Phase development in conventional and active belite cement pastes by Rietveld analysis and chemical constraints

High belite cements may be an alternative to reduce CO2 emissions. Although CO2 emissions may be depleted up to 10%, unfortunately, the hydration reactivity of belite phases is slow which leads to low mechanical strengths at early ages. In order to enhance their hydraulic reactivity, the activation of these cements by doping with alkaline oxides has been proposed. The authors synthesised a laboratory belite clinker without activation (47 wt.% of [beta]-C2S and 19 wt.% of [alpha]H'-C2S) and 2 alkaline oxide activated clinkers (one with 13 wt.% of [beta]-C2S, 24 wt.% of [alpha]H'-C2S and 19 wt.% of [alpha]-C2S; and the second with 12 wt.% of [beta]-C2S, 42 wt.% of [alpha]H'-C2S and 5 wt.% of [alpha]-C2S). A methodology to analyze quantitatively the phase evolution of cement pastes is developed and applied to these high belite cements. Rietveld quantitative phase analysis of synchrotron X-ray powder diffraction data, together with chemical constraints, is used to determine the phase development up to 1 year of hydration in the belite cement pastes. [beta]-C2S almost does not react during the first 3 months, meanwhile [alpha]H'-C2S reacts on average more than 50% in the same period. Moreover, the degree of reaction of [alpha]-C2S is slightly larger (on average about 70% after 3 months) than that of [alpha]H'-C2S. Full phase analyses are reported and discussed including the time evolution of amorphous phases and free water.