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The goal of the presented work was to find the most favorable conditions for the synthesis and stabilization of chemically pure ettringite and monosulfate. The reaction was carried out by mixing pure tricalcium aluminate (C3A) and gypsum (C S ¯ H2) in an excess amount of water. The impact of hydration time (2–7 days), C3A:C S ¯ molar ratio (1:1–1:3) and water vapor pressure of the selected drying agents (anhydrite-III and supersaturated CaCl2 solution) on the phase composition of the products was evaluated. After 7 days of hydration, either ettringite or monosulfate was obtained as the main product, depending on the C3A:C S ¯ molar ratio. The synthesis carried out at a C3A:C S ¯ molar ratio of 1:3 produced pure ettringite. In the case of the sample characterized by the ratio of 1:1 (typical of monosulfate), a considerable portion of ettringite (27.9%) was present in the final products along the AFm phase. Therefore, a different synthesis method has to be selected in order to obtain pure monosulfate. The results showed that thermal analysis, X-ray diffractometry and FTIR spectroscopy can be used to distinguish the characteristic features of ettringite and monosulfate. [ABSTRACT FROM AUTHOR]

The article evaluates the mechanical properties of concrete reinforced with hemp and alfa fibers to create an environmentally friendly concrete. The study shows that the use of hemp and alfa fibers improves the tensile strength of concrete compared to ordinary concrete. The addition of 0.25% hemp fibers optimally increases flexural strength, and the dynamic modulus of elasticity of HFC-0.25 is comparable to ordinary concrete. Hemp and alfa fibers are potential candidates for producing green concrete, offering high mechanical properties and low density, suitable for sustainable construction. [Extracted from the article]

The low water/binder ratio of ultra-high performance concrete (UHPC) often results in its high autogenous shrinkage. Our study explored the effect of the single or binary addition of a CaO-based expansive agent (CEA) and steel fibers on flowability, compressive strength, flexural strength, microstructure, and autogenous shrinkage of UHPC. X-ray diffraction (XRD), thermogravimetric (TG) analysis, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were applied to reveal the effects of CEA and steel fibers on hydration products and microstructure characteristics of UHPC. Experimental results show that the autogenous shrinkage of UHPC decreased markedly with the single or binary addition of CEA and steel fibers. Relative to the control group, autogenous shrinkage of UHPC with 2.5% dosage of single steel fibers, 6% dosage of single CEA, and binary addition of 2.5% steel fibers and 6% CEA decreased 17.8%, 10.9%, and 30.8% at 180 days, respectively. Steel fibers could enhance the mechanical performance of UHPC; nevertheless, they would decrease the flowability of UHPC. Meanwhile, the addition of CEA in the UHPC mixture not only maintained the mechanical properties and flowability but also decreased the autogenous shrinkage. Diffraction peak intensity and endothermic peak of Ca(OH)2 and the pore volume of 10-50 nm diminished with the content of CEA; however, that of C-S-H gel and ettringite increased. The prediction accuracy of nine shrinkage models (FHWA model, Lee model, Yoo model, JSCE model, B4 model, JonassonH model, Eurocode 2 model, CEB model, and DilgerW model) is analyzed with RE, R²new, and autogenous shrinkage of UHPC in this paper. [ABSTRACT FROM AUTHOR]

The World Bank study predicts that 4 °C warming will bring high temperatures, sea-level rise, and saltwater intrusion to coastal areas, damaging coastal concrete structures. Increased CO2 from industrialization exacerbates this, necessitating durable, low-carbon concrete. Combined use of fly ash (FA) and ground granulated blast furnace slag (GGBFS) as high-volume OPC replacements boosts performance while reducing concrete's carbon footprint. In this perspective current study examines the durability of concrete against aggressive agents (H2SO4, MgSO4, NaCl, and CO2) causing premature deterioration of concrete structures. Initially, three cost-effective sustainable concrete mix designs were developed, incorporating 50% replacement of OPC with locally available supplementary cementitious materials, specifically FA and GGBFS. These mixes were then evaluated for their mechanical and durability performances. The impact of aggressive ions (SO42−, Cl−, and CO32−) was studied by examining the changes in mechanical performance and phase assemblages. Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) techniques were used to estimate the phase compositions. Ternary blended concrete having 50% OPC+ 30% GGBFS + 20% FA exhibited optimal synergistic performance, enhancing pozzolanic and hydraulic reactions for better resistance to harmful ions. The sorptivity test confirmed that as the GGBFS content increased, the sorption rate decreased, indicating the higher reactive nature of GGBFS to that of FA. Deleterious compounds formed due to the action of SO42-, Cl-, and CO32- were identified to be ettringite (Ca6Al2(SO4)3(OH)12.32H2O, AFt) and gypsum (CaSO4.2H2O, Gy), Friedel's salt (Ca4Al2(OH)12Cl2.4H2O, Fs) and polymorphs of calcium carbonate (CaCO3), respectively through TG mass loss curve. These results were corroborated by FTIR analysis, which showed predominant characteristic bands at 662 cm−1 for SO42−, 459 cm−1 for Mg–O stretching, 790 cm−1 for Al–OH bending, and 1431-1443 cm−1 for C–O, confirming the presence of the deleterious compounds. [ABSTRACT FROM AUTHOR]

The hydration of high-alkali red mud-based ferroaluminate cement (RCFA) clinker with calcium sulfate needs to be regulated. This study explored the effects of the calcium sulfate type and dosage on the hydration and properties of high-alkali RCFA clinker. The research results show that when 4% gypsum was added, the 3 d compressive strength of cement was 39.1 MPa, and the 28 d compressive strength was 63.2 MPa. The 28 d strength increased by 61.6% compared with the 3 d strength. The properties of cement paste can be adversely affected by excessive anhydrite content. The exothermic hydration of clinker was accelerated by calcium sulfate at the beginning, but the rate declined as the process progressed. Sufficient sulfur supply can enhance the hydration of ye'elimite, thereby increasing the AFt content in the hydration product. The mass loss of the hydration product is mainly caused by the dehydration of ettringite, monosulfoaluminate, and AH3. In addition, the Na element in the RCFA hydration product is mainly present in monosulfoaluminate and unhydrated cement particles. [ABSTRACT FROM AUTHOR]

