Search

Alite calcium sulfoaluminate (ACSA) cement is an innovative and environmentally friendly cement compared to ordinary Portland cement (OPC). The synthesis and hydration of ACSA clinkers doped with gradient sulfur were investigated. The clinker compositions and hydrated pastes were characterized by X-ray diffraction (XRD), isothermal calorimetry, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) to analyze its mineral contents, hydration products, heat release, pore structure, and microstructure. The compressive strength and linear expansion of ACSA mortars were tested for their mechanical properties. Results showed that clinkers doped with 2 wt.% MgO can offset the hurdle that SO3 caused to the formation of C3S (tricalcium silicate). Clinkers with varying ratios of C3S and C4A3$ (calcium sulfoaluminate) were obtained, achieving 58–70% C3S and 2.0–5.6% C4A3$ in ACSA through adjusting the KH (lime saturation factor) values and SO3 dosage. ACSA cement showed better early mechanical properties. The 0.93 KH value with 3% SO3 dosage in the raw meal, which contains 63.9% C3S and 2.98% C4A3$ in the clinker, reached an optimal compressive strength level at 1d (26.35 MPa) and at 3d (39.41 MPa), marking 30.45% and 18.70% increases compared to PII 52.5. The excellent early strength of ACSA cement may offer promising applications t increasing the incorporation of supplementary cementitious materials, thereby reduce pollution and carbon emissions. [ABSTRACT FROM AUTHOR]

The performance of composite binders with high-volume steel slag (SS), granulated blast furnace slag (GBFS) and desulfurization gypsum (DG) were systematically investigated. The results show that the synergistic effects of high volume (40–60%) of SS with GBFS and less than 15% of DG could perform adequate setting time, higher strength, well volume stability and soundness as well as dense microstructure. The higher compressive strengths exceeding 70 MPa at 28 days and up to 80 MPa at 90 days could be obtained for composite pastes with high-volume SS, which exhibit tight relationships with [CaO]·[SO3]/[Al2O3]·[SiO2] and [CaO]·[Al2O3]/[SO3]2 molar ratios. More (15–19%) AFt and less (about 1%) portlandite produced during the hydration of composite binders. C–S–H gels and calcite crystals were formed. The composite binder possesses lower energy consumption, high carbon reduction and low cost compared with Portland cement in favor of the low-carbon cementitious material production and effective utilization of SS. [ABSTRACT FROM AUTHOR]

The hydration kinetics of Portland cement with and without nano-SiO₂ (NS) were evaluated via isothermal calorimetry and investigated based on the mathematic method of the Krstulovic-Dabic model in this paper. NS was used to substitute Portland cement at 0, 1 and 3 wt.% by mass. The test results obtained in this study indicated that the hydration process of Portland cement was dramatically promoted in terms of increasing the heat released and the heat release rate when NS was added. From the Krstulovic-Dabic model, the dynamics process of Portland cement was controlled by nucleation and crystal growth (NG), phase boundary interactions (I) and the diffusion (D) process, it does not matter whether there was NS or not, but the transformation of the controlling process occurred at a lower hydration degree for the NS-added pastes. Moreover, the increase in NS content increased the values of the kinetics parameters KNG and KI, but decreased KD and the reaction exponent to a different degree.

Calcium sulfoaluminate (CSA) cement has some attractive qualities, such as rapid setting, high early strength, and micro-expansion. It was commonly used to improve properties of Portland cement (PC). In this paper, the investigated results show that the setting times are notably reduced as the CSA cement content is increased. The compressive strength is slightly lower for the PC-CSA blends containing less than 10% CSA cement, but notably lower for the PC-CSA blend containing 20% CSA cement when compared to PC. High amounts of CSA cement significantly delay the hydration of alite, thereby increasing the porosity. Furthermore, the addition of CSA cement changes the ettringite morphology.

Alite(C3S)-Ye'elimite(C4A3$) cement is a high cementitious material that incorporates a precise proportion of ye'elimite into the ordinary Portland cement. The synthesis and hydration behavior of Alite-Ye'elimite clinker with different lime saturation factors were investigated. The clinkers were synthesized using a secondary thermal treatment process, and their compositions were characterized. The hydrated pastes were analyzed for their hydration products, pore structure, mechanical strength, and microstructure. The clinkers and hydration products were characterized using XRD, TG-DSC, SEM, and MIP analysis. The results showed that the Alite-Ye'elimite cement clinker with a lime saturation factor (KH) of 0.93, prepared through secondary heat treatment, contained 64.88% C3S and 2.06% C4A3$. At this composition, the Alite-Ye'elimite cement clinker demonstrated the highest 28-day strength. The addition of SO3 to the clinkers decreased the content of tricalcium aluminate (C3A) and the ratio of Alite/Belite (C3S/C2S), resulting in a preference for belite formation. The pore structure of the hydrated pastes was also investigated, revealing a distribution of pore sizes ranging from 0.01 to 10 μm, with two peaks on each differential distribution curve corresponding to micron and sub-micron pores. The pore volume decreased from 0.22 ± 0.03 to 0.15 ± 0.18 cm3 g−1, and the main peak of pore distribution shifted towards smaller sizes with increasing hydration time. [ABSTRACT FROM AUTHOR]

