Production and properties of the polyvinyl alcohol modified macro-defect-free α-hemihydrate gypsum composite

The polyvinyl alcohol (PVA) reinforced macro-defect-free α-hemihydrate gypsum composite (PMC) was prepared by compression molding technique. The mechanical strength from the age of 3 d to 120 d and water resistance of PMC were tested, and the phase composition, microstructure, pore structure of PMC was characterized by SEM, XRD, TGA and MIP. The results show that the compressive and flexural strength of PMC with 1 % PVA addition cured for 3 d reach 80.9 MPa and 23.2 MPa, respectively. A compacted gypsum crystal structure is obtained by using the compression molding technique and the PVA film in PMC matrix can absorb fracture energy, which makes PMC have high mechanical strength. Due to the low water to gypsum ratio, the main crystalline phases detected in PMC consist of CaSO4·2H2O and CaSO4·0.5H2O. The TG curves show that the hydration degree of PMC may be further limited by the excessive incorporation of PVA, which is not conductive to the strength development. The softening coefficient of PMC with 2 % PVA addition gets to 0.81. The diffusion rate of water in the matrix can be reduced because of the compact structure of PMC and the hindering effect of PVA film on water diffusion, which improves the water resistance of PMC.