hydration properties. Therefore, when PCE and HPMC are used to modulate the rheology of FR-SAC paste, it is necessary to combine the utilization process and compressive strength requirements to keep both the rheological parameters and compressive strength in the suitable condition. • The performances of FR-SAC paste were modulated by the PCE and HPMC. • The influence mechanism of polycarboxylate superplasticizer on the properties of FR-SAC paste was presented. • The influences of hydroxypropyl methyl cellulose on the properties of FR-SAC paste was demonstrated. [ABSTRACT FROM AUTHOR]

In this work, two PC pastes were used to carry out accelerated carbonation experiments, and phase assemblages of paste samples collected at different sampling depths were analyzed. The source of CaCO 3 originating from the carbonation reaction of hydrates in cement was quantitatively determined by a novel method based on the transformation of hydrates. Experimental results indicated that the phase assemblages in the carbonated zone of two PC pastes showed minor differences. C(A)SH, ettringite, AFm-CO 3 , and unhydrated cement particles in the complete carbonation zone of two cement pastes were carbonated and transformed into other phases. For the surface of two PC pastes (0–3 mm), calculations showed that the CaCO 3 originating from the carbonation of calcium hydroxide and C(A)SH accounted for 75.3% and 72.6% of the total CaCO 3 , respectively. A concentration of 20% CO 2 was not beneficial for the diffusion of CO 2 and the subsequent precipitation of CaCO 3 at the later period of carbonation. • The change in phase assemblages of two carbonated PC pastes collected at different depths was analyzed. • A calculation method was proposed to analyze the source of CaCO 3 originating from the carbonation of hydrates in PC. • For the surface, the CaCO 3 originating from the carbonation of portlandite and C(A)SH accounted for 75.3% of the total CaCO 3. [ABSTRACT FROM AUTHOR]

The original concrete subjected to environmental attack will change the properties of recycled aggregates, thereupon affecting the performance of recycled concrete. However, there is a lack of research that considers such impact. This study aims at providing knowledge on the performance of concrete containing sulfate-eroded recycled aggregates considering the effect of erosion age on workability, water absorption, mechanical, microstructure and interface characteristics. Results show that the incorporation of sulfate-eroded recycled aggregates increases the slump by up to 21.7%, and decreases the water absorption by approximately 9.9% given an erosion age of 126 days. With recycled aggregates under a long-term sulfate attack, the slump decreases and water absorption increases, which is due to the increased porosity of the attached old mortar on the recycled aggregate. Similarly, adding sulfate-eroded recycled aggregates leads to significant changes in compressive, split-tensile and flexural strength of concrete as well, with a sudden change occurring at an erosion age of 126 days. In addition, a large amount of erosion products, i.e., ettringite, remain in the residual mortar of the long-term eroded recycled aggregate, which results in excessive pore filling and causes more defects. Furthermore, it has a negative impact on the interface bond between recycled aggregates and the new mortar by reducing the proportion of C-S-H in the interface hydrates. • Concrete containing sulfate-eroded recycled aggregates is designed. • The effect of aggregate erosion age on concrete performance is evaluated. • Sulfate-eroded recycled aggregate negatively affects hydrates composition at the interface. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) members are subject to map cracking owing to the delayed ettringite formation (DEF) by excessive core temperature during the first few days of hydration. Studies on the mechanical properties of concrete with DEF expansion have been conducted to identify the variation in moduli and strength of the material. This paper provides a comprehensive review of the mechanical properties of plain concrete with DEF expansion to examine the degradation characteristics compared to the experimental conditions. The dynamic moduli, static moduli, and compressive strength of the plain concrete decreased up to 60%, 85%, and 65%, respectively, owing to large swelling, where the damage process showed colossal expansion. The reduction amount depends on the aggregate-to-cement ratio, chemical composition of the cement, aggregate type, water-to-cement ratio, and the mix. The static moduli is more sensitive to expansion in the initial expansion stage than other mechanical properties. The structural performance of DEF-affected RC members was also investigated, reviewing the relevant studies to include the structural performance of alkali-silica reaction (ASR) affected structures. The structural performance cannot be appraised using the mechanical properties of plain concrete. It was indicated that the DEF expansion of concrete core can be significantly confined by surrounding reinforcement in RC members. This inhibits the reduction in mechanical properties of concrete and results in maintaining the load-carrying capacity of RC members. Chemical prestress developed in concrete due to the swelling restraint by reinforcement, and anisotropic damage improves the stiffness of the RC members. • Comprehensive review of mechanical properties of DEF-affected plain concrete. • Structural performance of DEF-deteriorated RC members was studied. • Structural performance depends on DEF- induced stress in reinforced concrete. • DEF expansion confinement should be considered to assess structural performance. [ABSTRACT FROM AUTHOR]