Nanomaterials are widely used in cement to improve the various properties of cement-based materials. Nano-CSH is considered as an effective accelerator for the hydration of cement. This paper studied the effects of nano-CSH on the hydration behavior and compressive strength of cementitious materials with various mineral admixtures. Isothermal calorimetry test was used to study the early hydration of cement paste. Thermogravimetric and X-ray diffraction tests were used to study the evolution of hydration product and unhydrated mineral phases in the hydrated cement paste. The presentation of compressive strength can evaluate the influence of nano-CSH on the property of cement-based material at the macro level. The obtained results show that the used nano-CSH can promote the hydration of tricalcium silicate (C3S), and is more active on the hydration of tricalcium aluminate (C3A) in the samples with GGBFS. The presence of nano-CSH can definitely accelerate the secondary hydration reaction of mineral admixtures. [ABSTRACT FROM AUTHOR]

To further explore the effects of limestone powder (LP) on the hydration of Portland cement as well as the formation and transformation mechanism of calcium carboaluminate phases, the hydration reaction between LP and tricalcium aluminate (C3A) is thoroughly studied in this paper. Due to the crucial influence of gypsum on the hydration of C3A, two systems are well studied, viz. the C3A–LP–H2O and the C3A–LP–CaSO4·2H2O–H2O system. The replacement levels of LP are 0, 5 mass%, 15 mass% and 25 mass% by mass of total binder. The hydration behavior of these two systems are studied using isothermal calorimetry, XRD and quantitative XRD, and chemical shrinkage. The obtained results show that more LP delays the formation of hemicarboaluminate. In the C3A–LP–CaSO4·2H2O–H2O system, the formation of calcium carboaluminate phases only begins after gypsum is consumed. Moreover, LP can reduce the chemical shrinkage resulting from the volume increase of the formation of calcium carboaluminate phases. [ABSTRACT FROM AUTHOR]

This paper reports the effects of welan gum on the hydration and hardening behaviors of Portland cement (PC) by using X-ray diffractometry, mercury instruction porosimetry, thermogravimetry, differential scanning calorimetry and scanning electron microscopy. Results show that welan gum has notable influence on the setting time of PC paste and the formation of calcium hydroxide (Ca(OH)2); welan gum has little impact on the total hydration heat, AFt content and/or the morphologies of hydration products even though it delays the induction period of hydration and the second reaction of the aluminate phase. Welan gum retards the early hydration of C3S and accelerates the early hydration of C4AF. The compressive strength is improved, and pore size of hardened cement paste is reduced with at the studied period when welan gum is no more than 0.05%. In this research, the cement paste with 0.05% welan gum exhibits the highest long-term (90 d) compressive strength and lowest porosity. [ABSTRACT FROM AUTHOR]

Four types of alkanolamines (i.e., traditional alkanolamines represented by TEA and TIPA and new alkanolamines represented by DEIPA and EDIPA) were added to Portland cement as chemical additives, and their effects on the cement properties and hydration process were investigated. An isothermal calorimeter was used to track the hydration heat flow of the cement pastes with or without alkanolamines. Thermogravimetric analyses were performed to measure the degree of hydration over the course of 28 days. In addition, X-ray diffraction, MIP analysis and SEM were used as auxiliary tests. The results indicated that alkanolamines improved the compressive strength of the cement mortars. It was found that TEA increased the rate of the second hydration of CA, and TIPA accelerated the hydration of CAF. DEIPA and EDIPA promoted the hydration of both the aluminum and ferrite phases as well as catalyzed the conversion of AFt to the AFm phase. By contrast, the new alkanolamines represented by DEIPA and EDIPA expressed more superior properties. [ABSTRACT FROM AUTHOR]

The aim of this paper is to examine the effect of a blending dose of calcium sulfoaluminate (synthetic ye'elimite, C4A3$) and gypsum on the hydration and hardening properties of a new ye'elimite (C4A3$) blended PII 52.5 cement. By using X-ray diffractometry, mercury intrusion porosimetry, thermogravimetry and scanning electron microscopy, it was revealed that the hydration of tricalcium silicate (C3S) is significantly retarded by the lack of gypsum; the renewed hydration of tricalcium aluminate (C3A) was retarded in cement pastes containing a high amount of C4A3$. The dosage of C4A3$ and the ratio of gypsum/C4A3$ both have a notable influence on the hydration heat, hydration products and the compressive strength and linear expansion. The cement paste containing 3% C4A3$ at gypsum/C4A3$ ratio of 2 exhibited the best performance, in terms of both high early and long-term compressive strength and acceptable linear expansion (0·04% on 300 d); the cement can therefore be adopted in industrial structures. [ABSTRACT FROM AUTHOR]

