| A bubble is a vesicle formed by a liquid film wrapped around a gas,and the use of bubbles as a template to create pores is a very widely used method for constructing porous materials.However,the bubble system is a naturally thermodynamically unstable system,which evolves continuously since its creation until it breaks down.Therefore,the optimal regulation and topological design of bubbles become an important breakthrough direction for the high performance of porous materials.Cement-based material is a typical composite porous material.In addition to the intrinsic pores formed by cement hydration,air-void pores can be artificially introduced by means of air-entrainment or foaming to obtain cement-based porous materials with specific functions.The stability of bubbles determines the pore structure of cementitious materials,which in turn affects their various physical and chemical properties.At the same time,bubble stability is a prerequisite for the adjustable and designable pore structure of cement-based porous materials.Clarifying the bubble evolution process,stability mechanism and achieving controllable design of pore structure are the keys to the advanced,green and functionalization of cement-based porous materials.At present,the research to improve the stability of bubbles in cement-based materials mainly starts from optimizing the properties of cement slurry or surfactant properties,such as regulating the rheological properties of slurry,regulating the setting time of slurry,optimizing the properties of raw materials and enhancing the strength of bubble liquid film,etc.,thus enhancing the stability of bubbles.However,these methods cannot completely solve the problem of bubble destabilization in cementitious materials.With the increase of bubble volume or some harsh environments,the deterioration of properties due to bubble destabilization in cementitious materials becomes a difficult problem to avoid.In this thesis,four main aspects of research are carried out around the pore structure regulation,design and utilization of cement-based porous materials:firstly,the effect of extreme environment,i.e.highland low atmosphere pressure environment,on bubble stability and pore structure of air-entraining mortar is studied;then the bubble stability and pore structure optimization effect of modified nanoparticles in low atmosphere pressure environment is investigated;on this basis,amphiphilic nanoparticles are designed and synthesized to further enhance the bubble;finally,a controlled design method for the pore structure of cementitious materials was proposed based on the principles of bubble deformation and self-assembly.The main research results achieved through the above research are summarized as follows:（1）Aiming at the plateau low atmosphere pressure environment,the effect of atmosphere pressure on the pore structure of freshly mixed air-entrained mortar was investigated through a self-built low-pressure mixing device.It was found that the decrease of atmosphere pressure would lead to the decrease of initial air content of air-entrained mortar,the increase of bubble spacing coefficient,the increase of air content loss through time,and the obvious deterioration of pore structure.Increasing the amount of air-entraining agent can enhance the initial air content of mortar under low atmosphere pressure environment,but it cannot guarantee the pore structure of mortar after hardening.The main mechanism of pore structure deterioration of air-entrained mortar under low atmosphere pressure environment is that the low atmosphere pressure accelerates the Ostwald ripening process of the bubble system,which makes the small bubble gradually become smaller until it disappears,and the large bubble gradually increase until it ruptures,and the average pore size of the bubble system increases,which further accelerates the destabilization rate of the bubble system.（2）The method of using modified nanoparticles to improve the stability of bubbles in low-pressure environment was proposed.The use of silane modifiers to adjust the contact angle of nanoparticles at the gas-liquid interface enhances their adsorption properties on the bubble liquid film,which in turn improves the bubble stability performance of nanoparticles.When 3%of modified nano-silica was added,the air void structure of cement mortar did not deteriorate in a low atmosphere pressure environment of 60 k Pa（equivalent to the atmospheric pressure at about 4000 m altitude）.Studies on the bubble liquid film layer in liquid foam,freshly mixed and hardened cement slurries showed that the nanoparticles with proper wettability could adsorb at the air-liquid interface of the bubble film and form an adsorption layer around the bubble,thus reducing the gas permeability of the bubble film and preventing the occurrence of Ostwald ripening of the bubble.（3）To address the problem of difficult adsorption of homogeneous nanoparticles at the interface,amphiphilic（Janus-type）silica nanoparticles with a surfactant-like structure that is hydrophilic at one end and hydrophobic at the other were further synthesized.Ultra-stable foams were prepared by combining amphiphilic nanoparticles with a surface-active polymer.The lifetime of the foam was dramatically increased from 4 h to more than 5 days when the concentration of amphiphilic nanoparticles was 5%.the dense adsorption layer formed by the efficient adsorption of amphiphilic particles on the bubble film was the main factor for the excellent stability of the foam.Using ultra-stable foam,ultra-light cellular cements were successfully prepared with densities as low as low as 80 kg/m3 and thermal conductivity as low as 0.038 W·m-1·K-1.Meanwhile,this ultra-lightweight cellular cement exhibited excellent mechanical strength,fire resistance and thermal insulation capability.（4）Based on the principle of deformation of bubbles in cement paste under low atmosphere pressure environment and self-assembly of bubbles,highly regular honeycomb porous cementitious interfaces with orientation were prepared.Meanwhile,the connectivity and closure characteristics of pores of cement-based porous materials are controlled by controlling the viscosity of cement slurry.Cement-based interfacial solar evaporator for seawater desalination treatment was designed and prepared by making full use of the large specific surface area of the honeycomb porous cement-based material and the porous characteristics of the cement-based material itself.The results show that the honeycomb interfacial solar evaporator has 94.8%solar light absorption rate and the desalination rate reaches 2.63 kg·m-2·h-1,which exceeds many solar evaporators constructed with complex methods and expensive materials.It provides a broad prospect for the functional application of cement-based materials... |
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