Effects of high temperature on the pore structure and strength of plain and polypropylene fiber reinforced cement pastes

The effect of high temperature on the residual properties of plain and polypropylene fiber reinforced Portland cement paste was investigated. Plain Portland cement paste having water/cement ratio of 0.32 was exposed to the temperatures of 20, 50, 75, 100, 120, 150, 200, 300, 400, 440, 520, 600, 700, 800, and 1000degreesC. Paste with polypropylene fibers was exposed to the temperature of 20, 120, 150, 200, 300, 440, 520, and 700degreesC. Residual compressive and flexural strengths were measured and pore structure of the pastes was determined by mercury porosimetry. The total porosity of the pastes more than doubled when exposure temperature was increased from 20degreesC to 1000degreesC. The gradual heating coarsened the pore structure. The most notable coarsening of pore structure-together with strength loss took place at exposure temperatures exceeding 600degreesC. At 600degreesC, the residual compressive capacity (f(c600degreesC)/f(c20degreesC)) was still over 50% of the original. Strength loss due to the increase of temperature was not linear. Polypropylene fibers produced a finer residual capillary pore structure, decreased compressive strengths, and improved residual flexural strengths at low temperatures. According to the tests, it seems that exposure temperatures from 50degreesC to 120degreesC can be as dangerous as exposure temperatures 400-00degreesC to the residual strength of cement paste produced by a low water cement ratio.