This paper presents a novel low-carbon binder formulated from fly ash (FA), ground granulated blast furnace slag, steel slag, and desulfurization gypsum as a quaternary solid waste-based material. It specifically examines the influence of FA content on the mechanical properties and hydration reactions of the quaternary solid waste-based binder. The mortar test results indicate that the optimal FA content is 10%, which yields a 28-day compressive strength 11.28% higher than that of the control group without FA. The spherical particles of fly ash reduce the overall water demand and provide a "lubricating" effect to the paste due to their continuous gradation, improving the fluidity of the slag-steel slag-gypsum cementitious materials. The micro test results indicate that fly ash has minimal effect on the early hydration products and process of the solid waste-based cementitious materials, but after 7 days, it continuously dissolves silicon-oxygen tetrahedrons or aluminum-oxygen tetrahedrons, consuming Ca2+ and OH− in the system. After 28 days, the amount of ettringite and C-(A)-S-H gel generated increases significantly. The pozzolanic activity of fly ash is mainly stimulated by the Ca(OH)2 from steel slag in the later hydration stage. Additionally, spherical fly ash particles can fill the voids in the hardened paste, reducing the formation of cracks and weak zones, and thereby contributing to a denser overall structure of the hydrated binder. The findings of this paper provide data support for the development of low-carbon cement-free binders using fly ash in conjunction with metallurgical slags, thereby contributing to the low-carbon advancement of the construction materials industry. [ABSTRACT FROM AUTHOR]

To completely replace Portland cement for mine backfilling, MSWI fly ash (FA) and solid metallurgical wastes (ground granulated blast furnace slag, steel slag and flue gas desulphurisation gypsum) were used to prepare fly ash-slag-based binder (FBB). The effect of FA content on the hydration and Pb stabilization mechanisms of the FBB was investigated. When a small amount (15%) of FA was added to FBB, hydration product formation and compressive strength were inhibited during early curing (7 d). However, the additional Ca2+, SO42− and OH− carried by FA could partially promote the formation of ettringite, so that the compressive strength of 28 d was comparable to that of the treatment without FA addition (32.5 MPa). When a high amount of FA (30%) was added, the hydration process was strongly inhibited throughout the entire curing period, and the leaching concentration of Zn (1052 μg/L) exceeded the limits of the GB/T 30760–2014 standard (1000 μg/L). Pb2+ was also found to replace Ca2+ in ettringite to form insoluble Pb-ettringite and Pb2(SO4)O precipitates, resulting in remarkable Pb stabilisation in system. Therefore, if the amount of FA incorporated is appropriate, the use of FA as an additional cementitious material for mine backfilling is an ideal method for recycling FA.Highlights: FA as a cementitious material can meet backfilling strength requirements. FBM containing 15% FA met the CS and metal leaching limits for mine backfilling. FBM retains high Pb stabilization efficiency in acidic environment. Pb stabilization in FBM is achieved by the formation of Pb-ettringite and Pb2(SO4)O. The use of FA as a backfilling material is an ideal method to recover resources. [ABSTRACT FROM AUTHOR]

This study investigated a binary blend of calcium sulfoaluminate cement and fly ash with a target compressive strength of more than 32.5 MPa based on the EN 197-1 standard, using as much fly ash as possible. In the binder system of calcium sulfoaluminate cement and fly ash, a decrease in the replacement level of fly ash results in an increase in compressive strength. Mortar with a 40% replacement level of fly ash for calcium sulfoaluminate cement and added gypsum shows 49.41 MPa strength at 28 d. Its hydrates are hydroxy-AFm, ettringite, crystalline aluminum hydroxide, amorphous aluminum hydroxide, strätlingite, and monosulfate. They are attributed to the hydration of ye'elimite, C2S, and C3A in calcium sulfoaluminate cement. The reactivity of fly ash is not observed until 28 d of hydration. The formation of ettringite is the main cause of the gains in compressive strength. [ABSTRACT FROM AUTHOR]

Calcium sulfoaluminate belite (CSAB)-based cement exhibits useful properties such as low CO2 footprint, high early-age strength and shrinkage compensating behavior. Our previous study examined the mechanism of carbonation in CSAB-based binder and reported a higher degree of carbonation in CSAB-based binder compared to Portland cement (PC) binder. This study examines the admixing strategies to improve the carbonation resistance of CSAB-based binders. The addition of gypsum or calcium hydroxide reduced the carbonation depth (in natural and accelerated environments) in the CSAB binder. Gypsum addition reduced the pH and resulted in pore refinement. Whereas calcium hydroxide addition increased the pH and promoted the formation of monosulfate (AFm). Although the carbonation depth was reduced in the presence of gypsum or calcium hydroxide, the addition of calcium hydroxide offered a better admixing strategy due to its higher pH which is crucial for the passivation of reinforcement. [ABSTRACT FROM AUTHOR]

In view of the unclear action mechanism of modified alcohol amine in commercial reinforcers, the effects of modified alcohol amine (CSM, CSN and CSO) on setting time and strength of paste were investigated, and the effects of modified alcohol amine on cement hydration process, hydration products and microstructure of paste were investigated by means of hydration heat, cement quantitative XRD and microstructure SEM test, and compared with triisopropanolamine (TIPA). The results show that TIPA, modified alcohol amine all show retarding properties, and CSO has the strongest retarding property. In terms of strength, TIPA improves the strength development of cement paste during the test age, and CSM improves the strength development of cement paste for 7 - 28 d, and the improvement effect is better than that of TIPA, CSN and CSO have negative effects on strength. In terms of mechanism, the addition of CSM increases the hydration degree of cement mineral phase, optimizes the hydration rate, improves the microstructure of hydration products, promotes the interlacing of C-S-H, Ca(OH)2 and ettringite, and improves the compactness of paste. TIPA promotes the hydration of C4AF. CSN and CSO promote the hydration of C3 S and C4AF, but the rapid hydration of mineral phase and compact microstructure can not achieve coordination. In terms of modification measures, compared with amino and hydroxyl modification, TIPA molecular weight modification is more helpful to improve cement hydration process, product distribution structure and promote the development of cement paste strength. [ABSTRACT FROM AUTHOR]

We focused on the efflorescence induced microstructural evolution of ettringite-rich systems prepared with calcium aluminate cement (CAC) and anhydrite. The effects of anhydrite on the visible efflorescence, and the corresponding capillary absorption of CAC-anhydrite mortars were revealed. The composition and microstructure of efflorescence-causing substances were investigated by optical microscope, in-situ Raman spectroscopy, scanning electron microscope, energy dispersive spectrometer, thermogravimetric analysis, and differential scanning calorimetry, at multi-scales. Results indicate that, besides the calcium carbonate, ettringite is another main component of efflorescence-causing substances. Compared with the neat CAC mortars, the addition of anhydrite has a significant effect on the degree of efflorescence by acting on the composition of hydration products and pore structure. In addition, methods are proposed for the prevention of efflorescence of CAC-anhydrite binary system. [ABSTRACT FROM AUTHOR]

