Preparation And Properties Of Alkali-activated Fly Ash/Slag Foam Concrete

Foam concrete has some good engineering perpoties such as high flow,light weight,heat preservation,which make it widely used in the field of wall insulation.The dry bulk density of foam concrete is generally between 300-1200kg/m³,only 1/10-1/3 of ordinary cement concrete.The application of foam concrete as wall materials can reduce the weight of building by 25-40%.Foam concrete has good thermal insulation,fire resistance,high temperature resistance,sound absorption and other properties.Therefore,the use of foam concrete as wall insulation materials can not only achieve the purpose of thermal insulation,but also reduce noise pollution and avoid fire.Traditional foam concrete using cement as a cementitious material has the problems of low early strength,easy to collapse and cracking,etc.Alkali-activated material is prepared under alkaline conditions by using industrial product（fly ash,slag,etc）as raw material.Compared with ordinary cement,the advantages of alkali-activated material are as follows:（1）the preparation process only use industrial product and no cement cliker,which is more environemental friendly.（2）Alkali-activated material has some good properties such as high early strength,rapid setting,quick hardening,and ect.（3）The fresh pastes can better bind the bubbles inside the slurry and form a large number of closed uniform pores.Then,alkali-activated foam concrete has better thermal insulation properties.In this study,foam concrete was prepared by using alkali-activated material.The flowability,compressive strength,flexural strength,thermal conductivity and water absorption of alkali-activated fly ash/slag foam concrete with different fly ash content,water-cement ratio,the modulus of activator,Na₂O content and density grade were studied.Then,a mix proportion with comprehensive performance was obtained.Finally,the relationship between microstructure and macro-performance was discussed.The main results are as follows:The compressive strength of alkali-activated foam concrete decreases with the increase of fly ash content from 30%to 70%.The thermal conductivity gradually increases with the increase of fly ash content,and water-absorption rate decreases first and then increases.The fluidity of alkali-activated foam concrete increases gradually with the increase of water-cement ratio from 0.40 to 0.50,and the strength increases first and then decreases.The compressive strength of alkali-activated foam concrete increases increasingly with the increase of the modulus of activator from 1.0 to 1.5,and the thermal conductivity and water-absorption rate increase first and then decrease.The compressive strength and water-absorption rate of alkali-activated foam concrete decrease first and then increase with the increase of Na₂O content from4%to 6%,and the thermal conductivity gradually increases.The Na₂O content has little effect on the fluidity of fresh pastes.With the increase of density grade from 600kg/m³ to 1200kg/m³,the fluidity and compressive strength of alkali-activated foam concrete are better,the water-absorption rate is smaller,and the thermal insulation properties are worse.It was found that when the fly ash content is 30%,water-cement ratio is 0.45,activator modulus is 1.5,and the Na₂O content is 4%,the alkali-activated foam concrete has the best comprehensive performance.The 7 days compressive strength of the alkali-activated foam concrete is 3.96MPa,the water-absorption rate is 39.3%,the thermal conductivity is0.1685W/（m·K）;the 28 days compressive strength is 4.14MPa,the water-absorption rate is38.8%,and the thermal conductivity is 0.1356W/（m·K）.The compressive strength of alkali-activated foam concrete is related to the thickness of foam pore walls.The pore wall of the foam pores become thinner with the increase of fly ash content,which lead to a decrease of compressive strength.The alkali-activated foam concrete with the water-cement ratio of 0.45 has the thickest pore walls.Therefore,the highest compressive strength is obtained.The greater modulus of activator of the alkali-activated foam concrete,the compressive strength is higher with thicker pore walls.When Na₂O content is 5%,alkali-activated foam concrete with the thinnest pore walls is the lowest compressive strength.When the number of large pores（>1000μm）of alkali-activated foam concrete is more,the compressive strength is lower.When the number of large pores is similar,the compressive strength of alkali-activated foam concrete with greater the porosity is lower.The alkali-activated foam concrete thermal conductivity is not only related to the porosity,but also may be related to the roundness value and uniform distribution of pores.The more pores（0-500μm）of alkali-activated foam concrete,the higher water-absorption rate is.When the number of small pores is similar,the more mesopores（500-1000μm）content and porosity of alkali-activated foam concrete is the higher water-absorption rate.