# Mechanical Properties and Microstructure of Polyvinyl Alcohol (PVA) Modified Cement Mortar
## 1 Introduction  
Cement-based materials behave like typical porous, brittle materials [1; 2; 3]. Brittleness makes the cement material easy to crack under the action of external force, which adversely affects the bearing capacity and durability of cement concrete structures. Polymers have good elastic deformation properties, flexibility, acid and alkali corrosion resistance, and good compatibility with cement materials, which can effectively improve the mechanical properties, deformation properties and durability of cement-based materials [4; 5; 6; 7; 8; 9]. Therefore, polymers are widely used in repairing roads, bridges, reservoirs and dams and in the bonding engineering of facing materials [10; 11; 12]. Polyvinyl alcohol (PVA) is a typical water-soluble nonionic synthetic polymer containing vinyl [13; 14]. It is harmless and, therefore, considered safe and relatively environmentally friendly [15]. Commercial- grade PVA is usually divided into two categories based on its degree of polymerization and hydrolysis: (1) fully hydrolyzed groups (98% by mole of acetate or more) where the PVA group has been replaced by an alcohol group, and (2) hydrolyzed groups (approximately 87-89% by mole of acetate group) where the PVA group has been replaced by alcohol groups. Completely hydrolyzed PVA is soluble in hot water and has good film-forming properties (the formed film is insoluble in low-temperature water) and good adhesion properties [15; 16]. The polyvinyl alcohol after film formation has good deformation properties, toughness, and wear resistance [17; 18]. The physical properties of the polyvinyl alcohol film system are very stable. It not only has good heat (no obviouschange occurs under 140 \({}^{\circ}\)C), light, and chemical stability but also has excellent gas and water barrier properties (the inside of polyvinyl alcohol can remain dry under high humidity) [19; 20]. The good physical and chemical properties of polyvinyl alcohol and its formed films enable its wide use in the textile, construction, chemical, and papermaking industries, to name a few. Owing to the excellent performance as described above, researchers have utilized PVA to modify the mechanical properties of a cement-based material. [PERSON] et al. [21] found that the addition of 3% of PVA could increase the compressive strength of cement mortar about 12 % due to there exists chemical interaction between PVA and cement hydration. This chemical interaction is effective in improving the interfacial bond between cement hydration and aggregates. Moreover, the chemical products could fill the pores of the cement matrix, resulting in the mechanical properties of the cement matrix increase greatly. Similarly, [PERSON] et al. [22] demonstrated that the flexural strengths of cement mortar increased by 21% after loading 1% PVA when cured in dry condition, respectively. They stated that this improvement was mainly attributed to the formation of PVA film within the cement matrix. [PERSON] et al. [23] found that the improvement in bond strength after loading PVA seems to arise from suppression of the porous interfacial transition zone and inhibition of calcium hydroxide nucleation on the aggregate surface. Using PVA to enhance the properties of cement composites has attracted extensive attention. However, the previous research mainly focused on the hydration and mechanical properties after loading a mono content of PVA, there is limited information on the effect of PVA with different content on the fresh properties and strength development at the early ages. Moreover, the morphology and microstructure of cement composite with different PVA content lack systematic investigation. The goal of this study was to study the effect of PVA content on the compressive and flexural strength of mortar after 3 days, 7 days, 14 days and 28 days curing, in which PVA/binder weight ratios of 0% 0.2%, 0.6%, 1.0%, and 2.0% were adopted. Moreover, the fresh properties and water absorption of mortars were also investigated.