Interfacial Competitive Adsorption between the Polycarboxylate Superplasticizer and Hydrophobic Emulsion Mitigates Performance Deterioration in Calcium Sulfoaluminate Cement Mortars

Abstract Hydrophobic emulsions (HEs) are widely used to improve the moisture resistance of cement-based materials, but their adsorption on cement particle surfaces can form a physical barrier that retards hydration and compromises mechanical performance. Herein, a polycarboxylate superplasticizer (PCE) was introduced into hydrophobic emulsion-modified calcium sulfoaluminate (CSA) cement mortars to regulate the interfacial adsorption behavior between HE droplets and CSA cement particles. The coupled effects of PCE and HE were systematically investigated through fluidity, mechanical strength, rheology, hydration heat, pore structure, water absorption, adsorption capacity, and ζ-potential analyses. The results show that 1% PCE provided the best overall balance between the performance recovery and hydrophobic functionality under the investigated conditions. Compared with the hydrophobic mortar without the PCE, the addition of 1% PCE increased fluidity by 25% and enhanced the 28 day compressive strength by 129% while maintaining the inherently low water absorption. Adsorption capacity and ζ-potential analyses further supported competitive adsorption between the PCE and HE at the CSA cement interface. The incorporation of the PCE alleviated the hydration retardation induced by HE and improved the pore structure without compromising the hydrophobic functionality of the mortar. This work provides an interfacial regulation strategy for balancing workability, hydration, mechanical performance, and moisture resistance in hydrophobic CSA cement mortars.