This study uses an eco-efficient approach for sustainable phosphogypsum (PG) waste disposal by creating a mesoporous AFt binder. The results reveal that mixing PG waste with a synthetic sodium aluminate can induce the formation of the AFt phase through a cationic exchange reaction at room temperature. The morphology, expansive property, mesopore volume, and mechanical performance of the AFt-binder strongly depend on the activator (as Na 2 O) / PG weight ratio, the alumina modulus (M A) value, the mixing water content, and the curing temperature. The curing time has no significant effect on the performance of the AFt binder, as the majority of physico-mechanical properties are achieved within the first 24 h. For instance, a sample fabricated at a Na 2 O/PG weight ratio of 0.2 using SA with an MA of 1.0 recorded a 1-day compressive strength of 12.1 MPa. Increasing the curing time up to 28 days has no effect on strength development. Therefore, the AFt-based binder can be categorized as a rapid-hardening cementitious material. The proposed strategy mitigates the environmental issues caused by PG waste, contributes to the conservation of natural resources, and reduces the energy demand required to produce this type of cement. • Phosphogypsum was mixed with sodium aluminate to produce ettringite (AFt) binder. • Ettringite exhibits rapid hardening, high expansion, and a mesoporous structure. • Aluminum hydroxide and thenardite are byproducts resulting from the exchange reaction. • The performance of the AFt binder was evaluated under the influence of different parameters. [ABSTRACT FROM AUTHOR]

Global lithium salt price surges drive up costs for high-performance repair materials, creating demand for cost-effective alternatives. Calcium nitrate (CN), with its low production costs and capacity to enhance early cement strength, emerges as a preferred choice. Limited knowledge on combining calcium sulfoaluminate (CSA) cement and CN prompts further exploration for a potentially cost-effective solution in repair materials. This study investigated the impact of varied CN contents on fluidity, setting time, compressive strength, electrical resistivity, heat of hydration, hydration products, and pore structure of CSA cement. The results show that CN can enhanced fluidity, significantly reduced setting time, accelerated hydration process, shortened the hydration induction period, and promoted the nucleation and growth of ettringite. When CN content was 8%, the compressive strength of samples reached its peak at 40.8 MPa, 44.9 MPa, and 47.3 MPa at 3 d, 7 d, and 28 d, respectively. Exceeding 8% CN content will increase the cumulative heat release which reach the maximum of 314.5 J·g−1 at 72 h. The addition of CN will generate the NO 3 -AFm phase and improve the stability of ettringite, as well as refine the pore structure of the samples but increase their total porosity. • The hydration mechanism of CSA cement with high amounts of calcium nitrate (CN) was analyzed. • The optimal amount of CN to enhance the compressive strength of CSA cement samples was 8%. • Although high amounts of CN can refine the pores of CSA cement, it also led to an increase in its total porosity. • The presence of NO 3 -AFm in CSA cement samples affected its hydration process, promoting the stability and formation of AFt. [ABSTRACT FROM AUTHOR]

