Study on the evolutionary mechanism of shrinkage stress-strain behavior of EVA-modified cement mortar at an early age

Abstract

Understanding the early deformation characteristics is important to predict early cracks in cementitious materials and to serve structural engineering. Incorporation of polyethylene-vinyl acetate (EVA) dispersible polymer powder can improve the early cracking of cementitious materials. In this paper, the homogeneity, shrinkage stress-strain behavior and the evolutionary mechanism of EVA-modified cement mortar (EMCM) with different polymer-to-cement ratios (p/c) were investigated at an early age. The homogeneity and shrinkage stress-strain behavior of EMCM were characterized by a self-designed test setup. Isothermal calorimetric analysis, scanning electron microscopy and X-ray diffraction were used for the evolutionary mechanism analysis. The results show that the homogeneity of shrinkage of EMCM can be improved by adding EVA and prolonging the mixing time. The shrinkage stress of EMCM can be reduced by 83 % when p/c is 10 % and 15 %. Meanwhile, the elastic modulus of mortar can be reduced by 45 % when the EVA content is 15 %. Isothermal calorimetric analysis and characterization of microscopic properties show that EVA can inhibit both the generation and conversion of C4AH13, as well as the generation of AFt and CH, and retard the hydration of cement mortar. Within 3 h of hydration, the change in shrinkage stress of EMCM is only affected by the amount of AFt generated, while the change in shrinkage strain is affected by both CH and AFt generation, and the correlation with CH is greater. Finally, the interpenetrating network structure between the EVA and the cementitious matrix increases the interaction and bond between them, giving good deformability and thus reducing shrinkage. Therefore, the reduction in shrinkage stress-strain of EMCM is a result of the combined effect of the polymer on delaying cement hydration and increasing internal interaction and bonding.