Preparation And Heat Transfer Mechanism Of Porous Perlite Geopolymer Thermal Insulation Materials

Building industry is one of the most important area in social energy consumption.It is vital to develop wall insulation material and technology for building energy conservation.Porous geopolymer is a kind of green,environment-friendly and energy-saving building thermal insulation material with three-dimensional cross-linked skeleton and highly porous structure.In this paper,perlite tailings were chosen to prepare porous geopolymer thermal insulation material.The influence of additives and heat treatment on the main physical properties,structure and composition of materials were investigated.The heat transfer model for porous geopolymer thermal insulation material was selected.The main factors affecting thermal conductivity of porous geopolymer and their regulation method were investigated.The variation of the composition,structure and main physical properties of porous perlite geopolymers was studied by aging under the coupling condition of high temperature and moisture.The factors affecting the durability of porous perlite geopolymer and their affecting mechanism were studied,and the improvement methods for its durability were proposed.The main achievements are as follows:1.A porous perlite geopolymer with high porosity,low density and low thermal conductivity was prepared by room temperature foaming method with water glass as the activator,H₂O₂ as the foaming agent and CTAB as the foaming stablizer.It was found that the foaming agent is the main factor to affect the porosity,thermal conductivity,compressive strength and density of porous perlite geopolymer.CTAB is significant to improve the uniformity of pores and porosity.Rock wool can’t improve the strength of porous perlite geopolymer owing to the corrosion in alkali solution.Through the optimization of foaming process and raw material ratio,the porous perlite geopolymer with thermal conductivity of 0.040 W·m^-1·K^-1 and density of 0.2 g·cm^-3 was obtained.2.A method for producing hierarchical porous structure in geopolymer insulation materials was proposed—room temperature foaming and heat treatment.The primary porous structure（several hundred microns）was produced in the foaming process at room temperature.The secondary porous structure（about tens of microns in diameter）and the tertiary porous structure（micron and submicron in diameter）were formed by heat treatment.Owning to the hierarchical porous structure,the thermal conductivity and density of porous perlite geopolymer were significantly reduced by 10%and 15%,and the softening coefficient was improved by 80%.3.Several two-phase medium heat transfer models were compared.It was found that the novel effective medium theory（NEMT）model can describe the heat transfer mechanism of porous geopolymer thermal insulation materials better.Under the guidance of NEMT model,the main factors affecting the thermal conductivity of porous geopolymers were discussed,and the probable method to decrease the thermal conductivity of porous geopolymers were proposed:increasing the porosity and reducing the thermal conductivity of solid skeleton and uniform medium.4.The durability of porous perlite geopolymer insulation material was studied under the coupling conditions of high temperature and humidity.It was found that Na⁺has important effects on the stability of porous geopolymer.Under high temperature and high relative humidity,hydrated Na⁺in the pore reacted with CO₂ and H₂O in the air to form Na₂CO₃·H₂O,at the same time,some Na⁺entered and broke the gel framework,resulting in the deterioration of physical properties of porous perlite geopolymer.HCl or CaCl₂ solution soaking was adopted to improve the long-term stability of porous perlite geopolymer.When soaked in HCl or CaCl₂ solution,Na⁺in the pore of porous perlite geopolymer reacted with Cl^–to form Na Cl,which prevented Na⁺from entering and breaking gel framework and resulted in remarkable improvement of the long-term stability of porous perlite geopolymer.Aging process and HCl or CaCl₂ solution soaking promoted further polymerization in porous perlite geopolymer,which may contribute to some extent to the physical properties variation of porous perlite geopolymer.