The effect of pozzolans and slag on the expansion of mortars and concrete cured at elevated temperature

The expansive behaviour of heat-cured Portland cement mortars and concretes containing pozzolans and/or slag was investigated. The incorporation of any amount of these supplementary cementing materials into the mixture typically reduced the long-term expansion, slowed the rate of expansion, and delayed the onset of expansion. However, the efficacy of a particular material to control expansion was greatest with those that contained reactive Al ₂O₃. Metakaolin, which contains a large amount of reactive Al₂O₃, was the most effective at controlling expansion at relatively low cement replacement levels. Slag and fly ash, which also contain Al₂O₃, were effective at suppressing expansion at higher cement replacement levels. Silica fume was less effective at controlling expansion at conventional replacement levels, and even at higher replacement levels, expansion may only be delayed.; In general, curing at elevated temperature resulted in a high concentration of sulphate sustained in the pore fluid, and consequently a significant amount of sulphate incorporated in the calcium silicate hydrate (C-S-H) that formed at elevated temperature. With subsequent exposure to moisture at ambient temperature, this sulphate was liberated from the C-S-H, leading to the formation of ettringite in the hardened material, which under certain conditions resulted in deleterious expansion. With the incorporation of pozzolans or slag into the mixture, the sulphate concentration of the pore fluid, while decreased, remained sufficiently high that a significant amount of sulphate was still taken up by the C-S-H during the heat cure. Upon exposure to moisture, this sulphate was released from the C-S-H and a considerable amount of ettringite was also formed with these mixtures. However, if the pozzolan or slag contained reactive Al ₂O₃, and a sufficient amount was used, the subsequent formation of ettringite was not accompanied by expansion. It is believed that expansion only occurs when sulphate reacts with monosulphoaluminate that is finely intermixed with the C-S-H in the outer product, and is directly converted to ettringite. With the incorporation of an Al₂O₃-bearing supplementary cementing material, this reaction is apparently averted; ettringite still forms, presumably precipitating from solution into any available void space, but it is innocuous with respect to expansion.