Sulfate attack expansion mechanisms

A specially constructed stress cell was used to measure the stress generated in thin-walled Portland cement mortar cylinders caused by external sulfate attack. The effects of sulfate concentration of the storage solution and C₃A content of the cement were studied. Changes in mineralogical composition and pore size distribution were investigated by X-ray diffraction and mercury intrusion porosimetry, respectively. Damage is due to the formation of ettringite in small pores (10-50 nm) which generates stresses up to 8 MPa exceeding the tensile strength of the binder matrix. Higher sulfate concentrations and C₃A contents result in higher stresses. The results can be understood in terms of the effect of crystal surface energy and size on supersaturation and crystal growth pressure.