The utilization of coral waste is an economical way of using concrete in coastal and offshore constructions. Coral waste with more than 96% CaCO3 can be ground to fines and combined with supplementary cementitious materials (SCMs) such as fly ash, silica fume, granulated blast furnace slag in replacing Portland cement to promote the properties of cement concrete. The effects of coral sand powder (CSP) compared to limestone powder (LSP) blended with SCMs on hydration and microstructure of mortar were investigated. The result shows CSP has higher activity than LSP when participating in the chemical reaction. The chemical effect among CSP, SCMs, and ordinary Portland cement (OPC) results in the appearance of the third hydration peak, facilitating the production of carboaluminate. CSP-SCMs mortar has smaller interconnected pores on account of the porous character of CSP as well as the filler and chemical effect. The dilution effect of CSP leads to the reduction of compressive strength of OPC-CSP and OPC-CSP-SCMs mortars. The synergic effects of CSP with slag and silica fume facilitate the development of compressive strength and lead to a compacted isolation and transfer zone (ITZ) in mortar. [ABSTRACT FROM AUTHOR]

Alite-belite-ye'elimite (ABY) cement is considered to be eco-cements with adequate mechanical properties and fewer CO 2 emissions. The phase assemblage and ye'elimite content of ABY cement play a very important role in cement properties. In this study, ABY clinkers with increasing ye'elimite percentage (0.3 wt%-36.4 wt%) were prepared at 1300 °C and the phase composition was characterized by Rietveld Quantitative Phase Analysis combined with energy dispersive X-ray spectrometry. The ye'elimite formed with a calculated composition of Ca 3.92 Mg 0.02 Al 2.81 Fe 0.05 Si 0.04 S 1.16 , incorporating additional sulfur compared to the theoretical value. In addition, the hydration properties of heat evolution, hydration products and compressive strength of ABY cement are studied. The result shows that the increase of ye'elimite content leads to a more intense initial heat release and a higher cumulative heat release. The presence of ye'elimite inhibits the hydration of alite at early ages. ABY mortars with increasing ye'elimite (15.8 wt%) exhibited a higher strength (29.6 MPa) at 3 days by the contribution of ye'elimite hydration, and an increase (47.4 MPa) at 28 days. An adequate proportion of ye'elimite (<15 wt%) is favorable to the performance of ABY cement, and releases 12 % less CO 2. • ABY clinkers with a high coexistence of alite and ye'elimite were prepared. • Ye'elimite has a higher S atom and alite riches in the Zn atom in ABY clinker. • The presence of ye'elimite retards alite hydration adverse to later strength. • Adequate ye'elimite < 15 wt% favors strength development. [ABSTRACT FROM AUTHOR]

• THEED can be used as accelerator agent to shorten the Portland cement. • THEED has the advantages of TEA and TIPA, increasing the early compressive strength and late compressive strength. • THEED can promote the transformation of AFt into AFm, and change the morphology of AFt and C-S-H. N,N,N',N'-Tetrakis-(2-hydroxyethyl)ethylenediamine (C 10 H 24 N 2 O 4 ,THEED) is a different type of alkanolamine. In this work, the physical and chemical effects of small amounts of THEED added to Portland cement were investigated. THEED can be used as an accelerator for the setting of Portland cement. The compressive strengths of standard test mortars moist-cured for 1, 3, 28 and 90 days were enhanced in some cases by the addition of THEED. THEED promoted conversion of ettringite (AFt) to monosulfoaluminate (AFm). THEED had notable effects on the morphology of AFt and C-S-H at the early age, especially on the growth direction of portlandite(Ca(OH) 2) crystals. [ABSTRACT FROM AUTHOR]

• Sulfate resistance of PC, SRPC and CSAC with CSP compared to LSP were studied. • CSP filler (≥20 %) replacing PC and SRPC suffered severe thaumasite attack at 5 °C. • CSAC-CSP specimens possess greater sulfate resistance than PC and SRPC specimens. • CSP filler in contrast with LSP results in less sulfate resistance of mortars. Coral waste filler partial replacing Portland cement facilitates the utilization of coral debris waste, the reduction of the economic cost of oceanic construction as well as the CO 2 emission. Thaumasite-related sulfate attack needs to be concerned because of rich calcium carbonate in form of aragonite in coral waste filler and the sulfate resources in the oceanic environment. The sulfate resistance of Portland cement (PC), sulfate resistant Portland cement (SRPC), and calcium sulfoaluminate cement (CSAC) incorporating coral sand powder (CSP) were studied under corrosion in 5 wt% NaSO 4 solution at 5 and 20 °C up to 540 days. The results show that CSAC-CSP specimens possess greater sulfate resistance than PC and SRPC specimens incorporating with CSP filler. Cracking was the main way for PC and SRPC specimens' failure under sulfate attack at 20 °C. Exceeding 20 % of CSP filler replacing PC leads to server thaumasite-related sulfate attack exposure at 5 °C for 540 days. The formations of sulfate products including gypsum, ettringite, and thaumasite in PC-CSP and SRPC-CSP specimens were confirmed by XRD and FTIR tests. The ettringite-thaumasite cluster and deteriorated products were detected in PC and SRPC blended specimens. CSP filler in contrast with limestone powder (LSP) filler results in less sulfate resistance of blended specimens. [ABSTRACT FROM AUTHOR]

