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

Abstract

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 1000°C. Paste with polypropylene fibers was exposed to the temperature of 20, 120, 150, 200, 300, 440, 520, and 700°C. 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 20°C to 1000°C. The gradual heating coarsened the pore structure. The most notable coarsening of pore structure—together with strength loss—took place at exposure temperatures exceeding 600°C. At 600°C, the residual compressive capacity (fc600°C/fc20°C) 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 50°C to 120°C can be as dangerous as exposure temperatures 400–500°C to the residual strength of cement paste produced by a low water cement ratio.