A low-carbon geopolymer was prepared by using coal gasification coarse slag (CGCS), ground granulated blast-furnace slag (GGBS), soda residue (SR) and phosphogypsum (PG). The properties of CGCS based geopolymers were systematically studied under the combined effects of chemical activation and physical activation, mainly including binder-to-activator ratio, SR/PG ratio, water-to-binder ratio, CGCS grinding time, and curing humidity. Further, the contribution of hydration products to strength were revealed by X-ray diffraction (XRD), scanning electron microscopy (SEM-EDS), thermogravimetric analysis (TG-DTG), Fourier transform infrared spectroscopy (FTIR), and low-field nuclear magnetic resonance (LF NMR). The results showed that for the binder-to-activator ratio of 4:1 and the SR/PG ratio of 1:1, the slump of CGCS based geopolymers was 18.5 mm and the corresponding 28 d UCS was 20.38 MPa. Moreover, the matrix strength could be further enhanced by decreasing the water-to-binder ratio, reducing the grinding time of CGCS, and employing water curing. Therefore, the physico-chemical coupling activation could significantly promote the activation of CGCS and GGBS, with C-(A)-S-H gel and ettringite (AFt) being the primary hydration products. When the SR/PG ratio was 1:1, the synergistic effect of C-(A)-S-H gel and AFt was optimal, and the C-(A)-S-H gel formation compensated for the strength loss due to AFt reduced. The leaching rate of heavy metals from CGCS-based geopolymers was lower than the national standard and met environmental protection requirements. Nevertheless, the leaching concentration of Cl was significantly decreased, but the CGCS-based geopolymers could not meet the requirements used with steel bars. [Display omitted] • Low-carbon geopolymers were prepared by coal gasification coarse slag. • The propotion of PG determines the shapes of AFt: thin rod-like or rod-like. • With the decrease of SR/PG, the formation of Fs from existence to non-existence. • When the SR/PG was 1:1, the synergistic effect of C-(A)-S-H gel and AFt was optimal. [ABSTRACT FROM AUTHOR]

This study investigates the synergistic effects of metakaolin and dolomite on the chemical degradation and phase alteration of alkali-activated slag (AAS) binders exposed to various chloride and/or sulfate-containing solutions. The results indicate that the combined substitution of dolomite and metakaolin promotes gel precipitation and enhances the compressive strength of AAS at a fixed replacement level of 40%. Moreover, the AAS binders show strong stability and limited sensitivity to phase alteration when exposed to NaCl solution, Na 2 SO 4 solution, and artificial seawater. The phase composition and microstructure of the exposed AAS remain largely unchanged in these solutions, except for the trace ettringite precipitation. The exposure to MgSO 4 solution shows more severe deterioration on the AAS binders while the incorporation of metakaolin in AAS reduces the Ca/Al ratio of C-A-S-H, rendering better durability against MgSO 4 attack. Although the incorporation of metakaolin inhibits the formation of hydrotalcite in AAS, the aluminum dissolved from metakaolin promotes Friedel's salt formation in chloride-bearing solutions, which partially compensates for the weakened chloride binding capacity by reduced hydrotalcite formation. However, the potential release of carbonate ions from dolomite dissolution might occupy the hydrotalcite interlayer, leading to reduced chloride binding capacity in AAS with high dolomite content. • Combined use of dolomite and metakaolin improve the compressive strength of alkali-activated slag (AAS). • Metakaolin substitution enhanced the polymerization degree of the gel product but suppressed the formation of hydrotalcite. • AAS with 40% substitution of dolomite and metakaolin show strong phase stability in chloride and sulfate environment. • Higher content of dolomite promoted the hydrotalcite formation but with limited effect on chloride binding capacity. [ABSTRACT FROM AUTHOR]

The high content of heavy metals in electrolytic manganese residue (EMR) makes it difficult to utilize in engineering, and direct disposal can pose a threat to the environment. This study uses EMR as a raw material combined with steel slag (SS) and blast furnace slag (GGBS) to prepare electrolytic manganese residue-based cementitious material (ESGM) under the action of an alkali activator (NaOH), and studies the effects of the EMR to SS ratio and alkali activator concentration on the flowability, setting time, compressive strength, and toxic leaching of ESGM. In addition, XRD, FTIR, SEM EDS, and MIP were used to study the microscopic characteristics of ESGM and its stabilization/solidification mechanism for heavy metals. The results indicate that an increase in EMR content will reduce the fluidity and setting time of the slurry, while an increase in alkali activator concentration will increase the fluidity of the slurry and reduce the setting time. When the content of EMR, SS, and GGBS is 50%, 20%, and 30%, respectively, and the concentration of alkali activator is 0.2, the strength of ESGM reaches its maximum, reaching 23.0 MPa in 28 days. The heavy metal leaching in ESGM is lower than the allowable range of Chinese standard GB 89781–996, and the material meets environmental requirements. The microscopic analysis showed that the ESGM gelling products were mainly C-(A)-S-H and N-A-S-H gels, and the formation of ettringite was also observed. Mn2+ in EMR was mainly stabilized in ESGM in the form of MnO 2 and H 2 Mn 8 O 16 ·2.4 H 2 O. • A fully solid waste manganese residue-based cementitious material (ESGM) was prepared. • When the contents of EMR, SS, and GGBS are 50%, 20%, and 30%, respectively, the strength of ESGM is the highest. • The heavy metals leached from ESGM meet environmental safety requirements. [ABSTRACT FROM AUTHOR]