• The synergistic effect of MK and LS with different mass ratio under same substitution is evaluated. • The incorporation of MK and LS obviously influence early hydration and the formation of ettringite. • The strength development and the formation of carboaluminate are dependent on the mass ratio of MK to LS. • The Hc can still exist in the blends when the mass ratio of MK to LS is 2:1. The effects of simultaneous addition of limestone and metakaolin with different mass ratio on the hydration of Portland cement are investigated by a multi-method approach. The results indicate that the strengths of Portland cement mortars are depended on the mass ratio of metakaolin to limestone. When the ratio is as high as 1:2, the strengths of the blends are higher than those of Portland cement. The mixture of metakaoin and limestone stimulates the hydration of C 3 S and the formation of ettringite. The synergistic effect of MK and LS refined the pore distribution by consuming portlandite during the hydration. By incorporation metakaolin, hemicarboaluminate still exists in the late hydration, the hemicarboaluminate increases as the amount of metakaolin increases. [ABSTRACT FROM AUTHOR]

Highlights • Hydration behaviors of the CSP-OPC binary system are investigated comprehensively. • CSP can retard the conversion of AFt to AFm. • CSP can optimize the pore structure of the hardened cement paste at long term curing period. • CSP with 5% can contribute to the early compressive strength of cement mortar. Abstract This study presents the influence of coral sand powder (CSP) on the hydration and mechanical behaviours of Portland cement paste at the curing ages from 1 day to 90 days. The early hydration kinetics of Portland cement incorporated with CSP is studied by isothermal calorimetry test. The hydrates and pore structures of cement paste are determined by X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP), respectively. Experimental results show that CSP can promote acceleration period to end in advance. XRD results imply that CSP can react with tricalcium aluminate (C 3 A) to form monocarbonate (Mc) and hemicarbonate (Hc), which can suppress the conversion of Al-phase products, viz. from AFt to AFm. From the MIP results, the addition of CSP can refine the pore structure of Portland cement paste at long-term curing age, although negative effect of CSP on the total porosity can be observed. Low replacement level of CSP (5 wt%) improves the compressive strength of cement mortar at ages from 1 day to 7 days. However, from 28 days to 90 days, the addition of CSP decreases the strength of cement mortar regardless of replacement levels. [ABSTRACT FROM AUTHOR]

Abstract In this paper, an innovative method was proposed to synthesize an alite-ye'elimite clinker by adopting a secondary heat treatment step in a different sintering atmosphere after a traditional Portland cement (PC) clinker has been heated up at 1450 °C. It was found that a pure gaseous SO 2 or mixed gas containing the gaseous SO 2 used facilitated the decomposition of C 3 S and the consumption of C 3 A in the PC clinker. Simultaneously, anhydrite (CaSO 4), ye'elimite (3CaO·3Al 2 O 3 ·CaSO 4 , C 4 A 3 $) and ternesite (4CaO·2SiO 2 ·CaSO 4 , C 5 S 2 $) coexisted in the alite-ye'elimite clinker, resulting from gas-solid reactions during sintering. The experimental results also showed that the compressive strength of the synthesized alite-ye'elimite cement was higher than that of PC. Moreover, its setting times were shorter than that of PC. [ABSTRACT FROM AUTHOR]

The service life and durability of concrete structures strongly depend on the transport properties of the concrete. Water permeability is one of the important indicators for durability of concrete. The water acts as a carrier of aggressive agents, such as chloride ions, sulphate ions and acid that can activate many chemical reactions, speeding up the degradation process of concrete structures. Pore structure is the most important characteristics controlling the transport properties of the hydrated cement paste. Hence, knowledge of the relationship between water permeability and pore structure of cement paste is needed for engineers. Owing to the pozzolanic reaction, fly ash can refine the pore structure of cement paste, eventually affect the water permeability. In this paper the effect of fly ash on the water permeability of cement paste is investigated at ages of the cement paste up to 2 years. The basic pore parameters of cement paste blended with fly ash were already discussed in previous study. This paper only investigates the pore parameters, such as capillary porosity, connectivity of the capillary pores, related to transport properties of cement paste. The results show that at later ages, such as 180 days, cement paste blended with fly ash is less permeable than pure Portland cement paste. This mainly results from the refinement of the pore structure caused by the pozzolanic reaction of fly ash. Based on the comprehensive comparison of the pore structure and the water permeability between ordinary Portland cement paste and cement paste with fly ash, the crucial pore parameter governing the water permeability of cement paste blended with fly ash is the Connectivity of the capillary pores. Capillary porosity is not the significant factor affecting the permeability of cement paste. The comparison obtained in this study enables a good understanding of fly ash incorporated in cement-based materials, and provides information for the use of fly ash in practice. [ABSTRACT FROM AUTHOR]

