Superplasticiser for NaOH-activated slag: Competition and Instability between Superplasticiser and Alkali-activator

Alkali-activated slag (AAS), consisting of slag and alkaline activator, is a novel low carbon cementitious material and has received increased attention worldwide. This is mainly due to its environmentally friendly nature and superior performance than Portland cement (PC) system. However, current commercially available superplasticisers (SPs) are generally developed based on the chemistry of PC system which has been found incompatible with AAS due to 1) competitive adsorption between SPs and activators; and 2) chemical instability of SPs in highly alkaline environment. Two approaches, namely, separate SP addition methods (i.e. adding SP and activator separately) and synthesis of novel alkali-compatible superplasticiser, offers the potentials to tackle the above two issues, respectively. In this research, the effects of different addition methods of commercially available lignosulfonate and naphthalene superplasticisers on the properties of NaOH-activated slag paste, in particular the surface interaction among the SP, NaOH activator and slag particles (in terms of SP adsorption and zeta potential) and the fresh properties (such as minislump and rheology), were systematically investigated. The results demonstrated that, compared to the simultaneous addition, the separate addition methods improved the performance of lignosulfonate and naphthalene superplasticisers in NaOH-activated slag by avoiding the competitive adsorption between the SPs and the NaOH, which offers a chance to efficiently utilise the commercially available PC-based superplasticisers in AAS. Moreover, a novel alkali-compatible polymer was also synthesised and its optimal synthesis conditions were obtained through response surface methodology. The results indicated that the synthesised polymer exhibited better chemical stability in highly alkaline media, which provides an opportunity to develop a tailored superplasticiser for the alkali-activated slag system. However, the overall performance of the synthesised polymer in AAS under separate addition still needs to be improved by future research