Preparing beta-hemihydrate gypsum and then using it for gypsum-based self-leveling mortar (GSLM) in building engineering is one of the promising approaches to consume by-product gypsum such as flue gas desulfurization gypsum and phosphogypsum. In order to overcome the deficiencies of low mechanical strengths and large fluidity loss of beta-hemihydrate gypsum, solid wastes-derived sulfoaluminate cement (WSAC) was used to modify beta-hemihydrate gypsum and prepare GSLM in this work. The effects of various additives on the fluidity and setting characteristics of GSLM prepared using WSAC-modified beta-hemihydrate flue gas desulfurization gypsum was firstly studied, and an optimal formula was obtained. The WSAC-enhanced GSLM had good mechanical performance due to the well-connected structure of the hydration products gypsum and ettringite. However, the hydration speed and degree of GSLM prepared using WSAC-modified beta-hemihydrate phosphogypsum were seriously weakened by phosphorus impurities, which resulted in poor mechanical performance. PO 4 3- would react with Ca2+ and Al3+ dissolved from ye'elimite in WSAC to form CaHPO 4 ·2H 2 O and AlPO 4 precipitates, which seriously inhibited ettringite formation. This negative effect was found mainly acting on the hydration of WSAC and could last until later hydration. [Display omitted] • WSAC-modified GSLM has good mechanical performance. • Ettringite bridging between gypsum enhanced compactness and strength of GSLM. • H 3 PO 4 in phosphogypsum weakened GSLM mainly by severely inhibiting its hydration. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ground PG improves volume stability and mechanical properties of ESPSC. • PG-based binder and artificial aggregate were used to prepare the mixture. • The ITZ between the ESPSC and the artificial aggregate was narrower. • ESPSC increased the strength of the mixture containing artificial aggregate. • The mixture containing ESPSC and artificial aggregate has better volume stability. Phosphogypsum (PG) was used to produce an excess-sulfate phosphogypsum slag cement (ESPSC) and artificial aggregates and then co-utilized for road base mixtures to enhance their recycling efficiency. Their hydration process, strength development, volume stability, compatibility, etc., were investigated. The experimental results showed that the wet grinding process resulted in improved strength of the ESPSC mortar with finer ettringite hydrates. The base-course mixture prepared with ESPSC and artificial aggregate exhibited better volume and mechanical properties than that prepared with cement and artificial aggregate. In addition, it showed a narrower interfacial transition zone than that prepared with natural aggregate and ESPSC. The base-course mixture made of ESPSC and artificial aggregate containing 40 wt% PG showed a compressive strength higher than 3 MPa at 7 days, which met the highway base course specifications. This combination can significantly reduce material cost and improve the utilization of PG. [ABSTRACT FROM AUTHOR]

• Effect of alkanolamine-based grinding agent on OPC hydration has been revealed. • It enhanced C 4 AF reaction with slight C 3 S acceleration, resulting in more ettringite formation. • The C 4 AF reaction also led to more formation of amorphous phase and monocarboaluminate. • Those effects increased compressive strength of OPC mortar in both initial and later stages. This study aims to elucidate the origin of the improvement of mechanical properties of ordinary Portland cement (OPC) hydrates with the use of alkanolamine grinding agent (GA). It was found that the chemical activation induced by the use of 0.026 wt% GA led to improvements of the early strength as well as later strength. To be specific, GA predominantly enhanced ferrite (C 4 AF) hydration in both initial and later stages which resulted in more dissolution of calcite. As a result, the promoted ettringite formation at 1 day and monocarboaluminate (Mc) from 2 day were observed. Although the lower content of portlandite (CH) was found over the whole curing period due to the inhibition of CH crystal growth with GA, the amount of amorphous phase was higher from 1 day. More production of amorphous phase was most conspicuous during 7 to 28 days, which was possibly associated with more production of Fe-containing hydrates from the enhanced C 4 AF hydration. [ABSTRACT FROM AUTHOR]

