External sulphate attack on alkali activated slag and slag/fly ash concretes

Two types of alkali activated material (AAM) concretes were exposed to various sulphate bearing solutions for over two years. Physical changes to the concrete specimen and chemical changes in the exposure liquid were studied in an attempt to understand how sulphate attack occurs in such binders and also the role the mix variables play in offering resistance against such attack. The mix variables of alkali activated slag concrete (AASC) included water to binder ratio, percentage of alkali and the SiO2/Na2O ratio (silica modulus, Ms) while for alkali activated slag/fly ash (AA-S/F) concrete was slag/fly ash ratio and the SiO2/Na2O ratio. Exposure solution included water, magnesium sulphate (5%), sodium sulphate (5%), calcium sulphate (0.2%) and two concentrations of sulphuric acid solutions, pH3 and pH1. Physical changes studied were length and mass change, visual appearance and change in compressive strength. Exposure liquids were analysed for change in pH and ionic composition. Findings show that AA-S/F blend performs better than AASC in sulphate environments, based on strength and change in length. Exposure to water resulted in the most expansion/shrinkage in all mixes studied. An empirical model was proposed for predicting change in compressive strength for AAS&AA-S/F concretes based on mass gain. Further, a simple performance criteria is put forward for mixes in sulphate environments based on mass gain.