Phase development in cement in relation to the secondary ettringite problem

Phase relationships and mass balances in the quinary system CaO—Al₂O₃—CaSO₄—Na₂O—H₂O have been calculated at different temperatures and the results are applied to explain the formation of secondary ettringite. The sulphate phases in equilibrium with the pore fluid of a cement saturated with Ca(OH)₂ depend on temperature, water—cement ratio and the composition of the cement, especially its content of Al₂O₃, Na₂O, K₂O and SO₃. Results of the phase equilibrium and mass balance calculations are shown in the form of expansion potential diagrams, which relate potential for expansion to cement chemistry and temperature changes in a thermal cycle. These diagrams reveal critical regions of cement SO₃ contents with respect to expansion potential. A mechanism for the expansion is postulated: slow transformation of AF_m, preferentially formed at high temperatures, induces a swelling pressure within cement paste leading to its expansion as AF_m reverts to the more stable AF_t.