Experimental study on thermal insulation mortar with normal bearing capacity and its mesoscopic characteristics analysis

Low strength limits the application potential of thermal insulation concrete in high-temperature underground engineering. However, current research on enhancement methods for thermal insulation concrete is relatively limited, and there is a lack of appropriate evaluation indicators, which results in significant room for improvement in its overall performance. To address this issue, this paper develops a load-bearing and thermal insulation mortar (LBTIM) with both thermal insulation and load-bearing capabilities: based on M20 mortar, low-thermal-conductivity materials (aerogel, coal gangue, fly ash) and reinforcing materials (silica fume, basalt fiber, polypropylene fiber) were added. Orthogonal experiments with 6 factors at 5 levels and 6 factors at 3 levels were designed to investigate the thermal conductivity and uniaxial compressive strength of different material ratios. Subsequently, the "strength-to-thermal conductivity ratio" (RST) was defined, and an intensity-thermal conductivity scatter plot was drawn as an evaluation indicator. Based on the experimental results, the ratio of LBTIM was successfully developed and optimized, and its stability was verified through experimental replication. The optimal ratio of LBTIM was determined as follows: aerogel 0.26 %, silica fume 3.35 %, fly ash 7.06 %, coal gangue 30 %, basalt fiber 0.05 %, and polypropylene fiber 0.45 %. Experimental results showed that its thermal conductivity was approximately 0.8 W/m·K, and its uniaxial compressive strength reached 17 MPa. The strength-to-thermal conductivity ratio was improved by 60.7 % compared to M20 mortar. Furthermore, near-distance imaging and SEM tests revealed the mesoscopic mechanism of LBTIM: the high porosity and the formation of an effective porous shear-resistant structure in LBTIM significantly reduced internal air convection. Combined with the externally added porous materials, this effectively blocked the large heat flow channels within the mortar, thereby significantly reducing the thermal conductivity. Meanwhile, the internal porous shear-resistant structure and the reinforcing fiber materials jointly ensured the good load-bearing capacity and stability of LBTIM, providing an important reference solution for addressing the load-bearing and thermal insulation issues in high-temperature tunnels.