Reserach On Carbon Fiber Toughening Magnesium Phosphate Cement

Magnesium Phosphate Cement(MPC), also know as chemicaly bonded ceramics, is a cementitious material composited of magnesium oxide, phosphate and retarder. With characterstic of fast setting, high early strength, voloum stability, high bonding strength, MPC can be used in repairment of concrete structure and pavement, sodification of toxic and nuclear waste. However, MPC is bonded by covalent bond, ionic bond and hydrogen bond, which have high bility of resist distortion and movement of displacement. As the fast strength development, cracks and defects in process of hardening is inevitable.Moreover, some of ammonia, released in process of hydrotion, can cause defects such as stress cencentration, when it’s in subboundary. So MPC has a disadventage of high brittleness.Carbon fiber has characterstic of high modulus, high tensile strength and strong corrosion resistance, so it always be added to concrete to improve toughness. Taking the experience of carbon fiber modified concrete, this study try to use carbon fiber to improve toughness. Firstly, it explored the effect of W/C ratio, fiber content and length on MPC mortar’s fluidity and fiber dispersion. Secondly, the effect of fiber content and length on compressive, flexture and splitting tensile strength and some other mechanical properties of MPC mortar is studied. Finally, in order to improve surface properties of carbon fiber, Nitric acid is used, the effect of pretreating time, temperature and Nitric acid concentration on tensile strength of carbon fiber and compressive strength, flexture strength, spilitting tensile strength, frature properties of carbon fiber modified MPC mortar are analyzed.According to researches, the following outcomes come out. Increasing W/C ratio can improve fluidity of MPC mortar, but bad for fiber dispersion. Increasing carbon fiber content and length would reduce fluidity of MPC mortar and fiber dispersion. Adding carbon fiber has little effect on late compressive strength of MPC mortar,but can improve flexture and splitting tensile strength for further improving of short fibers. Nitric acid pretreating of carbon fiber has little effect on compressive and flexural strength of MPC mortar. Perlonging pretreating time can enhance splitting tensile stength, fracture energy and tensility of carbon fiber modified MPC mortar. And rising pretreating temperature can also improve fracture energy and tensility of carbon fiber modified MPC mortar, but at the temperature of 80 oC, carbon fiber would be over oxidized, splitting tensile and ultimate flexural strength of MPC mortar are slightly lower. Low concentration of Nitric acid pretreating fail to modify surface properties of carbon fiber, has little effect on splitting tensile stength, fracture energy and tensility of carbon fiber modified MPC mortar...