Forming mechanism and experiment of greening substrate bricks pressed by using agricultural organic waste

Abstract: This study aims to explore the compression forming of greening substrate bricks, covering the raw material preparation, pressing procedure, bricks characterization, finite element method (FEM) simulation, and the parameter optimization. 1) Raw material preparation: The rice straw was crushed by the grinder to mix with the cow dung. The aerobic fermentation was adopted to be completely decomposed the materials. Among them, the premixed materials was hold by hand and molded for reserve. Polyacrylic acid sodium, insecticide and fungicide were mixed to dissolve as the premixed liquid. The premixed materials and liquid were mixed in the proportion for the raw material of greening substrate bricks. 2) Pressing procedure: A pressing process of three-layer composite was utilized to make the greening substrate bricks. Among them, three layers was in the middle, including the upper and lower matrix layer, as well as the greening substrate layer. The greening substrate brick was made and dried below 60 ℃ for the subsequent transportation and storage. 3) Bricks characterization: The physical properties were characterized for the greening substrate bricks after molding and drying. Specifically, the bulk density, the total porosity, and the aeration porosity were 0.62 g/cm3, 63.09%, and 27.78%, respectively. The material properties of greening substrate brick were fully met the needs of plant growth, such as the bulk density and porosity. The physical and chemical indexes were within a reasonable range (pH=7) to balance the nutrition of greening substrate brick. 4) FEM simulation: A three-dimensional numerical model was established, where the materials to be compressed and naturally stacked in the forming mold were selected as the research objects. A systematic investigation was made to determine the variation in the stress along the height during compression and extrusion at different times. The distribution of compression stress was found: the extrusion pressure and friction force, the inner wall of the forming die produced the radial pressure on the material. The material was first extruded from the upper and outer layers to the central layer, and then flowed to the lower layer. 5) Parameter optimization: A pore channel model was constructed to determine the internal pore connectivity and dynamic water migration during compression. A pore channel and a skeleton were included, where the former was a solid matrix bonded and wound by the rice straw and cow dung, while, the pore channel included the channels and blind channels with the divergence and permeability. The compression molding of greening substrate brick was divided into the loose, compaction, pressure maintaining, combined with the pore channel model. It was verified the relationship between the pore coefficient ratio of the model and the porosity of the greening substrate brick after molding with the different holding time, indicating that the holding time posed an important influence on the compression molding. The longer the holding time was, the better the compression molding and molding stability were, particularly with the reduced total porosity. 6) The optimal molding process parameters of greening substrate brick were achieved using the single factor test method. Specifically, the moisture content of materials, compression time, compression stress, and cow dung mass percentage were taken as the experimental factors, whereas, the drying time, water absorption after drying and destructive strength were taken as experimental indexes. An optimal combination was achieved, where the moisture content was 60.4%-80.8%, the compression stress was 0.17-0.33 MPa, the compression time was 2-4 min, and the mass percentage of cow dung was 20%-40%. A four-element quadratic orthogonal rotation combination test was designed for the variance analysis and response surface analysis using Design-Expert 12.0 and Excel 2016 software. The optimal parameters were as follows: the moisture content of material was 75.7%, the compression stress was 0.29 MPa, the compression time was 2.5min, the mass percentage of cow dung was 30%, the drying time of greening substrate brick was 656.9 min, the water absorption after drying was 74%, and the destructive strength after drying was 11.8 N. The forming and cultivation tests were conducted to verify the optimal parameters. The germination rate of greening substrate brick was maintained at 88%, and the number of repeated water absorption was more than 50 times. The plant grew well and the root system was developed, which met the requirements of normal growth and development of plants. This finding can provide a strong reference for the production of greening substrate bricks, the comprehensive utilization of agricultural organic wastes for the upgrading of urban greening products.