• Utilization of CP with GGBFS to replace cement partially is feasible to mitigate the strength loss of cement due to the use of CP. • After exposed to seawater to 1 year, the compressive strength of the mixture (25% CP and 25% GGBFS) still can reach 54 MPa. • The pore structure of cement paste mixed with CP and GGBFS is densified. On island, the use of discarded coral reef is a way to resolve the lack of raw material for preparing concrete. Discarded coral reef can not only be used as aggregate, but be ground to powder to replace cement. Since the cement-based material prepared with discarded coral reef is exclusively used on island, the property of the cement-based material exposed to seawater condition should be studied. In this study, discarded coral reef is ground into coral powder (CP). CP is used together with ground granulated blast furnace slag (GGBFS) to replace Portland cement (PC) partially to prepare a mixture with good performance and the minimum use of PC. The compressive strength, solid phase composition and pore structure of the cement-based material exposed to seawater condition are investigated systematically. The results show that the addition of CP decreases the fluidity of the cement mortar since it has a more rough surface than other raw materials. Although the use of CP decreases the compressive strength of cement mortar, the coupled use of GGBFS can largely mitigate the strength loss of the cement mortar by comparing 9 mixtures. When the specimens are exposed to seawater, the strength of cement mortar only decreases slightly, but the pore structure of cement paste is densified. It suggests that the use of CP and GGBFS together in the concrete can meet the construction of island; the compressive strength of the sample made with 25% CP and 25% GGBFS is around 54 MPa after exposed to seawater to 1 year. [ABSTRACT FROM AUTHOR]

TiO 2 @SiO 2 nanoparticles with a core/shell structure widely used in photocatalytic fields were used in this paper to improve the hydration properties of the Portland cement. To this end, the core/shell TiO 2 @SiO 2 nanoparticles were synthesized first and then characterized by a series of techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR) and X-ray photoelectron spectra (XPS). Second, the influence of the core/shell TiO 2 @SiO 2 nanoparticles on cement hydration was investigated and compared with nano-TiO 2 through isothermal calorimetry, XRD, thermo gravimetric/derivative thermo-gravimetric (TG/DTG) and mercury intrusion porosimetry (MIP) analyses. The results showed that an amorphous SiO 2 layer can be deposited uniformly on nano-TiO 2 particles by forming new Si O Ti chemical bonds at the interface between the SiO 2 coating layer and nano-TiO 2 particle surface. This uniform layer was conducive to decrease the aggregation of nano-TiO 2 effectively. Compared with nano-TiO 2 , the core/shell TiO 2 @SiO 2 nanoparticles exhibited better hydration properties in terms of accelerated cement hydration, higher degree of hydration and lower porosity, even though both particles modified cement hydration. [ABSTRACT FROM AUTHOR]

In this work, six types of alkanolamines (i.e., TEA, TIPA, DEIPA, EDIPA, THEED and THPED) were added into Portland cement as chemical additives, and their effects on physical properties and hydration of Portland cement as well as mechanism were investigated. The results show that all the alkanolamines expect TEA can effectively improve the degree of cement hydration. TEA delays the hydration of alite because the formation of the TEA-Ca 2+ complex inhibits the growth of CH. TIPA accelerates the hydration of C 4 AF at late stage, but reduce the early strength due to its air-entraining effect. The lower dosages of DEIPA and EDIPA have the advantages of TEA and TIPA, which increase not only the early strength but also the late strength. They promote both the hydration of the aluminum and ferrite phase as well as catalyze the conversion of AFt to AFm, which lead to the optimization of pore size distribution. The effects of THEED and THPED on hydration of Portland cement are similar though their molecular structure are different which improve the late strength and have larger suitable dosage range. [ABSTRACT FROM AUTHOR]

