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This work aims to illustrate the impact of high concentrations of Cr(VI) (based on Na 2 CrO 4 ) on the hydration assembly and microstructural development of hydrated Portland cement, and the results also present the role of calcium sulfate on the immobilization of Cr(VI) in Portland cement. The results showed that the immobilization of Cr(VI) in hydrated Portland cement was attributed to the formation of CrO 4 -U phase, an analogue of SO 4 -U phase (3CaO·Al 2 O 3 ·CaSO 4 ·0.5Na 2 SO 4 ·15H 2 O). The growth of CrO 4 -U phase on the surface of clinker particles formed a diffusion barrier and hence increased the setting time. Increasing the calcium sulfate dosage impaired the Cr(VI) immobilization due to the competition between CrO 4 2− and SO 4 2− integrated into the U phase. The generalized acid neutralization capacity (GANC) test indicated that the Cr(VI) leaching behavior was a function of the leachate pH value. As the pH decreased to 11.8, the CrO 4 -U phase was converted quickly to CrO 4 -ettringite, which generated a slight increase in Cr(VI) concentration. The most leaching sector, approximately 89.3% of added Cr (1 wt% of cement), was found in the pH range 11.8–10.5 due to the dissolution of secondary CrO 4 -ettringite. It could also be shown that the C–S–H had little chemical binding for Cr(VI). [ABSTRACT FROM AUTHOR]

Abstract The high water absorption rate of alkali-activated slag (AAS) cement, which causes concerns to designers and constructors as "long-term" durability, was addressed using an alternative admixture, calcium stearate (CaSt). The macro- and micro-performance of AAS cement with two levels of CaSt dosage (4 wt%, 8 wt% of slag) were characterised under the water to binder ratio (W/B) ranging from 0.35 to 0.45 and benchmarked against corresponding Portland cement (PC) samples by sorptivity along with compressive strength, porosity, electrical resistivity, pore connectivity, pore size distribution and pore geometries. The interpretation of results showed that CaSt played an instrumental role in pore structure features of AAS and the use of CaSt could significantly reduce its sorptivity, even lower than that of the corresponding PC samples. It is also found that the recommended usage of CaSt is 4% of slag, beyond which no significant variation in sorptivity can be detected. The performance improvement was caused by two main mechanisms, optimising the pore structure (more entrained pores, less pore connectivity factor and less microcracks) and introduce of the hydrophobic film on the pore surface. One limitation of using CaSt was noted as well. That is, the strength development of AAS can be affected and it can be avoided through modifying mix proportions according to practical requirements. Therefore, CaSt can be used as a chemical admixture for AAS to solve the concern on its high sorptivity behaviour. [ABSTRACT FROM AUTHOR]

• Passivity of mild steel embedded in MKPC-CSA blended paste were investigated. • CSA addition significantly influences the degree of passivity. • Passivity increases with increased CSA addition and 30% CSA addition leads to a much higher passivity. • 30 % CSA addition benefits the precipitation of phosphate ions and formation of Fe2+ oxide in passive film. This study did a pioneer work focused on the passivation of mild steel embedded in magnesium potassium phosphate cement (MKPC)-calcium sulphoaluminate cement (CSA) blended paste via electrochemical measurements and materials characterization techniques. Results showed that 30 % CSA addition in MKPC leads to a passivity of mild steel much higher than that in Portland cement (PC) paste. The passivity of mild steel increased with an increase in CSA content in MKPC paste due to an increase in pH and phosphate concentration in pore solution. The formation of iron phosphate and Fe2+ oxide in passive film of MKPC-30 %CSA contributes to its highest passivity. [ABSTRACT FROM AUTHOR]

