Study On Properties Of Foam Concrete Modified By Nano SiO₂

Foamed concrete is a lightweight porous material made of precast foam into cement paste or mortar.Foam concrete has been widely used in buildings due to its low density,good fire resistance,high fluidity and low thermal conductivity.However,foam concrete also has disadvantages such as poor mechanical properties,high shrinkage and easy cracking,which limits its application and development.Therefore,how to improve and optimize the properties of foamed concrete is of great significance for promoting the application of foam concrete in the industry.Since the 1990s,some scholars began to use nano materials in cement-based products in order to improve the properties such as strength and durability.With the development of nanotechnology,nanoparticle technology has attracted extensive attention in recent years.Nanoparticle foam stabilization technology has been gradually applied in many fields,such as oil exploitation,food processing and so on.At present,there are few studies on the application of nano particles to foam concrete.Therefore,this paper studies the effect of nano fumed silica（NS）as foam stabilizer and additives on foamed concrete system,and studies its influence on the strength,fluidity,pore structure,water absorption,drying shrinkage,hydration product and microstructure of foamed concrete.Before adding foam concrete to NS as stabilizer,surface wettability of NS was modified by silane coupling agent.The effects of different wettability NS on foaming properties and foam stability of foaming agent were studied.The effect of modified NS content on foaming ability and foam stability was further studied.Finally,the influence of NS as foam stabilizer and additives on the properties of foam concrete is studied.The main results of this paper are as follows:（1）Under certain conditions,an appropriate amount of silane coupling agent can successfully modify the surface wettability of NS from hydrophilic particles to hydrophobic particles.Suitable wettability and proper dosage can make NS stable foam performance.（2）Modified NS as foam stabilizer has little effect on the fluidity of foam concrete when adding foam concrete.However,the foam with superior stability can optimize the pore structure not only reduces the production of macropores,reduces the average pore size,but also effectively reduces the generation of connected pores,which plays a positive role in restraining drying shrinkage.In addition,NS has high pozzolanic activity and can form Ca（OH）₂hydrated calcium silicate（C-S-H）gel,which is beneficial to enhancing the strength of the hole wall,thereby inhibiting the development of dry shrinkage cracks and improving the overall strength.（3）The addition of NS as admixture will seriously affect the fluidity of the new foamed concrete slurry and will lead to an increase in the number of macropores and connected pores,which will adversely affect the pore structure,and cause the dry shrinkage value to increase.However,because NS has high pozzolanic activity,it can react with Ca（OH）₂ rapidly in early curing,effectively consume Ca（OH）₂,and produce C-S-H gel with strong cementation strength and large surface area,which improves the structure of cement paste and significantly improves the compressive and flexural strength.（4）When the two methods are added together,although they will produce a superposition effect on the decline of fluidity,they can complement each other in improving pore structure and mechanical properties,especially the improvement of compressive strength.