Grinding aids have been researched and used for decades to reduce energy consumption in the process of cement production. In this work, the effects of new alkanolamines, such as N,N-bis(2-hydroxyethyl)isopropanolamine (DEIPA), N,N-bis(2-hydroxypropyl)-2-aminoethanol (EDIPA) and N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine (THEED), on the grinding, flowability and physical properties of Portland cement were investigated. The results indicated that DEIPA and EDIPA could significantly decrease the sieve residues (45 μm), and improve the flowability of the cement powder. They could decrease the compressibility and the magnitude of the pressure drop of the cement at a definite normal stress, thereby preventing cement particles from agglomerating. Notably, DEIPA and EDIPA increased both the early (3-day) and late (28-day) compressive strengths. DEIPA can be used as an accelerator to shorten the setting time. THEED just provides a greater contribution to the late compressive strength. [ABSTRACT FROM AUTHOR]

Sodium gluconate is commonly used to delay the setting of cement and concrete. This study investigated the effects of sodium gluconate on the physical properties and structure of Portland cement. Sodium gluconate improved the compressive strength at 3 and 28 days, delayed cement setting and increased the fluidity of the Portland cement mortar. Less than 0.03% sodium gluconate promoted the formation of ettringite (AFt) at early age. A dosage of 1.0% sodium gluconate significantly inhibited the reaction between C 3 A and CaSO 4 ·2H 2 O. Sodium gluconate delayed the hydration reaction of C 3 S, which increased the duration of the induction period. Sodium gluconate had only a slight effect on the hydration reaction of the ferrite phase. The pore distribution and porosity of the cement paste were not improved due to the decrease in hydration. [ABSTRACT FROM AUTHOR]

• Mechanical and structural properties of Ettringite and AFm phases were analyzed. • Atomic occupancy and coordinates of all the structures are refined. • Non-bonding electrons are found in the O- p orbitals of CO 3 2– in Monocarbonate. • Friedel's salt formation from Monocarbonate can increase the solid volume by 12.5% • Monocarbonate possesses the highest mechanical properties and structural stability. The aluminate containing hydration products of Portland cement, mainly referring to ettringite and AFm phases (AFm, Mc, Hc and Friedel's salt), are important for chloride chemical binding and microstructure development of cement pastes since they are easily transformed with different additives and solutions. First principles calculations are employed to find their thermodynamic equilibrium crystal structures. Various equivalent hydrostatic pressures are applied onto these structures to study their volume change, thermodynamic and elastic moduli. Among them, Mc shows the smallest interlayer space, highest volume stability, density, elastic modulus, and stiffness property. The volume of hydrated phase can increase by 12.5 % due to the conversion from Mc to Friedel's salt. Electronic properties suggest that there are non-bonding electrons in the O- p orbitals of CO 3 2– in the Mc and charge transfer from CO 3 2– to the Ca2+ made the bonding states between O- p of CO 3 2– and Ca- d stronger. This could provide a theoretical basis for improving the mechanical and durability properties of concrete by adding supplementary cementitious materials, such as limestone powders appropriately. [ABSTRACT FROM AUTHOR]

Diethanol-isopropanolamine (DEIPA) is a tertiary alkanolamine used in the formulation of cement grinding-aid additives and concrete early-strength agents. In this research, isothermal calorimetry was used to study the hydration kinetics of Portland cement with DEIPA. A combination of X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC)–thermogravimetric (TG) analysis and micro-Raman spectroscopy was used to investigate the phase development in the process of hydration. Mercury intrusion porosimetry was used to study the pore size distribution and porosity. The results indicate that DEIPA promotes the formation of ettringite (AFt) and enhances the second hydration rate of the aluminate and ferrite phases, the transformation of AFt into monosulfoaluminate (AFm) and the formation of microcrystalline portlandite (CH) at early stages. At later stages, DEIPA accelerates the hydration of alite and reduces the pore size and porosity. [ABSTRACT FROM AUTHOR]

Highlights: [•] SO3 hindered the formation of C3S by increasing the formation temperature by ∼100°C. [•] SO3 could promote the formation of C2S rims, indicating the dissolution of C2S. [•] MgO enhanced the formation of C3S in both the blank and SO3-bearing clinkers. [•] MgO favored the formation of M3 type alite, whereas SO3 could stabilize M1 alite. [•] Seven characteristic windows in XRD were confirmed to identify the M1 and M3 types of alite. [ABSTRACT FROM AUTHOR]

Abstract: Alite-ye''elimite cement is an alternative cement that combines desirable characteristics of calcium sulfoaluminate cements and Portland cement in that it shows improved strength development at early age while retaining high portlandite contents. The key problem in the clinkering process is to produce the alite-ye''elimite phase assemblage so that both phases can co-exist. In this study, a new synthesis method is proposed to achieve the coexistence of alite and ye''elimite consisting of a secondary heat treatment step at 1250°C after regular Portland clinker firing at 1450°C. Quantitative X-ray powder diffraction and electron microscopy were used to analyze the phase composition of clinker before and after the secondary heat treatment. The results show that ye''elimite develops during secondary heat treatment of calcium sulphate enriched clinker by reaction of C3A and sulphate phases. Additional ferrite is formed as result of rejection of Fe originally in solid solution with C3A during ye''elimite formation. [Copyright &y& Elsevier]

