Process optimization of green roof substrate reinforcement layer based on biomass ash

Abstract: A substrate of a green roof is the most important element in the roof landscaping system. In addition to providing basic functions, such as hydration, nutrition, and physical support for plants, the substrate also requires an excellent water retention capability to enhance rainwater interception, while reducing water evaporation. A fully functional system of a green roof generally includes a planting layer, filtering layer, water storage and drainage layer, moisture-retaining layer, root insulation layer, and an impermeable layer. Complex multi-layer structures are expensive and difficult to construct, while significantly increase the thickness and weight of the tectonic layer, resulting in a substantial building load. This design aims to use biomass ash as the main raw material supplemented by wheat straw and sludge, thereby compressing them into the bottom reinforcement layer to reduce the complexity and cost of the substrate in a green roof. Systematic optimization of binder, formulation ratio, and compression process parameters was utilized to improve the strength of matrix reinforcement layer. Attempts to optimize the matrix strength revealed that the Xanthan gum added at a proportion of 4% improved the structural integrity of biomass ash blocks, where the flexural and shear strength increased by 360.9 N (27.98 times) and 355.8 N (32.94 times), respectively, compared with the control group. The lignin addition group showed only minor improvements in the structural integrity of the reinforced layer, where the addition of 6% lignin only increased the maximum shear force by 29.5 N and the bending force by 41.6 N. The maximum flexural strength of the matrix reinforcement layer was 93.1 N, and the shear strength was 38.4 N when the ratio of biomass ash, wheat straw, and sludge was 4:1:1 by volume (the ratio of dry matter mass was 12.42:1:2.14). In the Scheffe ternary quadratic polynomial simulation, the main component influencing the relaxation density was the straw (coefficient 4.2). Specifically, the flexural strength was mainly affected by the biomass ash and straw (coefficient 451), and the shear strength was mainly affected by the straw and sludge (coefficient 197.2). The optimized ratio of biomass ash, wheat straw, and sludge resulting from this simulation was 3:1:1 by volume (the ratio of dry matter mass was 9.31:1:2.14). This ratio was used to optimize process parameters in the subsequent compression test. The orthogonal experiment showed that the optimal parameters for the compression process of the matrix reinforcement layer were a moisture content of 25% in the mixed raw materials, a molding temperature of 80℃, a molding pressure of 120 kN, holding the pressure for 5 min, when heating and drying under 105℃. In this case, an optimal performance was achieved, indicating a flexural strength of 184.6 N and a shear strength of 162.7 N. A field test was conducted to verify the mix ratio of raw materials and compression process parameters of the matrix reinforcement layer. A comprehensive investigation was made to evaluate the influence of binder type, mix ratio, and compression process parameters on the strength of the matrix reinforcement layer. The findings can provide new insightful ideas and an experimental basis to solve the transportation problem of roof-greening substrate, further to promote the resource utilization of agricultural waste, such as biomass ash, and thereby the optimize the compression process parameters of the substrate reinforcement layer. Subsequent research can be conducted on the combination process of the reinforcement layer and the planting layer in roof landscaping engineering.