This study examines the potential of using calcium‐rich precursors to improve the strength of alkali‐activated slag (AAS) cement at low temperature. To achieve this target, the effect of calcium‐rich precursors on the early strength development of AAS mortars cured at 5°C was assessed. The strength development was investigated along with its internal temperature, microstructure and reaction products. It is found that the calcium‐rich precursors can improve the early strength of AAS around 35% and the group with Portland cement clinker gave the highest strength. Calcium oxide can also increase the compressive strength of AAS cement, but its effectiveness is not as good as the sample with clinker. Adding calcium sulfoaluminate can increase the early strength, while no significant contribution to 28‐day strength could be found. The results highlight that compressive strength of modified AAS mortar can reach 15 MPa at 6 h and 20 MPa at 24 h at 5°C, suggesting that this binder could be a valuable alternative for low temperature construction. [ABSTRACT FROM AUTHOR]

The chemical compositions of the aqueous phases in alkali-activated slag (AAS) paste and Portland cement (PC) paste were determined up to 28 d with the aim of obtaining a better understanding of the stability of the hydration products in the two binder systems. The saturation levels with respect to the hydration products of PC and AAS were obtained by thermodynamic modelling. The main findings of this study were: the effective saturation index for portlandite in the AAS system was always below zero and the sulfate-bearing phases were not stable in the AAS system compared with those in the PC; strätlingite and hydrotalcite phases were stable in the AAS paste due to the high magnesium and silicon concentrations in the pore solution; both the ionic strength and alkalinity of the AAS pore solution were higher than those of the PC pore solution, which were responsible for more severe efflorescence in the AAS paste and the higher conductivity of the AAS pore solution. According to thermodynamic estimations, tobermorite-based C–S–H was dominant in the AAS system after 7 d, while jennite-based and tobermorite-based C–S–H gels were present in the PC system up to 28 d. The results suggest that in PC and AAS pastes, different solid phases are formed during the hydration, which change with time, and the reactions and equilibria in both binders are completely different. [ABSTRACT FROM AUTHOR]

[Display omitted] • The role of CGS on the workability of cement paste with and without PCE was investigated. • The decrease of interparticle force leads to the improvement of fluidity when added CGS. • The aggregation of PCE in pore solution leads to its efficiency decrease in the cement pastes blended with CGS. Coal gasification slag (CGS) is the main by-product of coal gasification technology, and the utilization of CGS as mineral admixture is environmentally sound. In this paper, the effect and mechanisms of CGS on the workability (i.e., fluidity and rheological behaviors) of ordinary Portland cement (OPC) paste with and without polycarboxylate superplasticizer (PCE) were studied. The zeta potential and surface free energies of OPC and CGS were tested to calculate the electrostatic force and Van der Waals force variations due to the addition of CGS particles. The results revealed that CGS mainly decreases the van der Waals attraction force between binder particles, enhancing the fluidity and reducing cement pastes' yield stress and plastic viscosity. When blended with PCE, the dissolution of CGS mainly increased the ionic concentrations of Si(OH) x (4-x), which promoted the aggregation of PCE and decreased its adsorption, thus decreasing the fluidity and increasing the rheological parameters. [ABSTRACT FROM AUTHOR]

From a material viewpoint, modern concrete’s frequent propensity to plastic shrinkage cracking can be attributable to a combination of low water-binder ratio use and ever-changing properties of binding materials. To obtain a better understanding of this phenomenon, this paper explores the effects of cement fineness and alkali content on the plastic shrinkage cracking of concrete manufactured with two water-binder ratios. Results indicate that within the range 275–385 m 2 /kg, cement specific surface area is approximately directly and inversely proportional to hydration rate and evaporation rate respectively; a trend generally leading to higher plastic shrinkage and resulting areas of plastic cracking. Similar effects were observed for alkali contents which resulted in increased levels of plastic shrinkage. Furthermore, while decreasing crack tendency was noted as alkali content increased from 0.4 to 0.8% by mass of cement, further increases in alkali content caused significant decreases of compressive strength and slump; thereby lowering overall concrete performance. It is also found that plastic shrinkage cracking is closely related to the kinetics of plastic shrinkage. In summary, the experimental programme confirmed that cement with relatively low surface area (less than 340 m 2 /kg) and low alkali content (less than 0.8%) is preferred for modern concretes with minimal plastic cracking problems. [ABSTRACT FROM AUTHOR]