Impact of calcium sulfate additions on the hydration of the belite phase in cements made from belite-ye'elimite-ferrite clinkers

“BYF” clinker, mainly composed of belite, ye’elimite and calcium alumino-ferrite, is a low-CO2 alternative to Portland cement clinker. Simple BYF cements are made by adding calcium sulfate to BYF clinker, typically in amounts larger than normally used in Portland cements. In this work the hydration kinetics of BYF cement pastes made with three different clinkers and three different calcium sulfate contents are studied. A dedicated Rietveld refinement file was developed in order to quantify the phase assemblage formed during hydration. Results show that increasing the ye’elimite/belite ratio in the clinker at constant added calcium sulfate results in greater formation of strätlingite at the expense of C-S-H. Increasing the level of calcium sulfate addition at constant clinker composition results in the formation of more ettringite at very early ages, and thus gives higher early strengths, but this tends to block the onset of belite hydration which can lead to slow strength development at layer ages. It is hypothesized that the blockage of belite hydration occurs due to the limited access of water resulting from the deposition of ettringite around belite grains.