• The coexistence of GGBS and cement improves the mechanical properties of β-HPG. • The presence of GGBS is beneficial for volume stability. • Morphology and location of ettringite altered by GGBS. Using Portland cement to improve the properties of β-hemihydrate gypsum (β-HPG) is a convenient and economical measure. However, the volume stability of the β-HPG-cement system is worrying because the hydration product ettringite is likely to cause expansion damage. This study introduced ground granulated blast furnace slag (GGBS) to enhance the β-HPG-cement system's volume stability. The influence of GGBS on the performance of composite systems was investigated, and the mechanism behind it was discussed. The results showed that when 5 % cement and 15 % GBBS coexisted, the compressive strength of the composite system reached 41.0 MPa, and the softening coefficient was 0.73. As for the volume stability, the 28 d volume change of the specimen containing 5 % cement and 5 % GGBS was consistent with that of the blank group. XRD, TG-DSC, and SEM results showed that the introduction of GGBS increased the content of ettringite but affected its crystal morphology and formation position. Most of the ettringites were thick rods and randomly distributed in the pores, which was speculated to be one reason for the composite system's enhanced volume stability. The altered crystallization driving force of ettringite due to the consumption of OH– by hydration of GGBS was considered responsible for its altered morphology and position. These research results showed that the morphology and location of ettringite had more influence on the expansion properties in the β-HPG -cement-GGBS system, which could provide theoretical guidance for designing composite materials with high-volume stability. [ABSTRACT FROM AUTHOR]

• Simplified up-scalable synthesis route of in situ ettringite in an inert matrix. • Impact of nano-granular ettringite on bulk rheology of a model suspension. • Correlation of particle's specific surface area with rheological properties. • Varying impact of ettringite particles' fineness on yield stress and plastic viscosity. Cement suspensions are subject to hydration processes causing dissolution of cement clinker, precipitation of hydration products and changes in the ion content of the carrier liquid. These mechanisms take place in parallel and control the workability of cement suspensions. The precipitation of nano-granular ettringite is currently believed to be one of the key influencing factors for rheological properties of fresh cement suspensions. In order to quantify the effect of ettringite precipitation from other precipitation or dissolution processes onto the rheological behaviour, a model suspension consisting of quartz powder and in situ formed ettringite is used, enabled by implementing a new upscalable synthesis route of ettringite. The quantity of formed ettringite in the model suspension is varied and derived qualitatively from X-ray diffraction as well as quantitatively from inductively coupled plasma optical emission spectrometry and thermogravimetric analysis. The influence of the ettringite content on the rheological properties is determined and linked to the specific surface of the formed ettringite particles. This allows a quantification of the controlling mechanism of ettringite and the dependency of these mechanisms on particle properties. The correlation of the results to a reference suspension without ettringite verifies the statement of influence. [ABSTRACT FROM AUTHOR]

• CaSO 4 had influenced on hydration and conversion of calcium aluminate cement (CAC). • The reduction of strength of CAC specimens was inhibited. • The incorporating of CaSO 4 inhibited the formation of unstable CAH 10 by direct precipitation of ettringite. Conversion from metastable phase to stable phase in calcium aluminate cement (CAC) causes volumetric instability, resulting in loss of compressive strength. This study investigated the effect of CaSO 4 on the phase conversion of calcium aluminate cement. The specimens with different dosage of CaSO 4 were exposed to 60 °C to trigger the phase conversion of CAC after curing at 20 °C for 7 days. Analysis with X-ray diffraction, thermogravimetry, mercury intrusion porosimetry, and scanning electron microscopy were conducted. The incorporation of CaSO 4 inhibited the formation of unstable CAH 10 by direct precipitation of ettringite. As a result, the reduction of strength of CAC specimens due to the increased pore volume by conversion was inhibited by ettringite and monosulfate formation. [ABSTRACT FROM AUTHOR]

