Experimental Study on the Thermal Performance of Rice Straw-Mortar Composite Materials

Abstract

Thermal properties of mortar block can be improved through adding straw, which is related to the type, size, amount of the straw, and so on. In order to test the thermal insulation performance of rice straw-mortar composite materials, this paper adopted response surface methodology (RSM), which employed a four-factor, three-level Box-Behnken design regarding gypsum content (% mass fraction of gypsum), straw size (mm), straw content (% mass fraction of straw) and water content (g) of rice straw-mortar composite materials as independent variables. Thermal conductivity, thermal diffusivity and thermal resistance of composite materials were response variables. The straw content and water content extremely significantly affected all the responses; the straw size had extremely significant effect on the thermal resistance as well as significant effect on the thermal conductivity. According to the optimum solution of regression equations, the optimum values of thermal conductivity, thermal diffusivity and thermal resistance were 0.048 W (m K)−1, 0.163 mm2 s−1, and 12.743 K W−1, respectively. The optimization options of gypsum content, straw size, straw content and water content were 5.01%, 20 mm, 22.7% and 0.06 g, respectively. Due to the state change of pore water, thermal conductivity of rice straw-mortar composite materials containing free water decreased seriously after complete freezing and increased slightly after 15 times freeze-thaw. During first freezing process, the thermal conductivity increased obviously with the increasing of freezing time, then increased slowly. A theoretical basis was provided by the experimental results, which was essential for the study on the thermal performance of rice straw-mortar composite materials as well as the application of similar materials.