Mechanistic pathways for early-age mechanical enhancement of high-belite cement via alkali-metal oxide doping from polymorph stabilization to strength gain: The role of R2O in stabilizing α-C2S

As a new type of low-calcium and low-carbon cement, high-belite cement has advantages such as low calcination temperature, lower CaO content, and low requirements for raw material. Given the low calcium requirement of high-belite cement, a large amount of solid waste containing CaO can be used to prepare it. This approach has dual advantages: first, it can reduce the consumption of limestone and improve the utilization rate of solid waste; second, the mineralization and lattice modification effects of impurity ions in the solid waste can decrease the calcination temperature, activate the clinker minerals, and enhance the reactivity of high-belite cement. This study aims to stabilize the high-temperature crystal form of C2S through the synergistic effect of phosphorus slag and alkali metal oxides(R2O), thereby enhancing the early mechanical strength of high-belite cement. The investigation focused on studying the combined effect of 8 % phosphorus slag and 0–2.0 % R2O on the activation and early performance of high belite clinker. Little amounts of P2O5 and F- in phosphorus slag enhanced the burnability of high-belite cement raw meal, but the burnability deteriorated after the addition of R2O. With the addition of alkali metal oxides, the contents of α-C2S, C3S, and C3A in high-belite cement clinker increased, while the contents of β-C2S and C4AF decreased. Adding 1.0 % Na2O and 1.5 % K2O made the α-C2S content in high-belite cement clinker reach 32.15 % and 40.82 %, respectively. It was found that the R2O phase preferred to dissolve in the intermediate phase, followed by C2S. Meanwhile, extensive Na2O entered the C3S crystals. Mechanical property test results showed that both 1.0 % Na2O and 1.5 % K2O can improve the early mechanical properties of high belite cement. Based on the obtained results, it could be concluded that P2O5 and F- in phosphorus slag significantly enhanced the role of R2O in the formation and stabilization of the high-temperature α-C2S polymorphs.