• New test setup to investigate sulphate attack and leaching on concrete powders. • Attack characterised by dissolution and precipitation reactions. • Ettringite and calcite formation in short time frame (7 days). • Eco-concretes: Important role of limestone quality due to reactive Al availability. • pH and superplasticiser influence ettringite nucleation/precipitation rate. The chemical resistance of eco-concrete with high levels (35–65 wt-%) of limestone substitution against Ca-leaching behaviour and sulphate attack was investigated by immersing powdered material in a 10 g L−1 Na 2 SO 4 solution. Time-resolved hydrochemical characterisation of the reactive solutions was coupled with chemical, structural and mineralogical characterisation of the experimental solid materials to describe and to quantify alteration phenomena like the dissolution of portlandite and AFm-phases, transformation of C-S-H gel and the precipitation of ettringite, calcite and hydrogarnet. Nucleation and crystal growth dynamics of ettringite are controlled by the availability of Ca, reactive Al, solution pH and potentially the amount of clay minerals and/or superplasticiser used in the eco-concrete material. The experimental approach allows revealing competing reaction pathways and kinetics causing concrete damage in sulphate-loaded environments and to tailor eco-concrete towards an elevated degree of chemical resistance. [ABSTRACT FROM AUTHOR]

• Strain and ettringite distribution are predicted for 2-D sulphate attack. • A durability-based limit state function is proposed for crack growth prediction. • Effects of specific influential diffusion-reaction factors are quantified. • Reliability approach is introduced to evaluate the sulphate-induced deterioration. This paper investigates numerically the two-dimensional diffusion-reaction behavior of sulphate ions in cement-based materials in accordance with Fick's second law of diffusion and reaction kinetics, and the tensile constitutive law, on account of ettringite formation. Further, a durability-based limit state function and a predictive model are proposed to forecast the crack growth in structures made with cementitious materials exposed to adverse sulphate environment. The results show that the amount of calcium aluminates in the binder and the initial diffusion coefficient are most influential in resisting external sulphate attack. Whereas, the initial porosity has minor significance, the sulphate concentration in the surrounding environment is inconsequential. Stochastic analysis confirms that sulphate-resisting Portland cement accords high reliability even after lengthy exposure, whereas an ordinary Portland cement or that blended with fly ash present a risk of failure in due course. [ABSTRACT FROM AUTHOR]

• The individual impact of calcined clay, limestone and gypsum in LC3 has been studied. • Additional ettringite is formed when gypsum content is increased in LC3. • The limestone reactivity reduces when gypsum is increased in LC3. This study aims to understand the impact of calcined clay, limestone and gypsum on the hydration mechanisms and phase development in limestone calcined clay cement (LC3). Crushed quartz was used as an inert filler to isolate the individual impact of calcined clay and limestone in LC3 systems. The hydration and the microstructure development in these systems were studied using X-ray diffraction (XRD), isothermal calorimetry, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) as well compressive strength experiments. The presence of calcined clay helped in achieving a well-refined microstructure in LC3 within 7 days of hydration. A reduction in the degree of hydration of clinker phases was observed due the presence of calcined clay. The AFt/AFm phase assemblage was seen to depend on the blend composition. Additional ettringite was seen to form when the amount of gypsum was increased in the LC3 system. However, the increased gypsum content also resulted in the reduction in reactivity of limestone. [ABSTRACT FROM AUTHOR]

• Effect of phosphogypsum on reactivity, strength and final matrix properties. • Faster initial and delayed final setting time. • Compressive strength is enhanced when sodium hydroxide is used as alkali activator. • The Al/Si and Ca/Si ratios in the C-A-S-H gels decreased. • Incorporation gave rise to a higher polymerized network. The effect of phosphogypsum (PG) in alkali-activated ground granulated blast furnace slag (GGBFS) on the reactivity, strength development and final matrix properties was investigated, as a function of alkali activator. The results were compared with alkali-activated reference samples from GGBFS. PG completely dissolves and takes part in solid reaction product formation. Upon PG incorporation, portlandite and ettringite are initially formed after dissolution. The hardened binder consists mainly of amorphous hydration products, intermixed with thenardite and minor amounts of secondary gypsum and merwinite. PG incorporation resulted in faster initial and delayed final setting time, while it enhanced the compressive strength and amorphous phase development when sodium hydroxide was used. PG gave rise to a decrease of Al/Si and Ca/Si ratios in the C-A-S-H gels and a higher polymerized network. [ABSTRACT FROM AUTHOR]

