Analysis of process and interface structure of straw geopolymer building material with heat preservation

Abstract: In order to develop a new insulation building material with high strength and better corrosion resistance, straws were added into geopolymer to synthesis straw-reinforced geopolymer composites, which can save building energy consumption and broaden industrial utilization way of straw because of its good thermal insulation and light weight properties. The composites also overcome the defects of low strength in traditional straw building materials. In this paper, modulus of the sodium silicate mixed with NaOH as alkaline activator was 3, and straws were cut to specific lengths after drying. After a certain amount of distilled water and alkaline activator added, the reaction was carried out in a cement paste mixer with constant stirring. When the geopolymer turned to be slurry, it was moulded into 20 mm×20 mm×20 mm cubes for 24 hours. Then the demolded straw-geopolymer would be placed into a humidity chamber and tested for compressive strength using TYE-300B Pressure Testing Machine when the schedule maintenance completed. The experiments which aimed to study the effect of A (molar ratio of SiO2 and Al2O3 in the reaction system), B (molar ratio of Na2O and SiO2 in the reaction system) value on the compressive strength of straw-geopolymer were carried out by adopting the matrix design. Besides, the effects of straw length(1-10mm), straw content(1%-10%), water-metakaolin ratio (0.09、0.10、0.11、0.12) and relative humidity(50%-90%)were studied under the best values of A and B. Process of straw-reinforced geopolymer was optimized based on compressive strength. The thermal conductivity was also measured to ensure the thermal insulation property. The eutectic phase, chemical bonding and microstructure were analyzed by XRD, SEM and FT-IR. Results showed that the better formula of straw-geopolymer was that molar ratios of SiO2 and Al2O3, and of Na2O and SiO2 were 3.9 and 0.30, respectively. The mixed straw accounted for 4% with the length of 1 mm, and the ratio of water and metakaolin was 0.10. The samples should be cured at temperate of (20±2) ℃ and relative humidity of 70%±2% for 7 d. The compressive strength of straw-geopolymer can be up to 54.58 MPa, and the thermal conductivities of straw-geopolymer which were synthesised under the optimal formula with different straw content (0-6%) were also been studied. Thermal conductivity measurement was based on steady-state method. The results showed that the thermal conductivity of straw-geopolymer decreased with the increase of straw content, and the thermal conductivity of straw-geopolymer at 4% straw content was 0.0681 W/(m·K). Comparing the thermal conductivity of straw-geopolymer material with other thermal insulation materials, it can be seen that the thermal conductivity of this material was close to that of cotton stalk insulation board and even smaller than foam concrete. Meanwhile, considering its high temperature resistance，cost saving and environmental contribution, the material synthesized in present work showed potential for building thermal insulation application. As a typical composite material, the interface between geopolymer and reinforcing straw had a significant correlation with the macroscopic properties of straw-reinforced geopolymer. The interface between straw and geopolymer was also studied through XRD, SEM, FT-IR analyses. XRD, SEM and FT-IR analyses results showed that the phases of straw-geopolymer were mainly composed of amorphous phase and crystalline phases of Quartz. The straw-geopolymer was semi-crystal, amorphous phase with Si, O and Al elements. Gaps and cracks were appeared near the straw/geopolymer interface. The cracks were diverged when there were straw, and secondary cracks appeared. The combination interface was week in the geopolymer matrix propagate and grow along the interface between straw and geopolymer. Moreover, new cracks generated approximately perpendicular to the strike direction of the original crack which could absorb plenty of fracture energy during crack propagation process. There was a clear imprint of the surface texture of the straw remaining on the geopolymer after straw pulled out. FT-IR analyses showed that the infrared peaks of straw-geopolymer are consistent with geopolymer. Overall，the bonding between straw and geopolymer mainly included mechanical locking, infiltration and vander waals forces. There was no chemical bonding between straw and geopolymer matrix. The study provided a new way to industrial utilization of straw, and provided guidance for research on green and insulation building material.