Effect of Surface Precoated Cementitious Materials on the Properties and Microstructure of PVC Mortars

The utilization of recycled polyvinyl chloride (PVC) in mortar production not only contributes to the recycling of waste plastics but also reduces the consumption of natural resources. However, the incorporation of PVC often leads to a decline in the mechanical properties of the mortar. This paper investigates the surface modification of waste PVC particles using precoated cementitious materials to improve the performance of PVC mortars. Mortars were prepared by replacing the fine aggregates with equal volumes of PVC particles before and after modification, with volume replacement ratios of 10%, 20%, 30%, and 40%. Experiments were carried out to investigate the changes in the surface properties of the particles before and after modification, as well as the density, consistency, compressive strength, and flexural strength of PVC mortars with different replacement ratios. Scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction, thermogravimetric analysis, and microhardness were used to analyze the morphology, relative elemental content, material composition, and interfacial strength of the interfacial transition zone (ITZ) between PVC particles and the cementitious material. The results showed that the density of the mortar gradually decreased as the PVC replacement ratio increased and the consistency of the modified mortar was significantly improved. The compressive strength and flexural strength of the mortar at all ages were significantly improved by the modification of the surface precoated cementitious material. Microscopic structural analysis indicated that the cement shell formed by the precoated cementitious material on the surface of PVC particles enhanced the bond between the particles and the cement matrix. After modification, the mass ratio of elemental calcium to silicon at the interface was significantly reduced. At the interface between modified particles and the cement matrix, compared to unmodified particles, there was significant reduction in the amount of low-strength, loosely structured Ca(OH)2, accompanied by an increase in higher-strength hydration products. The increase in microhardness indicated that the strength of the ITZ was improved. Overall, this study demonstrated the feasibility of using modified PVC particles as a replacement for fine aggregates in the preparation of mortar and provided a new solution for the development of new aggregates and the treatment of waste plastics.