Highlights • Expansion of concrete is enhanced by using an ettringite-based expansive agent. • Expansive concrete shows more significant effects with fly ash than without fly ash. • A high CaO, SO 3 , and free lime fly ash most significantly improves the expansion. • Mechanism of fly ash on the expansion enhancement is described by three hypotheses. • Compressive strength and pore structure of expansive concrete under confinement are enhanced. Abstract This paper evaluates the performances of expansive concrete containing fly ashes. In this study, both standard fly ashes and substandard fly ash are used as a cement replacing material in expansive mortar and concrete. A substandard fly ash which contains high CaO, free lime, and SO 3 contents is used in this study to compare it with high CaO and low CaO fly ashes which are standard fly ashes. Flow, water requirement, and setting times of the expansive mortars are studied together with unconfined and confined compressive strength, length changes, and Young's modulus of expansive concretes. Test results indicate that both free expansion and restrained expansion of the expansive concrete are enhanced by using fly ash, especially by the substandard fly ash. Three hypotheses supporting the mechanism of fly ash on the expansion enhancement of the mixtures are described. The first hypothesis is that expansive mixtures containing fly ash produce more ettringite than that without fly ash. The second hypothesis is that a smaller stiffness leads to a higher expansion of the expansive concrete containing fly ash when compared to that of the expansive concrete without fly ash. The third hypothesis is that fly ash delays the hydration of the expansive additive in the expansive mixtures. These hypotheses are clarified by the test results of XRD-Rietveld analysis, DTG, and Young's modulus. The use of fly ash in the expansive concrete enhances the expansion, the compressive strength, and the pore structure, especially for the expansive concrete under confinement. A mercury intrusion porosimetry (MIP) test of mortar is used to verify the enhanced compressive strength and the pore structure of the expansive concretes. [ABSTRACT FROM AUTHOR]

• ES-UHPC exhibits significant tensile strain-hardening behavior at early age. • ES-UHPC obtain high ductility and bonding between steel fiber and matrix at 1 d. • Damage evolution was monitored by acoustic emission method. • AE analysis proves that the ES-UHPC exhibits multiple cracking behavior at 28 d. • Ettringite is the main hydrates and the content increases rapidly from 4 h to 1 d. The properties of early age strength ultra-high performance concrete (ES-UHPC) made with calcium sulfoaluminate (CSA) cement, including tensile performance, damage evolution monitored by acoustic emission (AE) and microstructure were evaluated in this work. Results show that the ultimate tensile strain ε Utu of ES-UHPC can reach to 0.199% at 1 d, which is 4.23 times larger than that of normal UHPC (N-UHPC) at the same testing age. AE testing results indicate that both types of UHPCs exhibit multiple cracking behavior at 28 d. With the tensile strain developing, the two UHPCs show a similar damage evolution trend and multiple microcracks with crack-width smaller than 0.01 mm generated after 0.046% strain reached. Microstructure tests demonstrate that ettringite presented after 4 h and the content increased significantly from 4 h to 3 d, which contributed to the early mechanical properties for ES-UHPCs. [ABSTRACT FROM AUTHOR]

• Ettringite seeds remarkably accelerated the hydration of pastes in the presence of PCE. • 3-h compressive strength increased by 290% when containing 4% ettringite seeds. • The compressive strengths increased with seed content less than 1% at all curing ages. • Ettringite seeds improved the morphology and porosity of CSA pastes. Polycarboxylate superplasticizers have been applied in calcium sulphoaluminate (CSA) cement-based materials to obtain a good workability, while the concomitant retardation in hydration results in a slow early strength development after setting. Ettringite seed crystals synthesized in this research are expected to work as seed nucleus and accelerate the hydration of CSA at early ages in the presence of polycarboxylate-ether (PCE) superplasticizer. The effect of ettringite seed on setting time, rheological properties, compressive strength, volume changes, phase development and microstructure of CSA pastes was experimentally investigated. Results showed that the addition of ettringite seed crystals indeed promoted the hydration and significantly improved the compressive strength and microstructure of CSA pastes at early ages, while reduced the setting time, the yield stress and plastic viscosity. Compared with the control paste, 3-h compressive strength of CSA paste containing 4% ettringite seed increased by 290%. The compressive strengths increased with increasing ettringite seed content less than 1% at all curing ages, with an adequate setting time and rheological properties. [ABSTRACT FROM AUTHOR]

