Effect of Competitive Adsorption between Polycarboxylate Superplasticizer and Hydroxypropylmethyl Cellulose on Rheology of Gypsum Paste

High-fluidity gypsum-based materials have been widely used in grouting materials, self-leveling floors, wall materials, and three-dimensional (3D) printing materials. High fluidity means excellent workability without any segregation and bleeding, and generally, this can be obtained with addition of both polycarboxylate superplasticizer (PCE) and hydroxypropylmethyl cellulose (HPMC). However, the interaction between these two additives in gypsum paste has not been completely understood until now. In order to obtain a deeper insight into this interaction, this paper investigates the rheological performance of the gypsum pastes in the presence of these two materials with different orders of addition and discusses the adsorption behavior of these organic materials and the hydration process of the gypsum at very early age. Dispersion models are proposed to explain the mechanisms behind the interaction. The results show that both HPMC and PCE can adsorb onto the surface of gypsum particles. HPMC can hinder the adsorption of PCE due to competitively adsorbing behavior, resulting in a negative effect on dispersion of PCE. Preferential addition of PCE can effectively reduce this negative effect, whereas the preferential addition of HPMC has a positive effect on enhancing the viscosity of gypsum slurry. These results can provide useful guidance for the preparation of high-fluidity gypsum paste in practical engineering.