Preparation And Properties Of Advanced Porous Cement-based Materials

Since the concept of ‘sustainable development’ has been widely accepted in various fields,reducing energy consumption and greenhouse gas emission during the whole service life of buildings has gained more and more attention.Therefore,thermal insulation materials are widely utilized in various construction facilities.Compared with widely used organic heat insulating polymer materials,such as polystyrene board,polyurethane(PU)board and expanded polystyrene(EPS),foamed cement possesses advantages on superior fire-resistance,relatively low cost and less environmental impact.Foamed cement is applied to permeate every aspect of major engineering and national life,such as the roadbed fillers of high-speed railway,the protective and thermal insulation composite walls of island reefs,sound insulation and thermal insulation wall panels of buildings,etc.However,the thermodynamically unstable nature of aqueous foam is a challenging problem for fabrication of ultra-lightweight foamed cement-based materials which will cause the collapse phenomenon.Such materials are also facing many durability problems due to their hydrophilic property and porous microstructure.Therefore,the study of stability and hydrophobicity can make up for the existing deficiencies.It is of great significance for promoting the application of foam concrete and the development of foam concrete industry,as well as guiding practical engineering.In this dissertation,CS,NS and IBTES were used to prepare a foam concrete with good stability,high compressive strength and low water absorption.Firstly,the effects of different components on the properties of the slurry stage and hardening stage were studied.Secondly,the mechanism of bubble stabilization and enhancement in this system was analyzed.Finally,through the study of contact angle and water absorption,the hydrophobic properties of the material were explored.Based on the above experimental results,the following conclusions can be drawn:(1)Preparation of superhydrophobic foam concreteBased on the three-phase foam stability mechanism and the bionic design of superhydrophobic structure,through the roughness modification and chemical modification of the interface in porous cement-based materials,the interface functional modification is used to realize the matrix functionalization,and the matrix superhydrophobic foam concrete with good performance is finally prepared.At the same time,according to the experimental results,the adjustable range of the content of each component is obtained: calcium stearate,0.9% to 1.5%;nano-silica,1% to 3%;isobutyl triethoxysilane,3% to 4%.(2)Effects of nano-silica on pore size regulationNano silica has effects on the regulation of pore size.Through the incorporation of nano-silica,the pore size can be refined and homogenized to a certain extent.However,when the content of nano-silica is too high,the pore size refinement and homogenization caused by the agglomeration of nano-silica will decrease.(3)Stability and enhancement mechanismThere are two ways to improve the stability of foam concrete.One is to speed up the setting and hardening of slurry,and the other is to improve the stability of foam.In this study,nanosilica is used to adjust the viscosity of the slurry,so as to change the environmental limiting force on the bubble,further hindering the growth and movement of the bubble.At the same time,nano-silica adsorbed on the gas-liquid interface can reduce the surface tension,so the pressure in the bubble is reduced,thus preventing the Ostwald ripening.Moreover,nano-silica has the pozzolanic activity thus can react with cement to form C-S-H which can fill the gap and compact the pore wall structure.Calcium stearate,as a surfactant,forms a liquid film on the surface of bubbles after contacting with water,which hinders the fusion of bubbles.(4)The effects of nano silica and isobutyl triethoxysilane on superhydrophobic propertiesAccording to the Wenzel model and Cassie model,the two conditions needed to reach the superhydrophobic state are certain roughness and low surface energy.In this study,isobutyl triethoxysilane is the key component in the transition from hydrophilic to hydrophobic state,while nano silica is the key component in the transition from hydrophobic to superhydrophobic state.(5)Study on robust superhydrophobicity and water absorptionThe matrix superhydrophobic foam concrete prepared in this study has better mechanical stability compared with the coating.The new exposed surface after physical destruction still has robust superhydrophobic properties.At the same time,the matrix super hydrophobic foam concrete has better waterproof property than ordinary foam concrete,and can reduce its immersion water absorption to a minimum of about 6%.Compared with the results of capillary water absorption test and immersion water absorption test,it is found that nano silica can greatly affect the immersion water absorption while it has little effect on the capillary water absorption.