Highlights • Sodium silicate can promote the hydration of cement without retarder. • Sodium silicate can enhance the ability of retarder significantly. • Sodium silicate enhances the ability of retarder by retarded C 3 A. Abstract Sodium silicate is widely used as a coagulation accelerator for oil-well cement. However, the existing experimental results show that the addition of sodium silicate can significantly delay the setting of cement paste with retarder. This study investigated the effects of addition of sodium silicate on the setting time of oil-well cement. For oil-well cement with no retarder, the addition of sodium silicate shortened the setting time. In contrast, for oil-well cement with retarder, the addition of sodium silicate prolonged the setting time and served as a type of retarder. Moreover, the mechanisms involved in retarding the hydration of cement by adding sodium silicate were also examined in detail by XRD, BET nitrogen adsorption, and total organic carbon analysis. The results show that the addition of sodium silicate delayed the hydration of tricalcium aluminate and thus prolonged the setting process of oil-well cement. [ABSTRACT FROM AUTHOR]

Highlights • AS significantly accelerated the hydration of C 3 S. • AS notably refined the pore structure of the hardened C 3 S paste. • AS induced the ettringite-based nucleation seeds formed in bulk solution. • Ettringite induced C S H gel formed in the interspace among C 3 S particles. Abstract In order to obtain a deep understanding of the hydration of sprayed concrete, C 3 S pastes with and without aluminum sulfate (AS) were comparatively investigated. The results showed that AS significantly accelerated the hydration of C 3 S and refined the pore structure of the hardened C 3 S paste, by reacting with the calcium ions to produce ettringite; this acted as nucleation seeds to induce the formation of C S H gel. Differing from C S H based nucleation seeds, the ettringite-based nucleation seeds were formed in bulk solution where was away from rather than inside the rich silicate layer, thereby inducing the formation of C S H gel in the interspace among C 3 S particles. Furthermore, the formation of ettringite consumed a large amount of calcium ions, which not only expedited the dissolution of C 3 S to release the calcium ions into solution, but also perturbed the crystallization of calcium hydroxide (CH), resulting in formation of amorphous CH. [ABSTRACT FROM AUTHOR]

Highlights • Moderate TEA·HCl decreases the water requirement. • It also shortens the setting times and alters strengths. • It delays the end time of the induction period. • Excessive TEA·HCl accelerates AFt conversion to AFm. Abstract Triethanolamine (C 6 H 15 NO 3 , TEA) is one of the known organic amine accelerators for the cement-based materials. Triethanolamine hydrochloride (C 6 H 15 NO 3 ·HCl, TEA·HCl), part of whose molecular structure is the same as TEA, can also be regarded as chloride. Thus the water requirement for the normal consistency, setting time, strength, hydration heat liberation, calcium hydroxide (CH) and ettringite (AFt) contents in the hardened paste with TEA·HCl were analyzed and the effect of TEA·HCl on the performance of cement paste was studied to assess the feasibility of TEA·HCl used as an identical accelerator. The results indicate that moderate TEA·HCl decreases the water requirement for the normal consistency and this decline is related to the formation of the absorbed film on the cement particle. Zeta electric potential result also proves the appearance of the absorbed film. It also shortens the setting times and alters strengths. But the alterations of setting times and strengths depend on the TEA·HCl content. Heat liberation result implies that it delays the end time of the induction period and also postpones tricalcium silicate (3CaO·SiO 2 , C 3 S) hydration at early age, however moderate TEA·HCl increases the rate of C 3 S hydration in accelerated phase. Excessive TEA·HCl accelerates the AFt conversion to monosulfoaluminate (AFm) after several hours. The conversion product is the AFm with 14 H 2 O. TEA·HCl changes the total heat liberation, and these pastes with more TEA·HCl liberate more heat before about 12 h, but this case is reverse after about 12 h. X-ray powder diffraction (XRD) and differential thermal analysis (DTA) results conformably confirm that excessive TEA·HCl delays C 3 S hydration and decreases CH content in the hardened paste, but it increases the AFt content in the hardened paste. Hardened pastes with TEA·HCl provide the higher content of AFt than the control even at 28d. Heat liberation, CH and AFt contents are closely responsible for the results of the setting times and strengths. These results will provide a reference for the TEA·HCl application as an accelerator in the practice. [ABSTRACT FROM AUTHOR]