# Mechanical Properties and Microstructure of Polyvinyl Alcohol (PVA) Modified Cement Mortar
## 3 Results and Analysis
### SEM Investigation  
Scanning electron microscopy is an important means of analyzing the internal microstructure of materials. Scanning electron microscopy images can be used to determine the composition of the pore structure inside the cement matrix, the internal shape of the cement matrix and the type of hydration products. Figure 11 shows the microstructure of the modified mortar with a PVA content of 0.6% for 28 days of curing. As shown in Figure 11, the internal structure, pore distribution, and pore size composition of the modified mortar changed with the incorporation of PVA. The polymer was evenly distributed in the cement matrix while the cement mortar was being agitated and intertwined with the cement hydration product. As the cement hydration reaction progressed, the polymer continuously precipitated in the cement to form a polymer film. The presence of these films can play the following roles: (1) The films can combine with the hydration product to enhance the connection between the hydration products, thereby improving the mechanical properties of the cement mortar. (2) Part of the film can become interspersed with the cement base. Between the pores, it can fill the pores and form bridges in the cement plate. Under a load, it can absorb the energy generated by the external force so that the mechanical properties of the cement matrix are improved [23]. (3) The presence of the polymer can refine the pores, making a large number of harmful pores decrease into medium-sized or even harmless pores, thereby enhancing the mechanical properties of the cement mortar. In addition, it can be clearly observed in Figure 11 that the polymer film spans the crack between the cement plates to form a load transfer bridge. Under the action of an external force, the film is torn, and the energy generated by the external load is absorbed so that the crack generation and development process of the cement plate are delayed. Figure 12 shows the microstructure of the modified mortar with a PVA content of 1.0%. As shown in Figure 12, as the amount of PVA increased, polymer film combines with the hydration product in the cement matrix to form a stable continuous network-like structure. This structure improves the mechanical properties of the mortar, especially the bending performance. Figure 11: Polymer film stretched in open space in 0.6% PVA modified mortar at 28 days. Figure 12: Network-like structure in cement mortar with the PVA content of 1.0% at 28 days. Figure 13 is the SEM image of PCM4 containing 2.0% PVA. Figure 13 shows that the morphology and microstructure of PCM4 are quite different from PCM2. The excessive incorporation of PVA led to the formation of heave polymer sheets within the cement matrix. These heavy PVA film might coat cement particles and prevent them from having contact with water. This coated effect of PVA might delay the hydration rate of cement, as demonstrated by DSC and FTIR test results as shown in Figures 9 and 10. The lower compressive strength of PCM4 over PCM0 might be due to its lower hydration rate caused by the coated effect of PVA. In addition, Figure 13 shows the positive effect of PVA on bridging cracks, which lead to the increase of flexural strength of mortar as shown in Figure 6, where the three-day flexural strength of PCM4 was about 36.6% and 1.8% higher than that of PCM0 and PCM2, respectively. The results of the scanning electron microscopy analysis show the following: (a) The proper amount of PVA incorporation (0.6%-1.0%) can be evenly distributed inside the matrix with an appropriate aggregate stirring process. A uniformly distributed polymer film can be formed that has a certain tensile strength. The polymer film bridging between the cement plates can delay or inhibit crack development of the cement mortar. Additionally, the film also can combine with hydration products to form a network-like structure. The structural system improves the mechanical properties of the cement mortar, especially for the flexural strength.