Abstract
This paper reports findings of ongoing investigation into the effect of high alumina ground granulated blast furnace slag (GGBS) on sulphate resistance of concrete. Slags used in most countries contain low alumina contents and provide high resistance to sulphate attack among other durability improvements. It is however known that slags of high alumina contents do not necessarily improve sulphate resistance but may otherwise adversely influence concrete performance. South African slags have moderately high alumina contents but hardly any studies have been conducted to determine its influence on sulphate resistance of concretes. In this investigation, commercially available slag widely used in South Africa was used. Mortar prisms 25 x 25 x 285 mm of 0.5 water-binder ratio incorporating 30%, 50%, 70% GGBS were prepared and immersed in sodium sulphate solutions of different concentrations of 28 g/L and 50 g/L as SO4. Expansion and mass change of the cementitious systems were monitored. Variables examined were compressive strengths prior to immersion in Na2SO4 solution, slag replacement levels, concentrations of sulphate solutions. It was found that the moderately high alumina slag improved resistance to sulphate attack in correspondence with increase in the replacement levels of the extender. Mixtures that were not cured to develop 20 MPa initial strength prior to exposure in Na2SO4 solution, showed elevated early age expansion while their cured counterparts did not expand. The long-term expansions of mixtures that had not been cured were much higher than expansions of the respective cured mixes.
Interestingly, the use of GGBS in proportions exceeding 50% mitigated the adverse effects of early age expansions giving no long-term expansions in any of the mixtures containing the extender.