Compressive strength and hydration of wastepaper sludge ash–ground granulated blastfurnace slag blended pastes

Abstract

Compressive strength and hydration characteristics of wastepaper sludge ash–ground granulated blastfurnace slag (WSA–GGBS) blended pastes were investigated at a water to binder (w/b) ratio of 0.5. The strength results are compared to those of normal Portland cement (PC) paste and relative strengths are reported. Early relative strengths (1 day) of WSA–GGBS pastes were very low but a marked gain in relative strength occurred between 1 and 7 days and this increased further after 28 and 90 days. For the 50% WSA–50% GGBS blended paste, the strength achieved at 90 days was nearly 50% of that of the PC control paste. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analysis were carried out to identify the mineral components in the WSA and the hydration products of WSA and WSA–GGBS pastes. The principal crystalline components in the WSA are gehlenite, calcium oxide, bredigite and α′-C 2S (stabilised with Al and Mg) together with small amounts of anorthite and calcium carbonate and traces of calcium hydroxide and quartz. The α′-C 2S and bredigite, which phase separate from liquid phase that forms a glass on cooling, are difficult to distinguish by XRD. The hydration products identified in WSA paste are CH, C 4AH 13, C 3A.0.5CC̄.0.5CH.H 11.5 and C-S-H gel plus possible evidence of small amounts of C 2ASH 8 and C 3A.3CS̱.H 32. Based upon the findings, a hydration mechanism is presented, and a model is proposed to explain the observed strength development.