The use of cementitious materials to improve clay is a common technique in engineering. However, the effectiveness of these materials, particularly desulfurized gypsum, on clays with different mineral compositions remains unclear, resulting in a lack of theoretical basis for their application in engineering. This study investigated the synergistic effects of clinker–metakaolin–desulfurized gypsum on clays with various mineral compositions through a series of macroscopic and microscopic laboratory tests. The results revealed that the stress–strain relationships of all clay samples exhibited softening characteristics. The softening was most pronounced in kaolinite samples, followed by illite and bentonite samples. For single-phase clays, the unconfined compressive strength followed the order of kaolinite > illite > bentonite. For multiphase clays, the order was illite + kaolinite > bentonite + illite + kaolinite > bentonite + kaolinite > bentonite + illite. The strength enhancement in the improved soils was primarily due to kaolinite and illite. As the content of desulfurized gypsum increased, the ettringite crystals in the improved soils transformed from cluster-like to framework-like structures. When the gypsum content exceeded 10%, the macroscopic performance of the improved soils decreased. These findings provide valuable insights for related engineering applications. [ABSTRACT FROM AUTHOR]

This article provides information on ettringite, a hydrated calcium sulfoaluminate mineral that belongs to the ettringite group. It describes the physical characteristics of ettringite, such as its hexagonal, prismatic crystals and color variations. The article also mentions the origins of the name ettringite and its cotype localities in the Ettringen district of Germany. Additionally, it discusses the village of Ettringen and its association with outdoor activities like rock climbing in basalt quarries. The article further explores the Bellerberg volcanic lava flows and their use in manufacturing querns and millstones. It highlights the Kalahari manganese field in South Africa as the primary natural locality for ettringite specimens, with larger crystals than any others known. The article focuses on the identification and properties of yellow ettringite found in the Kalahari manganese field, emphasizing the challenges in visually distinguishing it from other minerals in the area. It also delves into the role of ettringite in the deterioration of concrete and its stability under different conditions. The research for this article was funded by the National Research Foundation and CIMERA. [Extracted from the article]

Supersulfated cements (SSCs), as potential green alternatives to Portland cements, are in demand for developing new kinds of precursors instead of blast-furnace slag. A mixture of kaolin, quartz and sodium hydroxide was calcined to prepare SSC by grinding clinkers with lime and gypsum. Preparation factors were investigated and multiple characterisation methods were adopted to explore the calcination and hydration mechanisms. The sodium hydroxide additive was found to reduce the activation temperature of the kaolin and a moderate addition of sodium hydroxide improved the compressive strength of SSC made from calcined kaolin. Quartz added into the kaolin calcined with a moderate amount of sodium hydroxide also promoted the compressive strength. Comprehensive characterisations indicated that the alkali-added calcination of the kaolin generated pozzolanic zeolite and sodium aluminosilicates along with inert nepheline at a calcination temperature ≤950°C. The hydration of SSC pastes tended to produce C(N)–A–S–H/N(C)–A–S–H gels and crystalline ettringite. During sodium hydroxide-added calcination, quartz added into the kaolin hindered the generation of inert nepheline and produced cementitious N–S gels. The research facilitates the utilisation of the massive discharged coal gangues rich in kaolinite and quartz by synthesising SSCs economically. [ABSTRACT FROM AUTHOR]

This paper describes the first ever reported case in Brazil where delayed ettringite formation (DEF) has been confirmed as the sole cause of deterioration in field concrete. Still under construction and less than 2 years old, numerous cracks were observed in the foundation structures of a building located in Recife, a city known for its history of alkali–aggregate reaction (AAR) cases. In addition to a field survey of cracks, microstructural analyses, tests for residual expansion and compressive strength, a review of the structural design, determination of sulfate content in the groundwater and simplified thermal analysis were carried out. The results confirmed that DEF was the sole cause of the damage present in cast-in-place concrete structures, with no evidence of AAR or any other deterioration process. This work corroborates previously published studies on the high risk of the occurrence of DEF in the region, demonstrating the urgent need for a preventive technical standard in the country, considering the chemical composition of cements and prior thermal analyses to adopt possible preventive measures to limit the maximum internal temperature of concrete. [ABSTRACT FROM AUTHOR]

Incineration bottom ash (IBA) is a by-product generated from incineration of municipal solid wastes, and it is mostly landfilled, especially for fine IBA. Reusing IBA with waste soils as filling materials in civil engineering can be a potential solution. However, the existing studies only focused on the air-cured IBA-soil mixtures in the laboratory, whereas in the field, the mixtures may contact water or immerse in water. Because IBA is rich in sulfates, the stability of IBA-soil mixtures in water may be compromised. Therefore, this study investigated the stability of IBA-marine clay (MC) mixtures in water. Ordinary portland cement (OPC) and granulated blast-furnace slag (GGBS) served as binders to stabilize/solidify IBA-MC. The appearance, strength, mineralogy, and microstructure of IBA-MC, OPC-IBA-MC, and GGBS-IBA-MC were studied. The results showed that all mixtures performed well when cured in air, but some deteriorated after soaking in water. IBA-MC and OPC-IBA-MC subjected to a short precuring period developed significant cracks and strength loss. Long precuring can reduce or eliminate the cracks on specimens during soaking, leading to higher strength. However, GGBS-IBA-MC had no surficial defects and only a slight strength decrease under both precuring periods. The mineralogy and microstructure of IBA-MC and stabilized IBA-MC revealed that the formation and growth of ettringite were responsible for the deterioration of IBA-MC and OPC-IBA-MC soaking in water. [ABSTRACT FROM AUTHOR]

The "returning to yellow" phenomenon usually occurs in treated chromium residue, which is related to the re-release of Cr(VI) in undissolved Cr(VI)-bearing minerals. In this regard, the complete dissolution of unstable Cr(VI)-bearing minerals and the release of Cr(VI) are crucial to the long-term stability of chromium residue. In this study, the dissolution of ettringite was facilitated by microwave (MW), and then the efficient stabilization of chromium residue was achieved via MW combined with FeSO4 treatment. In general, the apparent activation energy of ettringite dissolution (35.8 kJ/mol) was significantly reduced by MW irradiation, and hence promoting the rapid dissolution of ettringite to release Cr(VI). Furthermore, the dissolution of hydrotalcite was promoted by H2SO4 from FeSO4 hydrolysis in MW field, leading to the complete dissolution of Cr(VI)-bearing minerals in chromium residue. Moreover, the reduction efficiency of Cr(VI) by FeSO4 was significantly improved by MW, which reached up to 100% with the treatment of MW (462 W) combined with low dose FeSO4 (w = 21.5%). More importantly, the formation of crystalline Cr(III)-bearing minerals, including FeO · Cr2O3 and Cr2O3 were promoted by MW irradiation. There was no "returning to yellow" phenomenon occurred after one year. The stabilization and safe disposal of chromium residue was obtained. Therefore, this study provided data support and theoretical basis for the application of MW-coupled FeSO4 treatment in the field of chromium residue stabilization. [ABSTRACT FROM AUTHOR]

