Fly ash and granulated blast‐furnace slag as ‘stabilisers’ of portland cements with added periclase

Abstract

AbstractOne method proposed for utilisation of high‐magnesia Portland cements involves the addition of active siliceous admixtures such as fly ash or ground granulated blastfurnace slag. This addition enables otherwise unsound cements to pass the ASTM autoclave test (C151–74a) for expansion of Portland cement. Cements were made from an OPC, magnesia (2 and 3%) and varying proportions (0–70%) of fly ash or ground granulated slag. The expansions of pastes made from the cements stored at 50°C were measured over periods of 3 and 4 years, respectively. The cements were also subjected to the ASTM autoclave test. The results of the autoclave test support the view that expansion in the autoclave is to some extent related to the strength of the matrix, and best results were obtained with CaO:SiO2 ratios of about 1.25 corresponding to formation of large quantities of 1.1 nm tobermorite. However, it has been shown that even a high strength matrix cannot stabilise too much free magnesia either in the autoclave or at 50°. Slag was less effective than fly ash in producing a reduction in autoclave expansion although results at 50° were comparable. This work on cements with added periclase suggests that the stabilisation of ‘normal’ high magnesia cements, in both the autoclave and under more normal conditions, with any practical level of stabiliser addition, will depend on factors such as the ‘blocking effect’ described by Rosa, and the dilution effects of stabiliser addition being sufficient to reduce the effective free magnesia content to the small amount which the strength of the matrix can control.