• The Portland cement clinker was modified in a sintering atmosphere containing SO 2. • SO 2 could affect the content and structure of alite in Portland cement clinker. • SO 2 could change the grain size and micromorphology of alite. • The clinker sintered in SO 2 atmosphere had higher hydration activity. This paper explored the influence of gaseous SO 2 at different temperatures on the composition and structure of the secondary sintered cement clinker. The combination of X-ray powder diffraction (XRD) and rietveld refinement was used to study the composition and content changes of clinker phase. The morphology and clinker sintering history of the clinker phase were determined by the petrographic analysis. The structure of alite was identified by XRD and transmission electron microscopy (TEM) analysis. The experimental results showed that the presence of gaseous SO 2 promoted the decomposition of alite and increased the value of M1-alite%/(M1-alite% + M3-alite%). The hydration exothermic curve showed that the activity of M1-alite was higher than that of M3-alite. [ABSTRACT FROM AUTHOR]

• C-S-H-PCE nanocomposites improve the fluidity of the PC-MK mortar. • C-S-H-PCE nanocomposites enhance the compressive strength of the PC-MK mortar. • The optimal content of C-S-H-PCE nanocomposites is 3 wt%. • C-S-H-PCE nanocomposites accelerate the hydration process of the PC-MK paste. • C-S-H-PCE nanocomposites promotes the pozzolanic reaction of MK. In this work, the effects of the synthetic calcium silicate hydrates(C-S-H)/polycarboxylate(PCE) nanocomposites (CPNs) on the hydration behaviour of Portland cement (PC) containing 20 wt% of metakaolin (MK) were evaluated. Six mixtures with different CPNs replacement levels ranging from 0 wt% to 4 wt% were designed to cast CPNs-modified pastes and mortars, respectively. The fluidity and compressive strength were tested and micro-analysis including hydration heat, hydration products, pore size distributions and microstructures for CPNs-modified pastes was also conducted to disclose the potential mechanism for modification of mortar properties due to CPNs. The current results showed that an increased CPNs content contributes to a continuously improved workability for the PC-MK binary mortars. The corresponding compressive strength development for the mortars also benefits from CPNs addition. The optimum CPNs content was determined to be 3 wt%, which contributed to the highest compressive strength obtained for all the mortars at each curing age: 36.41%, 21.37%, 14.17% and 5.66% higher compared with the control PC-MK binary mortar at 1 day, 3 days, 7 days and 28 days, respectively. It is also worth noting that mortars incorporated with 2.0 ~ 4.0 wt% of CPNs also presented higher compressive strength values than 100 wt% cement mortar at each curing age. Based on the micro-analysis results, it was speculated that CPNs not only accelerate cement hydration at the early age, but they also promote the pozzolanic reaction of MK with calcium hydraxide (CH). This synergistic effect between CPNs and MK effectively results in a much denser microstructure with a refined pore structure for the PC-MK paste, which is, thus, helpful to further improve the final compressive strength. The findings of this study provide theoretical guidelines for modifying the properties of blended cements by using nanoparticles. [ABSTRACT FROM AUTHOR]

• The DSF agglomeration remains intact in the cement paste. • The addition of DSF may slow down the hydration of C 3 A. • Coarse DSF is beneficial to the formation of AFm crystalline. • Finer DSF accelerates the hydration of C 3 S significantly. To be easily transported and stored, silica fume is commonly densified before using in concrete. The size of densified silica fume (DSF) agglomeration is up to several millimeters. In general, the agglomeration is hard to be dispersed into individual silica fume sphere. The properties of concrete are closely associated with the fineness of DSF. In order to explore the effects of DSF fineness on the hydration of Portland cement, four techniques, such as isothermal calorimetry, X-ray diffraction (XRD), thermogravimetry (TG) and scanning electron microscope (SEM) are carried out. In the end, the effect of the fineness of DSF on the mechanical property of cement mortar is discussed. Four series of DSF with different particle size distribution are investigated. Three dosages (0%, 10% and 20%) are considered. The results show that the screened DSF is difficult to be dispersed in the cement paste regardless of fineness. The fineness of DSF has obvious effect on the hydration of cement. With the increase of particle size, the delay action of DSF agglomeration on the hydration of cement becomes noticeable. The rate of cement hydration is decreased and the acceleration period is prolonged. The large DSF agglomeration shows less pozzolanic activity. In this binary system, calcium hydroxide cannot be consumed completely by DSF under high replacement level at 28 days of curing. Finer DSF definitely contributes to the improvement of the compressive strength of cement mortar, but only at a curing age of 28 days. [ABSTRACT FROM AUTHOR]

