Effect of mixing sequence of calcium ion and polycarboxylate superplasticizer on dispersion of a low grade silica fume in cement-based materials

Abstract

Effective use of low grade silica fume (SF) is of both environmental and economic significance. However its low pozzolanic reactivity, presence of large fused aggregates and poor adsorption of dispersant such as polycarboxylate superplasticizer (PCE) hinder its application in cement and concrete. Recent studies have focused on effect of cationic ions on PCE adsorption on silica fume, but attempts have not been made on low grade SF. In this paper, effect of different mixing sequences of cationic calcium ions (Ca2+) and anionic polycarboxylate superplasticizer (PCE) on dispersing a low grade SF was compared in both aqueous suspension and cement paste. In the aqueous suspension, adsorption and dispersion behavior of low grade SF suspension mixed with different sequence of calcium ion and PCE were characterized by Total Organic Carbon (TOC) test, free settling test and laser particle size test. Results showed that priority adsorption of Ca2+ followed by PCE to low grade SF suspension (sample SF2) favored adsorption of PCE on SF. After sequential addition of Ca2+ and PCE to the SF, sample SF2 had the best anti-free settling ability and particle dispersion. In the cement paste, the sample that had priority mixing of Ca2+ ions with low grade SF followed by addition of PCE, water and cement (CS2, corresponding to SF2) was verified to have better fluidity than the control sample (CS0, corresponding to SF0) that was prepared by mixing PCE, water, SF and cement (including abundant Ca2+ ions). The hardened cement paste with pretreated SF had higher strength and more effective silica fume dispersion in the cement matrix.