# Experimental Study on Flexural Properties of Polyurethane-Cement Composites under Temperature Load
## 7 Conclusions  
This paper studied the flexural properties of polyurethane-cement composites under temperature load. Firstly, the raw materials for preparing polyurethane cement composites were introduced. Polyurethane cement composites' molecular structure, reaction process, and action mechanism were modeled and analyzed from a microscopic perspective. Secondly, a three-step process for preparing polyurethane cement is proposed, and a suitable ratio is selected. Finally, the effect of temperature on the bending and tensile strength of polyurethane cement composites was studied experimentally. The indexes affecting the bending performance of polyurethane cement composites and the damage modes under different temperature loads were studied and analyzed. The main conclusions are as follows:  
1. Polyurethane cement composites consist mainly of cement and polyurethane where polyurethane is composed of isocyanate, combined polyether, chain extender, and catalyst by the reaction. The critical step of the polyurethane cement preparation process is to dry and dehydrate the cement; otherwise, the formulated polyurethane cement will show a foaming phenomenon. 2. Polyurethane is a block copolymer formed by the addition polymerization of isocyanate and polyether polyol under the action of electrostatic attraction. The main reactive groups are the isocyanate group and the hydroxyl group. The silicate cement particles can fill the tiny gaps inside the polyurethane colloid and eliminate the internal stress concentration. At the same time, high energy is required to excite the silicate cement to make an electron leap. The silicate cement will not react with the polyurethane molecules under a normal state, so the two are more compatible. The composed polyurethane cement composites have better macroscopic mechanical properties. 3. Polyurethane is divided into hard chain segments and soft chain segments. During the cooling process, the interval between the soft and hard chain segments gradually increases, the molecular chain movement ability gradually increases, and the mechanical index of polyurethane cement composites improves. However, this mechanical change is not always inversely proportional to the temperature. Polyurethane tends to crystallize and solidify when the temperature is too low, and the mechanical properties will be reduced instead. 4. With the increase in temperature, the flexural and tensile strength of polyurethane-cement composites showed a trend of decreasing first, then increasing, and then decreasing. Their mechanical properties changed from brittleness to viscoelasticity and plasticity. The flexural failure stiffness modulus of polyurethane cement composites decreases with the increase in temperature; the failure load shows a trend of first decreasing, then increasing, and then decreasing with the increase of temperature, and the failure displacement shows that with the increase in temperature for an increasing trend. 5. The damage strain of polyurethane cement composites increases with the increase in temperature, and the damage strain curve is L-shaped, with an obvious turning point near 30 \({}^{\circ}\)C. The damage modes of polyurethane cement composites under different temperature loads are divided into three types: Type I damage is brittle damage, Type III damage is flexible damage with flow deformation of polyurethane cement, and Type II damage is elastic-plastic damage between Type I and Type III. Conceptualization, B.L., H.L. and H.G. ; Methodology, B.L. ; Software, B.L. ; Validation, H.G. ; Formal analysis, B.L. ; Investigation, J.J.; Data curation, H.D. ; Writing--original draft, B.L. ; Writing--review and editing, B.L. and H.G. ; Supervision, H.L. ; Funding acquisition, H.L. and H.G. All authors have read and agreed to the published version of the manuscript.