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Lower early strength has been one of the bottlenecks of low-carbon supersulfated cement (SSC), consisting of slag (∼85 wt.%), gypsum (∼10 wt.%), and a small amount of an alkali activator (Portland cement (PC), ∼5 wt.%). The work aims at improving the early strength of SSC with the addition of calcium formate (CF). Different dosages (1 wt.%, 2 wt.%, 3 wt.%) of CF were added to the SSC. Then, compressive strength, heat flow, hydration product assemblage, and hydration degree of slag were obtained to reveal the role of CF. The results show that CF would significantly increase the compressive strength of the SSC, and the higher the dosage of CF, the more pronounced the effect is. 3 wt.% of CF would increase the 3-day and 28-day compressive strength of the SSC by 126% and 172%, respectively. CF would accelerate the hydration of slag, increase the hydration degree of slag and production of C-(A)-S-H gels; however, it would inhibit the rapid production of AFt during the first 3 days, and have little influence on the final production of AFt. In addition, the lower the crystal-gel ratio (the ratio between AFt and C-(A)-S-H gel by mass) is, the higher the compressive strength. The enhancement effect of CF was attributed to a more appropriate hydration environment (higher Ca2+ concentration and lower pH value), which could promote the precipitation of C-(A)-S-H gels, inhibit the rapid formation of Aft, and reduce the crystal-gel ratio of the system.

Limestone, calcined clay and cement (LC³) are one of the low carbon cement materials which are used as a partial substitution of clinker and Portland cement. In this study, the effects of limestone and calcined clay (LC²) on the mechanical properties and microstructure of mortars were studied under multi-scales. It could be concluded that the compressive strength decreased slightly with an increase in the LC² content, but the flexural strength increased significantly due to the greater formation of crystalline aluminates in the LC³. The x-ray diffraction and thermal analysis results prove the pozzolanic activity of the calcined clay in the LC³ system which is in agreement with the results of the decreasing calcium hydroxide and the increasing ettringite content, and the LC² could reduce the heat of the hydration of the LC³ significantly. Also, the formation of calcium hemicarboaluminate (3CaO.Al₂O₃.Ca[(OH)(CO₃)0.5].xH₂O) and calcium monocarboaluminate (3CaO.Al₂O₃.CaCO₃.xH₂O), as well as the pozzolanic activity of the calcined clay contributed to the development of the mechanical strength.

The purpose of this study is to explore the effect of MgO (1 - 9 wt. %) on the composition and properties of belite-barium calcium sulphoaluminate cement with additions of Na2O and K2O. The results show that 1 - 5 wt. % content of MgO can stabilize crystal types of M3-C3S, R-C3S and β-C2S. Moreover, MgO can promote the formation of C3S and C4AF, but has little effect on the formation of C2.75B1.25A3$ and C3A. The C3A/C4AF ratio is reduced by 22 % at 5 wt. % MgO, which indicates that appropriate MgO can decrease the liquid viscosity. In the presence of Na2O and K2O, the highest limit of incorporated amount of MgO is about 3 wt. %, which is higher than that in Portland cement clinker of 2 wt. %. Besides, MgO favors the formation of small C3S crystals in size of 4 - 20 μm. MgO enhances the hydration rate and mechanical property of cement at an optimal dosage (1 - 5 wt. %), beyond which an adverse effect could be resulted. At a MgO dosage of 5 wt. %, the compressive strengths of the cement at 1, 3, 7 and 28 days are 15.8, 39.3, 68.6 and 97.3 MPa, which increases by 116 %, 17 %, 10 % and 6 % respectively compared to the cement without MgO dopant. This study could lead to the effective use of magnesia-rich limestone in industrial production of belite-barium calcium sulphoaluminate cement.

The rapid repair of concrete pavement under severe cold environment requires the repair materials to have high early strength and high bond strength with concrete. Using magnesium phosphate cement (MPC) with high early strength to prepare rapid repair materials is beneficial for achieving rapid repair of concrete pavement under non curing conditions in the winter. The effects of magnesium phosphorus ratio (M/P), boron magnesium ratio (B/M), and water cement ratio (W/C) on bond strength between MPC mortar and ordinary Portland cement concrete (OPC) in severe cold environment were studied, and the interface bonding mechanism were analyzed. The results show that M/P 4 ~ 5, B/M 0.02 ~ 0.03, and W/C 0.12 ~ 0.14 are more conducive to improve bond strength between MPC mortar and OPC in severe cold environment. The 3 h bond strength between MPC mortar and OPC can exceed 2.5 MPa, meeting the requirements for rapid repair of concrete pavement in severe cold environment. [ABSTRACT FROM AUTHOR]

Reactive MgO cement (RMC), which undergoes hardening through the reaction of MgO with water and CO2, is a novel cementitious material that has gained significant attention in recent years. The hydration and carbonation processes of RMC samples are significantly affected by the curing method and water content. This work compared the effects of standard curing ((20 ± 1) °C, 95% relative humidity), water curing ((20 ± 1) °C, and carbonation curing ((20 ± 3) °C, 70% relative humidity, 20% (volume fraction) CO2) on the properties of RMC with water-cement ratio (w/c) of 0.5, 0. 6, and 0.7, respectively. The types and content of the phases of the hydration and carbonation reaction products as well as their micro-morphological characteristics were investigated. The results show that the residual content of MgO in samples is 10% ~ 15% (mass fraction) at 3 d and disappears at 14 d under standard curing and water curing. When w/c is 0.6, the compressive strength of paste reaches 7.7 and 3.2 MPa at 14 d under standard curing and water curing, respectively. The strength of paste under carbonation curing is superior to that under standard curing and water curing (3 d compressive strength, 16.19, 0.42, and 0.43 MPa, 14 d compressive strength, 22.34, 7.46 and 7.23 MPa). The quantitative analysis results show that the carbonation product is mainly MgCO3 ⋅ 3H2O. When w/c is 0.6, the content of MgCO3 ⋅ 3H2O is 12.12% and 11.29% higher than that of samples with w/c = 0.5 and 0.7. The main reason for the optimal compressive strength could lies in that these needle-rod MgCO3 ⋅ 3H2O are bonded to each other form a dense microstructure. [ABSTRACT FROM AUTHOR]