Study On Thermal Properties Of Rice Straw Mortar Composites

In recent years,due to the shortage of land resources and the need to build a conservation-oriented society,straw composite building materials,which combine crop straw with traditional building materials,have been studied and popularized.It not only effectively reduces the environmental and social problems caused by straw burning,but also provides material support for the construction of green energy-saving buildings with good thermal insulation performance of composite materials.It is very important for the popularization and application of the composite material to study the influence of the main factors on the thermal insulation performance of the straw composite.In this paper,the response surface test(RSM)was designed by using Design-Expert software to study the effect of different quality gypsum and rice straw on the properties of straw composites,the thermal conductivity,thermal diffusivity and thermal resistance were measured under different gypsum content,straw size,straw content and different moisture content in the composite.By fitting the data,the regression equations of thermal conductivity,thermal diffusivity and thermal resistance were obtained,and the significance of each factor and its interaction on thermal conductivity,thermal diffusivity and thermal resistance was determined.It’s found that the order of the influence of each factor on the thermal conductivity,thermal diffusivity and thermal resistance was following as:moisture content,straw content,straw size and gypsum content.By solving the equation optimally,the optimum values of each factor and the corresponding target values are obtained,in order to facilitate the engineering application,the optimum technological parameters were 5%gypsum,20 mm straw,23%straw,and the composite was completely dried.The theoretical optimal response target values were obtained,which were 0.048 W/m·K thermal conductivity,0.165 mm~2/s thermal diffusivity and 12.82 K/W thermal resistance.The heat preservation and insulation properties of straw-mortar composite materials under freezing and thawing conditions were analyzed in combination with the cold weather conditions in winter in north China,the thermal conductivities of 29 groups of straw-mortar composite specimens were measured at-20℃and 15 times of freeze-thaw cycles.The thermal conductivities of the specimens were 0.111～0.992 W/m·k when frozen completely,compared with the straw composites at room temperature,the water-containing composites have lower thermal insulation performance when frozen completely,the reason was the thermal conductivity of the dry materials was much less than that of ice.The thermal conductivity of29 groups was 0.111～0.810 W/m·K after 15 freeze-thaw cycles,and the thermal insulation performance of the moisture-containing straw composite material after 15 freeze-thaw cycles was higher than that of the normal temperature,the main reason was that water expanded after freezing in the material,and more pores were produced after melting.The increase of pores leaded to the decrease of heat transfer rate.In order to study the thermal conductivity of straw composites during freezing,one group of experimental data was used to analyze the thermal conductivity of straw composites.The thermal conductivity curve of the composite could be divided into two stages,the first stage was from 1h to 8h,the thermal conductivity was stable;the second stage was from 8h to12h,the moisture of the composite material was gradually frozen,which made the thermal conductivity of the material present an obvious upward trend during the initial freezing process.After freezing for a period of time,the freezing rate gradually slowed down,and the internal structure of the material gradually stabilized,at this point the thermal conductivity of the material also tended to stabilize.In this paper,the thermal properties of straw composite mortar are studied comprehensively,and the change rule of thermal properties of straw composite under freeze-thaw condition are discussed,which provides a reference for the study of thermal properties of straw composite material.