Abstract Marl soil is highly prone to erosion when exposed to water flow, posing a potential threat to structural stability. The common practice of stabilizing soil involves the addition of cement and lime. However, persistent reports of severe ruptures in many stabilized soils, even after extended periods, have raised concerns. In stabilized marls, unexpected ruptures primarily result from the formation of ettringite, which gradually damages the soil structure. This article aims to assess the impact of nanosilica on the formation of ettringite and the nanostructure of calcium silicate hydrate (C-S-H) during the marl soil stabilization process with lime. To achieve this, marl soil was stabilized with varying percentages of lime and nanosilica. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were collected to observe changes in mineralogy and microstructural properties. Various geotechnical parameters, including granularity, Atterberg limits, compressive strength, and pH, were measured. The results indicate that the uniform distribution of nanosilica in marl-lime soils enhances pozzolanic activities, calcium aluminate hydrate growth (C-A-H), and the nanostructure of calcium silicate hydrate (C-S-H). According to XRD and SEM experiments, the presence of nanosilica reduces the formation of ettringite. Moreover, the compressive strength of modified samples exhibited an upward trend. In the experimental sample manipulated with 1% nanosilica combined with 6% lime, the compressive strength increased by 1.84 MPa during the initial 7 days, representing an approximately 18-fold improvement compared to the control sample.

Ultra-high performance concrete (UHPC) contains a significant number of unhydrated cementitious material particles due to its extremely low water-binder ratio. This makes UHPC highly susceptible to the external water supply. This paper examines the correlation between the moisture transfer and compressive strength development of UHPC under three conditions: sealed standard curing, sealed steam curing, and conventional standard curing. This study analysed the corresponding mechanisms using XRD, TG, SEM, MIP and other microscopic characterisation methods. The UHPC exhibited the best mechanical properties at the deepest moisture transfer positions, and the strength increase was most pronounced under sealed steaming. However, the UHPC experienced a reduction in strength under all three curing conditions. The XRD and SEM analyses indicated the presence of ettringite in the UHPC under various curing conditions. This was closely related to the strength reduction, and UHPC was more likely to produce ettringite in areas with higher water contents, which adversely affected its strength. When steamed, the reaction degree of the fly ash inside the UHPC intensified, resulting in a more favourable strength performance.

Mixing cement with seawater will cause the hydration process of cement to be different from that of ordinary cement, which will significantly affect cement's mechanical properties and durability. This article investigates the effects of chloride ion concentration, curing temperature, and nano-metakaolin content on the evolution process of Friedel's salts and ettringite (AFt) crystals in cement pastes. The study was conducted using X-ray diffraction (XRD), thermal analysis (TG), scanning electron microscopy (SEM), and mercury-intrusion porosimetry (MIP). The results show that chlorine salt can increase the production of Friedel's salt and ettringite, and the delayed AFt production increases by up to 27.95% after the addition of chlorine salt, which has an adverse effect on cement-based materials. Increasing the curing temperature and increasing the nano-metakaolin dosage increased the generation of Friedel's salt and decreased the delayed AFt generation, which resulted in a decrease in the length and diameter of the AFt crystals. After 28 days of high-temperature curing and the addition of nano-metakaolin, Friedel's salt production increased by 13.40% and 14.34%, respectively, and ettringite production decreased by 9.68% and 7.93%, respectively. Increasing the curing temperature and adding nano-metakaolin can reduce the adverse effect of delayed ettringite increases due to chloride ion binding. [ABSTRACT FROM AUTHOR]

Marl soil is highly prone to erosion when exposed to water flow, posing a potential threat to structural stability. The common practice of stabilizing soil involves the addition of cement and lime. However, persistent reports of severe ruptures in many stabilized soils, even after extended periods, have raised concerns. In stabilized marls, unexpected ruptures primarily result from the formation of ettringite, which gradually damages the soil structure. This article aims to assess the impact of nanosilica on the formation of ettringite and the nanostructure of calcium silicate hydrate (C-S-H) during the marl soil stabilization process with lime. To achieve this, marl soil was stabilized with varying percentages of lime and nanosilica. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were collected to observe changes in mineralogy and microstructural properties. Various geotechnical parameters, including granularity, Atterberg limits, compressive strength, and pH, were measured. The results indicate that the uniform distribution of nanosilica in marl-lime soils enhances pozzolanic activities, calcium aluminate hydrate growth (C-A-H), and the nanostructure of calcium silicate hydrate (C-S-H). According to XRD and SEM experiments, the presence of nanosilica reduces the formation of ettringite. Moreover, the compressive strength of modified samples exhibited an upward trend. In the experimental sample manipulated with 1% nanosilica combined with 6% lime, the compressive strength increased by 1.84 MPa during the initial 7 days, representing an approximately 18-fold improvement compared to the control sample. [ABSTRACT FROM AUTHOR]

One of the most important hydration phases of Portland cement is ettringite, a calcium sulfo-aluminate mineral ( C a 6 A l 2 (O H) 12 (SO 4) 3 · 26 H 2 O ) showing a great capacity of adsorbing radionuclides and other contaminant cationic and anionic species, or incorporating them into its crystal structure. In this work, the X-ray diffraction pattern and infrared spectra of a synthetized ettringite sample are recorded and simulated, employing theoretical methods based on Density Functional Theory. Despite the complexity of this phase, the calculated structure, X-ray diffraction pattern and infrared spectrum are in excellent agreement with their experimental counterparts. Since the calculated and experimental spectra are consistent, the main infrared bands are assigned using a normal coordinate analysis, some of them being completely reassigned with respect to other experimental works. The good agreement found provides strong support for the computational methods employed towards their use for studying the surface adsorption properties and the incorporation of contaminations in its structure. The density of reactive groups at the surfaces of ettringite is reported, and the surface adsorption of water molecules is studied. These surfaces appear to be highly hydrophilic, in agreement with the experimental finding that the ettringite structure may include more water molecules, at least up to 27, one more than in its standard formula. [ABSTRACT FROM AUTHOR]

