Experimental Research on the Preparation of Alkali-Activated Ultrahigh-Performance Concrete Matrix with Low Water-to-Binder Ratio Using Water Reduction Agents

The water reduction agents (WRA) are the necessary admixtures for preparing traditional ultrahigh-performance concrete (UHPC), whereas the effects of WRA on the properties and microstructure of alkali-activated ultrahigh-performance concrete (AAUHPC) are still unknown. The slump flow of AAUHPC with a water-to-binder ratio (W/B) of 0.28 reaches 150–170 mm when the polycarboxylate superplasticizer (PCE), naphthalene sulfonate formaldehyde condensate (FDN), and sulfonated melamine water reducer (SM) are used, whereas the AAUHPC cannot be formed without using WRA. In addition, greater addition of PCE, FDN, and SM leads to poorer fluidity. These results show that PCE, FDN, or SM possesses certain water reduction outcomes for the preparation of AAUHPC; however, partial ineffectiveness of PCE, FDN, or SM is also found. The water reduction mechanism is that PCE, FDN, or SM can combine with Ca2+ and further absorb on the surfaces of the precursor particles to generate the repulsive effect; however, the poor dispersity of PCE, FDN, or SM in a strong alkaline solution also causes partial PCE, FDN, or SM to be ineffective. The dissolution of more invalid PCE, FDN, or SM consumes the partial free water, thus resulting in a decrease in fluidity. When the PCE, FDN, or SM content is 0.5%, the FDN is most conducive to the increase in the degree of polymerization and the generation of reaction products, leading to the lowest porosity and densest matrix, followed by SM and finally PCE. In addition, the 0.5% PCE is most beneficial for the mitigation of autogenous and drying shrinkage of AAUHPC, followed by FDN and finally SM, which is because fewer meso and capillary pores are generated in the AAUHPC matrix with the addition of 0.5% PCE. This research provides the reference for using WRA in the preparation process of AAUHPC and a route for preparing AAUHPC with higher mechanical properties and lower shrinkage.