Gradient Structure Design And Co-extrusion Peparation Of Cement-based Insulation Materials With Graded Structure

The reform of wall materials and building energy saving has become the basic national policy of our country. The development of new insulation materials meet the national specification of energy-saving, has become the mission of all researchers in material field. The performance defects in durability and mechanic of traditional thermal insulation materials, has become the bottleneck for its large-scale application. This study applied the functionally gradient materials principle to insulation materials. The gradient structure was composed of core insulation layer, middle transition layer and outer protective layer, designed for gradually dense from the inside out. The composition of paste for each layer was based on rheological properties and performance. Co-extrusion technology and computer simulation was applied on this study. Through the research of co-extrusion rheological behavior and computer simulation, the rheological relation between each gradient layer was revealed, and appropriate process parameters for co-extrusion and rheological model for matching was established. Also, this paper investigated the influence of co-extrusion process on physical, mechanical, thermal and microstructure properties of product. The research work and main achievements are shown as follows:(1) Extrusion property of cement paste has been studied, which was embodied in three aspects: consistency moderate state of fresh paste, stable pressure during extrusion and apparent complete for extrusion product. The water-cement ratio was the main factor infect extrusion property, when water-cement ratio is 0.21, the fresh cement paste is suitable for extrusion. The incorporation of fly ash, methyl cellulose and fiber was also important means to improve extrusion property. Content of stair fly ash reach 40%, can reduce the extrusion pressure significantly, and improve extrusion products. Methyl cellulose as thickener, can increase the viscosity of cement paste, enhance paste’s stability during extrusion, adding amount of MC is 3.3% of mixing water consumption was appropriate. PVA fiber can improve paste with the ability of water retention, more sticky and plastic, suitable content of PVA fiber is 2 over one thousand of the total volume.(2)The rheological properties of cement-based material, and the rheological principle for matching in co-extrusion has been studied. Benbow nonlinear equation under constraint conditions, has been used to analysis extrusion rheological property of paste and quantitative analysis was carried out, the parameters characterization the extrusion rheological property including initial volume yield stress σ0, rate to yield stress factor α, linear index of yield stress m, initial wall shear stress τ0, rate to wall shear stress β, linear index of wall shear stress n. By adjusting the water-cement ratio and content of fly ash, methyl cellulose, hollow glass cenosphere, the mix proportions for each layer with different apparent density and extrusion rheological properties was designed. The rheological principle for matching of multi-layer co-extrusion was founded: in plug flow phase, paste’s volume yield stress shall be greater than the wall shear stress, and for differential flow phase, it is required that each layer’s rheological property must be coordinate with others, the matching of volume yield stress is the crucial factor. In double-layer co-extrusion experiment, the yield stress ratio of outer and core layer should between 1.6 and 2.8; in triple-layers co-extrusion experiment, the outer layer, middle layer and inner yield stress of the optimal matching relation should be 1: a:(0.7 + 1.37), a∈(1.6, 2.3).(3)Physical properties, mechanical properties, thermal properties and microstructure of extrusion products has been studied. For the outer protective layer, the apparent density is between 1.73~1.87g/cm3, 46.8~54.8Mpa in compressive strength, 11.9~13.1 MPa in flexural strength is between, 3.6~4.8Mpa in tensile strength, 0.60~0.67W/Mk in coefficient of thermal conductivity, respectively. The middle transition layer is 1.51~1.57g/cm3 in apparent density, 32.4~38.1Mpa in compressive strength, 6.75~7.98 Mpa in flexural strength, 1.96~2.64 MPa in tensile strength, 0.3722~0.4659W/mk in coefficient of thermal conductivity. For core insulation layer, is 1.03~1.28g/cm3 in apparent density, 9.5~11.9MPa in compressive strength,, 3.01~3.17 MPa, 7.1~9.0MPa in flexural strength, 1.7~2.4MPa in tensile strength, 0.2062~2148W/m K in the coefficient of thermal conductivity. Triple-layer functionally gradient thermal insulation material composed by outer protective layer, middle transition layer and core insulating layer, the apparent density is between 1.42~1.47g/cm3, 31.3~33.7MPa in compressive strength, 7.1~9.0MPa in flexural strength, 1.7~2.4MPa in tensile strength, 0.2062~2148W/m K in coefficient of thermal conductivity, gradient functional material prepared with proper mechanical properties, and thermal insulation performance is significantly enhanced, realized the combination of self-protection and self-thermal. The micro morphology of extrusion product’s interface region has been analysis, uniform transition of component has achieved.(4)Finally, the flow of paste during extrusion process was simulated with finite element method, velocity field, pressure field and the streamline simulation results has been achieved, which provides theoretical support for analysis paste flow and force situation during extrusion.