Interaction of PCE and Chemically Modified Starch Admixtures with Metakaolin-Based Geopolymers—The Role of Activator Type and Concentration

Water-reducing admixtures are of enormous importance to adjust the workability of alkali-activated materials. Especially in geopolymers activated by highly concentrated alkaline solutions, the polycarboxylate ether (PCE) superplasticizers are less effective than in conventional cementitious systems. The aim of this study was to clarify the reasons for the lower dispersing performance of PCE and the synthesis of alternative dispersing agents based on the biopolymer starch to improve the workability of highly alkaline geopolymers. Furthermore, the focus of investigations was on the role of activator type and concentration as key parameters for geopolymer reaction and interaction of water-reducing agents. Therefore, in this study the conformation of three different types of PCE (MPEG: methacrylate ester, IPEG: isoprenol ether, and HPEG: methallyl ether) and synthesized starch admixtures in sodium and potassium hydroxide solutions (1 mol/L up to 8 mol/L) were studied. Furthermore, the dispersing performance, adsorption behavior, and influence on reaction kinetics in metakaolin-based geopolymer pastes were investigated in dependence on activator type and concentration. While the PCE superplasticizers show coiling and formation of insoluble aggregates at activator concentrations of 3 mol/L and 4 mol/L, the synthesized starch admixtures show no significant change in conformation. The cationic starch admixtures showed a higher dispersing performance in geopolymer pastes at all activator concentrations and types. The obtained adsorption isotherms depend strongly on the activator type and the charge density of the starch admixtures. The reaction kinetics of geopolymer pastes were not significantly influenced using the synthesized starch admixtures. Especially the cationic starch admixtures allow the reduction of liquid/solid ratios, which leads to higher flexural and compressive strengths.