Study On The Effect Of Admixture On The Properties Of Alkali-Activated Slag Based Foamed Concrete For Backfilling

Foamed concrete with controllable quality,high fluidity and construction convenience has a broad application prospect in the backfill field.However,due to the low strength and heavy pollution of traditional foamed concrete,its further development and application are limited,but foamed concrete prepared with alkali slag cement is not only of high strength,but also energy-saving and environment-friendly,which meets the requirements of sustainable development.In addition,in order to adapt to the backfill engineering in rainy areas and cold areas,it is important for backfill materials to reduce drying shrinkage,increase water resistance and improve frost resistance.Therefore,in this paper,two mineral admixtures,glass powder(GP)and fly ash(FA),were used to modify the alkali slag-based foamed concrete,and the effects of mineral admixtures on compressive strength,drying shrinkage,water resistance and frost resistance were studied.The reasons for the change of macroscopic properties were discussed by combining ultrasonic nondestructive testing,image analysis,SEM test of pore structure and XRD analysis of hydration products.The main results are as follows:(1)The compressive strength of the alkali activated slag-based foamed concrete mixed with GP and FA at different ages(7-day and 28-day)was investigated.With the increasing replacement of GP,the 7-day and 28-day compressive strength began to increase at first,however the amount of replacement exceed 20%,the 7-day and28-day compressive strength decreased.When the replacement of FA was 10%,the7-day and 28-day compressive strength was maximum,and then the amount of FA continued to increase,the 7-day and 28-day compressive strength began to decrease rapidly and was lower than the control group.(2)The drying shrinkage property of the alkali activated slag-based foamed concrete mixed with GP and FA at different ages was studied.The drying shrinkage and water loss at 56 days decreased at first and then increased as both GP and FA replacement increased.However,the shrinkage reduction effect of FA was more significant than that of GP,and the optimum content of GP and FA was 20% and 10%,respectively.(3)The water resistance of the alkali activated slag-based foamed concrete mixed with GP and FA was investigated.The water absorption was decreased firstly and then increased,but the softening coefficient increased at first and then decreased with increasing replacement of GP and FA.The water resistance of the foamed concrete can be improved by adding GP and FA,and GP was more effective.However,there was an optimal replacement,which was 20% GP and 10% FA,respectively.(4)The frost resistance after different freeze-thaw cycles of the alkali activated slag-based foamed concrete mixed with GP and FA was explored.The content of GP should be controlled within 20%,because under this condition the improvement effect of frost resistance was better,the change of mass loss was smaller,and the strength loss rate was lower than that of the control group.The optimum content of FA was10%,and in this content,the loss rate of mass and strength decreased the most.(5)The pore structure of the alkali activated slag-based foamed concrete mixed with GP and FA was evaluated by ultrasonic non-destructive test,image analysis and SEM,and the relationship between GP and FA on the microstructure and macro performance of alkali slag-based foamed concrete was discussed.When the replacement of GP and FA was respectively 20% and 10%,the ultrasonic velocity was the highest,the porosity was the lowest,the pore distribution was the most evenly distributed,and the degree of pore deformation was reduced,and the micron-sized pores were smaller and more homogeneous.Among them,the optimization effect of FA on pore structure is better than that of GP.(6)The variation of hydration products of alkali activated slag-based foamed concrete with different contents of GP and FA were investigated by using XRD.The results showed that the appropriate amount of GP can promote the hydration reaction,while the addition of FA reduced the degree of the hydration reaction.Adding 20% GP,the peak height of C-S-H gel increased,indicating that GP can promote the hydration reaction of the matrix.With the increasing replacement of FA content,the height of C-S-H peak decreased continuously and the hydration degree of matrix decreased.