To provide a theoretical basis for reducing thaumasite sulfate attack (TSA) and enhancing ground limestone utilization in cement-ground limestone cementitious materials, this study examines the mechanisms underlying thaumasite formation and its relationship with temperature, cations, and ground limestone, utilizing thermodynamic principles. The findings indicate that thaumasite formation is constrained above 60.35 °C. Between 48.15 and 60.35 °C, only direct route is feasible, while below 48.15 °C, both direct and woodfordite routes occur spontaneously. As temperature decreases, Gibbs free energy changes decrease for both routes, promoting thaumasite formation. Mg2+ facilitates thaumasite formation via woodfordite route, while Na+ promotes it via direct route. With both Mg2+ and Na+ present, thaumasite formation depends on Na+ to Mg2+ ratio: below 5 favors woodfordite route, exceeding 5 favors direct route. Ground limestone reduces sulfate required for thaumasite formation, favoring the woodfordite route and exacerbating TSA; however, excessive ground limestone has a dilution effect on cement, thereby reducing thaumasite formation. [ABSTRACT FROM AUTHOR]

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The addition of calcium (Ca)-based stabilizers to sulfate-rich expansive soils is associated with the formation of ettringite, a deleterious reactant that can cause moderate-to-severe swell-related damage to overlying lightweight infrastructures. This research study was conducted to understand the effects of combining nanosilica admixtures with a traditional Ca-based stabilizer to effectively treat high-sulfate soils with an intent to suppress the ettringite formation. Engineering and microstructural studies were thus performed to gain a comprehensive understanding of the behavior of sulfate-bearing soils treated with lime in the presence of amorphous nanosilica. The engineering studies on treated and untreated soils included strength tests before and after capillary soaking, free swell strain tests, and resilient moduli studies that were performed to study and understand the macrostructural behavior of these soils at different curing periods. Supplemental studies using scanning electron microscope imaging and energy dispersive X-ray spectroscopy, thermal analyses using differential scanning calorimetry, and X-ray diffraction studies were also conducted to determine the microstructural changes that occur within these sulfate-rich soils. The results showed that additional silica phases furnished from nanosilica suppressed the precipitation of ettringite and correspondingly increased the formation of cementitious phases. This study also provided ample evidence that the application of amorphous siliceous nanomaterials positively impacts chemical treatments and reduces the precipitation of ettringite in sulfate-rich soils, thus enhancing their engineering performance. [ABSTRACT FROM AUTHOR]

This study explores the impact of weak acid salt agents (citrate and tartrate) in the mixture and various curing conditions on the properties of supersulfated cement (SSC) mortar. Compressive strength tests were conducted on prism-shaped samples measuring 4 × 4 × 16 cm. A comprehensive analysis of the phase assemblage of hardened products was carried out using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The results reveal the abundant formation of ettringite (E) and hydrotalcite (Ht) when weak acid salt agents are added, especially in chloride and sulfate-rich curing solutions, leading to a notable increase in compressive strength, up to 200% compared to the control sample. Furthermore, the study demonstrates a substantial improvement in early and overall strength under temperature-stimulated curing conditions, accelerating the hydration reaction of SSC. These findings provide deeper insights into the factors influencing SSC hardening and strength development, offering valuable information for enhancing the efficiency and utilization of SSC in the construction industry, aligning with the goals of sustainable development. [ABSTRACT FROM AUTHOR]

One significant aspect that affects the performance of high‐ferrite cement is the incorporation of iron (Fe) atoms and its stabilization into the cement phases. However, the influence of Fe doping on the stability of key phases, particularly on the ettringite, remains largely unexplored requiring deeper insights into the fundamental mechanisms. The present study explored the stability of Fe‐doped AFt structure based on density functional theory calculations at different Fe dosage. The obtained results showed that the stability of AFt structure decreases by increasing Fe dosage, which is in good agreement with the previous experimental observations. The incorporation of Fe into ettringite induced a series of changes in AFt structural and electronic properties. Bond order analysis underscored stronger covalent Fe–O bonds compared to Al–O, further emphasizing changes in the material's bonding network. Notably, density of states (DOS) analysis highlights the emergence of new occupied states near the Fermi level, primarily contributed by Fe and O atoms. This increased DOS, coupled with higher electron mobility, correlates with a reduction in material stability, as observed through shifts in bonding interactions and alterations in the bonding network. These findings point toward a complex interplay of factors, including altered bonding characteristics, increased electron mobility, and changes in the electronic structure, contributing to the observed instability of Fe‐doped ettringite. [ABSTRACT FROM AUTHOR]

Calcium sulfoaluminate belite (CSAB) cement has gained prominence as a viable environmentally friendly substitute for conventional portland cement (PC). The current study investigates the relative performance of expansive and nonexpansive CSAB cement-based concretes, and PC-based concretes when subjected to chloride-rich conditions. Multiple testing methodologies, including surface resistivity measurements, chloride migration, rapid chloride penetration, long-term bulk diffusion, and water sorptivity, were employed to assess the durability of these concrete systems. Surface resistivity measurements indicated that CSAB systems exhibited significantly higher resistivity when compared with PC-based counterparts. Increased resistivity in CSAB systems was influenced by the conductivity of the pore solution, emphasizing that the formation factor provides a more accurate representation of the pore structure within the system. The expansive and nonexpansive CSAB cement–based concrete outperformed PC-based concrete in migration-based tests and rapid chloride penetration test (RCPT). In contrast, the CSAB and PC systems demonstrated similar performance in the long-term bulk diffusion-based test. Although CSAB systems exhibit finer pore structures than PC-based counterparts, their performance in chloride-rich environments is affected by their reduced binding capacity. The study emphasizes that the conclusions drawn from accelerated tests and resistivity measurements of CSAB cement–based concrete must be cautiously interpreted because these results may not indicate real-world performance. [ABSTRACT FROM AUTHOR]

As the core admixture of shotcrete, the accelerator promotes the rapid setting of shotcrete. In order to promote the rapid development of strength, the synergistic effect and the synergistic mechanism of early-strength agents [ Al(NO3)3 (AN), and Mg(NO3)2 (MN)], and aluminum sulfate liquid accelerator on the strength of mortar were investigated. The aluminum sulfate liquid accelerator used in this paper was named AF. The results show that the 1-day strength of the mortar reached 8.8 and 10.73 MPa when AF was added with 2.4% AN and 1.8% MN, respectively. When AF and 2.4% AN were used, the initial and final setting times of the cement paste were 1 min 32 s and 3 min 50 s, respectively, whereas the AF and a very small amount of MN would prolong the setting time of the paste. The synergistic mechanism of AN/MN and AF is as follows: AF provides a large amount of SO42− and Al3+ , which promotes the rapid formation of ettringite (AFt). In addition, NO3− in AN and MN promotes the formation of the AFt phase 3CaO·Al2O3·Ca(NO3)2·XH2O (NO3-AFt). Compared with AN, Mg2+ dissolved by MN will first precipitate Mg(OH)2 with OH− , providing crystal nuclei for the hydration system and accelerating the hydration of C3S. It is concluded that AF and MN have a better synergistic effect on improving early strength. [ABSTRACT FROM AUTHOR]

