Phase development in cement in relation to the secondary ettringite problem

Abstract

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 AFm, preferentially formed at high temperatures, induces a swelling pressure within cement paste leading to its expansion as AFm reverts to the more stable AFt.