• Hydration and mechanical properties of CSP-CSA system are studied up to 90 days. • CSP can promote the formation of AFt and inhibit the formation of AFm. • The conversion from Hc to Mc does not happen in CSP-CSA binary system. • CSP can accelerate the carbonation of AFt. • CSP can optimize the pore structure of the hardened cement paste in the long term. Recently, the studies on the application of coral reef debris to construction attracted wide attention and interest of researchers, especially in China. Coral reef debris and its powder can be used as fine aggregate and mineral admixture in concrete, respectively. The main component of coral reef debris is calcium carbonate (CaCO 3), up to 90%. In our previous research, the effects of coral sand powder on the hydration behavior and mechanical properties of Portland cement are well investigated. It is well known that calcium sulfoaluminate cement, with low production energy, reduced CO 2 emissions, fast bonding and rapid strength development, etc. has been used for decades in concrete. This paper will give a detailed fundamental research on the hydration kinetics, microstructure development and mechanical properties of the coral sand powder-calcium sulfoaluminate cement binary system up to 90 days of curing. Isothermal calorimetry test, X-ray diffraction analysis, thermogravimetric analysis, mercury intrusion porosimetry and compressive strength tests are used to evaluate the properties of the calcium sulfoaluminate cement incorporated with different contents of coral sand powder (5%, 15% and 25%). The obtained results show that coral sand powder reacts with ye'elimite to generate hemicarboaluminate. Coral sand powder optimizes the pore structure of calcium sulfoaluminate cement at long time curing. [ABSTRACT FROM AUTHOR]

• Particle size, nano-TiO 2 content, w/s ratio directly affected the compressive strength. • A small dosage of nano-TiO 2 improved the compressive strength. • Nano-TiO 2 with smaller size can better improve the compressive strength. • Nano-TiO 2 would agglomerate in the paste especially at high dosages. The effects of using different sizes of nano-TiO 2 (NT) on the hydration characteristics of Portland cement with different water to solid ratios (w/s, solid: cement + NT) were investigated in this study. Cement was partially replaced by NT with two sizes, 5 nm and 25 nm, at weight fractions of 0, 0.5%, 1.0%, 1.5% and 2.0%. The compressive strength, hydration products, hydration degree, pore structures and morphologies of the modified cement pastes were systematically evaluated. The experimental results showed that a suitable NT content effectively enhanced the compressive strength of the cement paste, and this enhancement directly depended on the NT particle size. Cement paste mixed with 5-nm NT resulted in a higher compressive strength than 25-nm NT, irrespective of the w/s ratio. The cement paste with a maximum compressive strength at all the curing ages was obtained using a 0.3 w/s ratio and 0.5 wt% of 5-nm NT (which were therefore identified as the optimal NT parameters). The compressive strength of this cement paste increased by 41.4%, 15.9%, 18.7% and 24.35% at 1, 3, 7 and 28 days, respectively, compared to a control sample. For both NT sizes, the w/s ratio and the NT content controlled the compressive strength development of the paste during the early and late ages, respectively. Compared with 25-nm NT, replacing cement by 5-nm NT within the optimum content range was more effective in accelerating cement hydration, increasing the hydration degree, decreasing the crystal size of Ca(OH) 2 , refining the pore structure and densifying the cement paste at the microscale, all of which increased the compressive strength at the macroscale. [ABSTRACT FROM AUTHOR]

• TiO 2 @SiO 2 nanoparticles can better improve compressive strength of cement than nano-TiO 2 did. • Nano-TiO 2 particles formed microscale agglomerates in the hardened cement matrix. • TiO 2 @SiO 2 can limit the agglomeration of TiO 2 nanoparticles in cement matrix. • TiO 2 @SiO 2 nanoparticles resulted in a considerable improvement in the microstructure of cement matrix. This paper presented the contribution of core/shell TiO 2 @SiO 2 nanoparticles to the hydration properties of Portland cement and the contribution was compared with that of nano-TiO 2. Both nanoparticles were used to partially replace cement at 1 wt%. The mechanical property and microstructure of the cement pastes were experimentally characterized using compressive strength and scanning electron microscope (SEM), respectively. Specially, thermodynamic modeling was also applied to predict the phase assemblages and calculate the corresponding volumes. The results showed that the paste with 1 wt% TiO 2 @SiO 2 developed a much higher compressive strength than the paste with or without nano-TiO 2 added at each curing age. Both SEM and mapping analyses showed that obvious agglomeration of nano-TiO 2 could be observed in nano-TiO 2 modified cement paste, which decreased its efficiency in the cement paste. However, nano-TiO 2 was well distributed when TiO 2 @SiO 2 was added because of its limiting effect on the agglomeration of nano-TiO 2 , further densifying the microstructure. The calculated hydrates volumes by thermodynamic modeling also indicated that TiO 2 @SiO 2 would result in producing most hydrates of all the pastes, thus contributing to a denser microstructure and a much higher compressive strength development. [ABSTRACT FROM AUTHOR]