Low-temperature circulating fluidised bed combustion (CFBC) of oil shale results in Ca-rich ashes with low pozzolanic properties, raising concerns regarding the long-term stability of ash depositories. This paper presents findings from a long-term field study investigating the mineral and chemical transformations of total CFBC ash sediments over a 15-year period. The study reveals that the pozzolanic properties of CFBC ashes are primarily influenced by the formation of Ca-Al sulphate mineral ettringite and the compactness of sediment. The compaction of ashes during deposition contributes to the development of a dense microstructure, providing uniaxial compressive strength that exceeds 20 MPa. Our findings suggest that compacted and hydrated CFBC ashes are sufficiently stable in ash deposits, holding promise as a sustainable alternative for cement-free construction materials. [ABSTRACT FROM AUTHOR]

Calcium sulfoaluminate belite (CSAB) binder is an alternative low CO2 binder. CSAB cement has ye'elimite as the primary clinker phase which hydrates to form monosulfate or ettringite as the main hydration product. The hydration and mechanical characteristics of CSAB cement and PC-CSAB blended cement have been reported in several studies; however, studies on the durability characteristics such as carbonation are limited. This study focuses on evaluating the microstructural and mechanical alterations due to carbonation in a CSAB binder system and understanding the underlying mechanism of development of the carbonation front. CSAB binder was found to carbonate rapidly compared to PC binder, accompanied by a reduction in compressive strength. This was attributed to the increase in the pore volume due to the carbonation of the ettringite-rich microstructure, facilitating further ingress of CO2 into the microstructure. The rate of carbonation in CSAB binders diverged significantly from the square root of time model used for PC binders. [ABSTRACT FROM AUTHOR]

The activity and expansibility of circulating fluidized bed coal-fired sulfur-fixed ash (CFB fly ash) were utilized to prepare compaction slurry. The effects of CFB fly ash content and water-binder ratio on the workability, strength and expansion rate of the compaction slurry were investigated. CFB fly ash and mineral powder were mixed in different ratios, and the high-strength and micro-expansion characteristics of CFB fly ash compaction slurry were studied and verified by XRD and SEM analyses. The results show that, with the increase of CFB fly ash replacing cement content, the water demand of the compaction slurry increases and the compressive strength decreases. The 28 d compressive strength of the specimen with 20% (mass fraction) CFB fly ash is close to that of the base sample, up to 85.8 MPa, while the flexural strength rises first and then decreases, and the expansion rate rises sequentially. With the increase of water consumption, the expansion rate decreases but is positive. With the increase of the rate of CFB fly ash replacing mineral powder, the workability decreases, and the 28 d compressive strength rises first and then decreases. The specimen with 20% (mass fraction) CFB fly ash and 10% (mass fraction) mineral powder has the highest 28 d compressive strength, which is 99.8 MPa, and the expansion rate has been increasing. The various properties of compression slurry with CFB fly ash and mineral powder meet the requirements of the iron standard. CFB fly ash not only expands by its own hydration, but also can activate the activity of mineral powder, resulting in a composite enhancement effect. [ABSTRACT FROM AUTHOR]

Abstract Ettringite is a hydrous calcium aluminum sulfate mineral present in cement. In this study, ettringite was synthesized via co-precipitation to remove methyl blue, dichromate, and copper ions from solutions with various pH values. The synthesized ettringite was characterized using scanning electron microscopy, X-ray photoelectron spectrometry, Fourier transform infrared spectroscopy, zeta potential analysis, and the Brunauer–Emmett–Teller method. Equilibrium adsorption experiments were performed using methyl blue and dichromate at pH values ranging from 5 to 11. To avoid precipitation, equilibrium adsorption experiments were performed on copper ions in solutions with pH values of 3, 4, and 5. The adsorption kinetics experiments for each contaminant were performed at pH 5. The results showed that ettringite was successfully synthesized, and calcite might have precipitated during the synthesis process. The point of zero charge was at pH 8.6. The maximum adsorption capacities for methyl blue, dichromate, and copper ions at pH 5 were 406, 321, and 365 mg/g, respectively. The adsorption kinetics fitted the pseudo-second-order model well. The properties of the contaminants affect their equilibrium and rate constants. Ion exchange is regarded as the primary adsorption mechanism, whereas the other mechanisms include complexation, hydrogen bonding, surface precipitation, π-interaction, and van der Waals forces. This study revealed a new adsorbent, ettringite, for the removal of contaminants from wastewater, which is a promising alternative adsorbent that can be used under specific conditions.

Super sulphated cement (SSC) is a very promising substitute for traditional construction materials (i.e. Portland cement), due to its enhanced durability and particularly low environmental impact. This paper explores the microstructure and certain properties of SSC, focusing on the particular complexities of its microstructure and the difficulties of microanalysis of its hydrates. To do so, SSC paste samples were first cast to identify hydration products using X‐ray diffraction, then observed at early age using confocal laser scanning microscopy (CLSM) and at early and late age using scanning electron microscopy. In addition, concrete cores impregnated with fluorescein in order to highlight porosity, cracking and aggregates debonding were observed under UV light using optical microscopy (OM), showing a complete absence of cracking and aggregate debonding. Both microscopy techniques (CLSM and UV light OM) have been applied to this type of binder for the first time. The results show that SSC microstructure is characterised by a sophisticated intergrowth of various phases, including ettringite and amorphous calcium‐(alumina)‐silicate hydrate gels. Finally, Monte–Carlo simulation of electron‐matter has been provided for a better understanding of EDS analysis. LAY DESCRIPTION: This work focuses on the study of a particular construction material known as super sulphated cement (SSC). SSC appears to be a promising alternative to traditional building materials as it offers improved durability reduced and environmental impact attributed to its composition comprising at least 75% of a by‐product of the steel industry. This study investigates the microstructure and properties of this type of cement, focusing particularly on its complex microstructure and the challenges posed by the analysis of its hydration products. To do this, samples of SSC cement paste were first cast to identify hydration products using X‐ray diffraction. They were then examined at early stages using confocal laser scanning microscopy (CLSM) and at early and late stages using scanning electron microscopy (SEM). In addition, fluorescein‐impregnated SSC concrete cores were observed under ultraviolet (UV) light using optical microscopy (OM) to highlight porosity, cracking and delamination of aggregates, phenomena that may be present in such materials. These observations revealed the absence of these problems on this type of cement in this work. This research marks the first application of CLSM and OM under UV light to this type of cement. These microscopic techniques enabled us to understand the hydration phenomenon of this type of cement and its highly complex morphology. Finally, Monte–Carlo simulation was used in this study to model electron‐matter interaction, enabling us to understand the elemental analyses obtained and to gain a better understanding of the microstructure of SSC samples. [ABSTRACT FROM AUTHOR]

