Dicalcium silicates doped with strontia, sodium oxide and potassia

The early-age mechanical property of belite cement can be improved by the inclusion of the high early-strength mineral calcium strontium sulfoaluminate (C_1·5Sr_2·5A₃$, 1·5CaO·2·5SrO·3Al₂O₃·SO₃). In this paper, the effects of strontia (strontium oxide), sodium oxide, potassia (potassium oxide) and their combination on the burnability and phase composition of belite are discussed with the purpose of enhancing hydration activity of dicalcium silicates (C₂S), which lays the foundation for improvement in the early hydraulic activity for belite-C_1·5Sr_2·5A₃$ cement. Results showed that, contrary to strontium oxide, sodium oxide and potassium oxide can greatly reduce free-calcium oxide (f-CaO) content in dicalcium silicates and lower the calcium carbonate decomposition temperature. The dicalcium silicate polymorphism was influenced by minor elements, α-dicalcium silicate, α′-dicalcium silicate, β-dicalcium silicate and γ-dicalcium silicate coexisting in different proportions for each doping sample and non-doping sample, of which the most abundant polymorphs were β-dicalcium silicate and γ-dicalcium silicate. According to the Rietveld quantitative phase analysis, the β-dicalcium silicate contents in the control sample, 3% sodium oxide-added, 3% potassium oxide-added, 5% strontium oxide-added and a combination of three elements were 16·49%, 70·13%, 59·34%, 72·78% and 91·04%, respectively. The stabilising effect of β-dicalcium silicate for minor elements can be listed as composite, strontium oxide, sodium oxide and potassium oxide. Scanning electron microscope photographs of doped dicalcium silicate showed granular shape β-dicalcium silicate of dimension of 1–10 μm and prism or plate-like shape γ-dicalcium silicate.