| Cement-based material is a kind of heterogeneous material, high temperatures induce changes in physical and chemical properties, such as loss in quality, strength, elastic modulus and durability due to forming a large number of holes and cracks. It is very important to improve the high temperature resistance of cement-based materials and to repair the damaged cement-based materials in a scientific and economic way, which will reduce the loss caused by fire and benefit both the economy and society.The high temperature behavior and rehydration characteristics of the cement-based materials and their mechanisms have been studied in this paper. A kind of mortar with fire resistance and easy to rehydrated repair performance is prepared in this experiment and its mechanism is studied. There are three aspects of conclusions in this paper as follows.1.Some main conclusions about high temperature characteristics of cement-based materials can be drawn as follows:1)With the temperature increased, the compressive strength of hardened cement paste (HCP) first increased and then decreased, while the compressive strength of mortar decreased gradually, above400℃the compressive strength of mortar decreased sharply. The addition of mineral materials was disadvantageous to keep mechanical properties and to maintain resistance to carbonizing when mortars exposed to fire.2)There are a large number of cracks in the surface of HCP and even part of paste splitting when HCP exposed to high temperature. While there are less cracks in mortar surface and there is none of splitting phenomenon when mortar exposed to high temperature.3)According to micro measurements, in400℃the porosity and average pore diameter of HCP increased slightly, crystal structure of CH keep well and relative content of CH increased. In addition, in800℃the porosity and average pore diameter of HCP increased sharply, most of hydration products decomposed and the microstructure of HCP appeared broken.2.Some findings about rehydration of cement-based material may be summarized as follows:1) After post-fire-curing, the compressive strength of most HCP decreased. When mortars were cured after exposing to400℃, its compressive strength decreased slightly and carbonation resistance became weaker. In addition, When mortars were cured after exposing to600℃and800℃, its compressive strength increased and carbonation resistance improved.2)When HCP was cured after exposing to400℃, its pore structure and phase composition had no change, while HCP was cured after exposing to to800℃, there are new hydration products, and its pore structure may be more fine but it can not fully recover to original state.3.The main conclusions of mortar with the characteristics of heat resistance and easy rehydration repair can be drown as follows:1)The mortars with the characteristics of heat resistance and easy rehydration repair were prepared in this paper.2)Compared to the way of replacing sand with kaolinite slag by same weight, it is better to preparing mortar by the way of replacing sand with kaolinite slag by same volume, and the compressive strength of all mortars were larger than44Mpa.3)With the increase in replacing level of kaolinite slag, the relative residual compressive strength increased after mortar subjected to elevated temperature. In600℃, the relative residual compressive strengths of mortars with more than60%kaolinite slag were higher than100%, and in800℃the relative residual compressive strength of mortar with100%kaolinite slay was more than90%.4)With the increase of replace level of kaolinite slag, the relative residual compressive and bonding strengths of mortars first increased then decreased after post-fire-curing, the relative residual compressive strengths of all simples with20%replace level of kaolinite slag were more than78%and far higher than that of control mortar.5)Considering the heat resistance and rehydration characteristics, the comprehensive performance of mortar with about40%kaolinite slag by volume is best. |
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