Metakaolin-based geopolymers have substantial potential as replacements for cement, but their relatively inferior mechanical properties restrict their application. This paper aims to enhance the mechanical properties of metakaolin-based geopolymers by incorporating appropriate amounts of calcium sources. CaCO3, Ca(OH)2, and CaSO4 are three types of calcium sources commonly found in nature and are widely present in various industrial wastes. Thus, the effects of these three calcium sources on the performance of metakaolin-based geopolymers were studied. Through the analysis of the mechanical properties, heat-release behavior during hydration, hydration products, and microstructure of geopolymers, the effectiveness of the aforementioned calcium sources in improving the performance of metakaolin-based geopolymer was evaluated, and the mechanisms of action were elucidated. The results indicate that the pozzolanic reaction between CH and MK could promote MK hydration and increase the proportion of CASH gel in the hydration products, thereby facilitating the setting of the geopolymer and enhancing its strength. CS could react with the active aluminates in MK to form ettringite, thus forming a higher early strength. CC had a lower reactivity with MK and does not improve the performance of MK-based geopolymers. [ABSTRACT FROM AUTHOR]

When calcium-based stabilizers (e.g., lime and cement) are used to treat sulfate-bearing soil, ettringite will be produced under wet conditions, causing swelling of the soil and damage to pavements constructed on sulfate-bearing soils. In order to reduce the swelling of soils caused by ettringite, this study attempted to use the non-calcium-based activators (sodium silicate and NaOH) to activate ground granulated blast-furnace slag (GGBS) to stabilize the sulfate-bearing soil. A series of experiments were carried out: unconfined compression strength, swelling test, leaching test, X-ray diffraction (XRD), and scanning electron microscopy (SEM) tests, to study the performance of soils treated with sodium silicate–NaOH-GGBS. The results showed that the swelling percentage of sodium silicate–NaOH-GGBS-treated soils (0–0.2%) were smaller than that of cement-treated soil (1.2%), and their strengths (1.35–3.35 MPa) were higher than cement-treated soil (0.79 MPa). This was attributed to the formation of ettringite in the cement-stabilized soil after soaking. The calcium ion (Ca2+) concentration in the cement-stabilized soil was higher than that in the sodium silicate–NaOH-GGBS-stabilized soils, and the Ca2+ concentration in the sodium silicate–NaOH-GGBS-stabilized soils kept at a low and stable level. The rapid consumption of Ca2+ in sodium silicate–NaOH-GGBS-stabilized soils for the synthesis of calcium silicate hydrate (CSH) caused the absence of Ca2+ and prevented the production of ettringite. XRD and SEM tests did not detect ettringite in the sodium silicate–NaOH-GGBS-treated soils, which was the reason for insignificant swelling and high strength of the soils. Overall, the sodium silicate–NaOH-GGBS were effective in suppressing swelling due to ettringite and enhancing the strength of sulfate-bearing soils. [ABSTRACT FROM AUTHOR]

There is a huge reservation of loess in the Shanxi mining area in China, which has great potential for preparing supplementary cementitious materials. Loess was modified via mechanical and thermal activation, and the pozzolanic activity was evaluated using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Moreover, the workability of grouting materials prepared using modified loess was assessed. The experimental results revealed that the number of ultrafine particles gradually increased with the grinding time, enhancing the grouting performance. The coordination number of Al decreased upon the breakage of the Al–O–Si bond post-calcination at 400 °C, 550 °C, 700 °C, and 850 °C. Moreover, the breaking of the Si–O covalent bond produced Si-phases, and the pozzolanic activity of loess increased. Furthermore, the modified loess was hydrated with different cement proportions. With increasing grinding time, the overall setting time increased until the longest time of 14.5 h and the fluidity of the slurry decreased until the lowest fluidity of 9.7 cm. However, the fluidity and setting time decreased with increasing calcination temperature. The lowest values were 12.03 cm and 10.05 h. With the increase in pozzolanic activity, more ettringite was produced via hydration, which enhanced the mechanical properties. The maximum strength of the hydrated loess after grinding for 20 min reached 16.5 MPa. The strength of the hydrated loess calcined at 850 °C reached 21 MPa. These experimental findings provide theoretical support for the practical application of loess in grouting. [ABSTRACT FROM AUTHOR]

Super-sulfated cement (SSC) is known for its low-carbon footprint, energy efficiency, and eco-friendliness (mainly derived from industrial by-products) with promising applications. However, SSC's slow early strength development results in inadequate initial hardening, compromising its durability and limiting its use in practical engineering projects. This study aims to enhance SSC's early performance by incorporating ordinary Portland cement (OPC), carbide slag (CS), and sodium silicate as alkaline activators alongside anhydrite. The effects of varying proportions of OPC, CS, and sodium silicate on SSC's compressive strength and hydration mechanism have been investigated experimentally in this study. Results show that using 2% OPC, 2% CS, and 1% sodium silicate as alkaline activators effectively activates slag hydration in SSC-2, achieving a compressive strength of 9.6 MPa at 1 day of hydration. As hydration progresses, SSC's compressive strength continues to increase. In the early hydration stage, OPC and CS create an alkaline environment for SSC, facilitating rapid slag reaction with anhydrite and sodium silicate, resulting in ettringite and C-S-H formation. Simultaneously, slag hydration produces C-S-H and OH-hydrotalcite, filling voids in the ettringite-formed skeleton structure, leading to a denser microstructure and significantly enhancing SSC's early compressive strength. From 28 to 90 days of hydration, the ettringite formation rate decreases in the SSC system, but some anhydrite remains, while C-S-H production continues to rise, further enhancing late-stage compressive strength. [ABSTRACT FROM AUTHOR]

The fast aging of existing building stock requires effective and sustainable strengthening solutions. Textile-reinforced mortars (TRM) have already proved to be very effective as well as versatile retrofitting solutions for reinforced concrete and masonry structures. TRMs can enhance the load bearing capacity of reinforced concrete structures; however, current TRM systems are based on standard Portland cement-based binders, which largely contribute to global human-induced CO2 emissions. This work, for the first time, explores the use of belite calcium sulfoaluminate (BCSA) binder for carbon textile reinforcement through a cross-disciplinary study combining structural engineering and materials science. An experimental study was carried out on concrete block members with externally bonded strips of carbon textile-reinforced mortars, similar to a typical TRM retrofitting system for concrete beams. The textiles were embedded in an ordinary Portland cement-based (OPC) binder or in a BCSA-based binder to compare the bond behaviour to the concrete substrate. The tests revealed a superior bond between the BCSA mortar and the concrete, as well as outstanding adhesion to the textiles achieved using the BCSA binder, with performance levels largely surpassing those measured in their counterparts that used the OPC-based binder. Scanning Electron Microscopy, X-ray diffraction, and thermogravimetric analyses were used to understand this behaviour difference and it was concluded that the ettringite phase is responsible for the enhanced performance in the studied system. The results of this study suggest that BCSA binders have the potential to be a more effective and "greener" alternative to the standard binders based on Portland cement in TRM strengthening applications. [ABSTRACT FROM AUTHOR]

This work discusses the effect of rich alumina bauxite on the mineralogical composition of calcium sulfoaluminate (CSA) clinker cement on their performances. After preparation of different local raw materials (rich alumina bauxite and marble), they were mixed with 15 wt% of commercial gypsum and pressed at 2 MPa. The obtained pellets were thermally treated at 1200°C to produce clinker, which is use to synthesize the CSA cements. The raw materials as well as products were characterized by many analyses, such as FT‐IR, X‐ray fluorescence, X‐ray diffraction, differential scanning calorimetry (DSC)/thermogravimetry analysis (TGA), isothermal calorimetry (ICC), scanning electron microscope, and physico‐mechanical tests. ICC analysis of the powder of clinker cement showed that the reactions are strongly affected by gypsum content. XRD results revealed that clinker and cement present a strong intensity of ye'elimite as main mineral phase. Moreover, the hydrated cement exhibited ettringite and monosulfate as the new phases formed. The compressive strength of hydrated cement reached ∼21 and 28 MPa after 1 and 28 days of curing, respectively. From the aforementioned results, this local rich alumina bauxite can be used to produce high‐strength cement for self‐leveling materials, which allow their use in engineering and building applications. [ABSTRACT FROM AUTHOR]

Ettringite is a hydrous calcium aluminum sulfate mineral present in cement. In this study, ettringite was synthesized via co-precipitation to remove methyl blue, dichromate, and copper ions from solutions with various pH values. The synthesized ettringite was characterized using scanning electron microscopy, X-ray photoelectron spectrometry, Fourier transform infrared spectroscopy, zeta potential analysis, and the Brunauer–Emmett–Teller method. Equilibrium adsorption experiments were performed using methyl blue and dichromate at pH values ranging from 5 to 11. To avoid precipitation, equilibrium adsorption experiments were performed on copper ions in solutions with pH values of 3, 4, and 5. The adsorption kinetics experiments for each contaminant were performed at pH 5. The results showed that ettringite was successfully synthesized, and calcite might have precipitated during the synthesis process. The point of zero charge was at pH 8.6. The maximum adsorption capacities for methyl blue, dichromate, and copper ions at pH 5 were 406, 321, and 365 mg/g, respectively. The adsorption kinetics fitted the pseudo-second-order model well. The properties of the contaminants affect their equilibrium and rate constants. Ion exchange is regarded as the primary adsorption mechanism, whereas the other mechanisms include complexation, hydrogen bonding, surface precipitation, π-interaction, and van der Waals forces. This study revealed a new adsorbent, ettringite, for the removal of contaminants from wastewater, which is a promising alternative adsorbent that can be used under specific conditions. [ABSTRACT FROM AUTHOR]

Belite sulfoaluminate cement is attracting increasing attention and being considered as a potential alternative to ordinary Portland cement owing to lower preparation temperature and carbon dioxide (CO2) emission. However, it still has disadvantages of poor workability and low early strength. This study investigated the effects of compound polycarboxylate superplasticiser (PCE) and early strength agent, calcium nitrite (Ca(NO2)2), on flowability and early strength of belite calcium sulfoaluminate (BCSA) cement. Variation patterns of setting time, fluidity, strength and polycarboxylate superplasticiser (PCE) adsorption of the paste were measured; the hydration products and microscopic morphology were also analysed by the heat of hydration, X-ray diffraction and scanning electron microscopy. Results showed that calcium nitrite effectively enhanced adsorption of PCE onto the surface of BCSA cement particles, leading to notable improvement in the fluidity of the paste (reaching up to 275 mm). In the initial hydration stage, 0.7% PCE compounded with 1.2% calcium nitrite inhibited the formation of ettringite (AFt), resulting in prolonged setting time. However, it deepened the degree of hydration of BCSA for 3 days and refined the hydration product, AFt crystals. Consequently, the compressive strength was increased to 95.75 MPa and 107.13 MPa for BCSA cement at 3 days and 28 days, respectively. [ABSTRACT FROM AUTHOR]

The high geothermal environment causes a temperature gradient inside shotcrete, which affects the development of its mechanical strength. This research investigated the strength evolution of shotcrete in simulated high geothermal environment by applying a unidirectional heat source. The influence of temperature gradient on the pore structure, morphology, phase composition, and hydration degree of shotcrete was also studied. It was found that the early strength of shotcrete was improved but that the later strength was reduced in the high geothermal environment. It was attributed to the increase in harmful porosity and total porosity of the concrete caused by the high-temperature curing. Cracks and pores can be observed in the microstructure near the heat source region. Besides, the ettringite content in shotcrete was lower than that under standard curing environment and presented gradient distribution in the direction of the temperature gradient. [ABSTRACT FROM AUTHOR]

Supersulfated cement (SSC), a low-carbon, energy-efficient, eco-friendly cementitious material, is mainly made from industrial byproducts. However, SSC's slow early strength development leads to inadequate initial hardening and reduced durability, which restricts its practical application. This study investigated the potential enhancement of SSC by incorporating calcium aluminate (CA) and carbide slag (CS) alongside anhydrite as activators to address its slow early strength development. The effects of varying CA and CS proportions on the mechanical property and hydration mechanism of CA-CS-SSC were examined. Results indicate that employing 1% CA and 4% CS as alkaline activators effectively activates slag hydration in the 1CA-4CS-SSC, achieving a compressive strength of 9.7 MPa at 1 day. Despite the limited improvement in early compressive strength of other mixtures with higher CA and lower CS proportions in the CA-CS-SSC system, all mixtures exhibited enhanced compressive strength during long-term hydration. After 90 days, ettringite formation in the CA-CS-SSC system decelerated, whereas anhydrite remained. Concurrently, the formation of C-S-H continued to increase, promoting late compressive strength. The mechanism for enhancing the early compressive strength of the CA-CS-SSC system is attributed to the swift hydration of CA with anhydrite, dissolution of fine slag particles, and reaction with anhydrite under conditions with suitable alkali content to augment the ettringite production. This process also generates a C-S-H and OH-hydrotalcite to fill the void in the skeleton structure formed by ettringite, resulting in a dense microstructure that improves early compressive strength. [ABSTRACT FROM AUTHOR]