Preparation And High Temperature Resistance Of Alkali-activated Cementitious Materials

In this paper,alkali slag-fly ash mortar was prepared by using industrial solid wastes as the main raw materials.On this basis,the alkali activated ultra-high performance concrete(AUHPC)was prepared by optimizing the mix proportion and adopting the heat curing method.The effects of high temperature on the mechanical properties of alkali slag-fly ash and AUHPC were investigated.The effects of high temperature on the phase composition and pore structure of the two materials were studied by means of XRD,FTIR,MIP and BET,which provided a reference for the application of alkali activated cementitious materials in engineering.The main achievements were as follows:The variation of mechanical properties of the alkali slag-fly ash mortar with different fly ash content at 20℃,200℃,400℃,600℃and 800℃was studied.The results indicated that adding fly ash can optimize the pore structure of the slurry,reduce the internal porosity,and improve the high-temperature resistance performance of the alkali slag mortar,the suitable blending amount is 30%～50%.The highest compressive strength with 30% fly ash mortar at 200℃which is 49.3% higher than 20℃;At 600℃ the mortar maintains the same compressive strength as 20℃,but the solid phase reaction occurs in the slurry,the number of less harmful pores increases and the C-S-H decreases.When the temperature is 800℃,the compressive strength of mortar is 11.5% at 20℃due to a large amount of calc feldspar is produced in the slurry and the workability was basically lost.The results show that the temperature of alkali slag fly ash mortar should not exceed 600℃.The results show that both steam curing and 200℃ high temperature curing can significantly improve the early compressive strength of AUHPC.The compressive strength of steam cured specimens increased little in the later period,while the compressive strength of high temperature cured specimens decreased significantly in 7d and 28 d.Increasing the steam curing temperature can accelerate the hydration rate at the early stage.The aluminum silicate gel produced by steam curing at the temperature of 90℃is the most,so that the pore size is refined and the porosity is reduced.The effect of improving the pore structure is most obvious.High temperature curing at 200℃ will result in pore coarsening and porosity increase,which may be the reason for the compressive strength of auhpc to shrink in the later stage.It was found that the compressive strength and flexural strength of AUHPC increased first and then decreased with the increase of temperature,and promoted the internal reaction of slurry at 200℃,with the compressive strength and flexural strength reaching235.1MPa and 47.8MPa,respectively.The compressive strength of the specimens at 400℃did not change significantly,but the C-S-H gel removed some of the bound water,resulting in a significant reduction in the flexural strength of the specimens.At 600℃,the compressive strength of the specimen is almost the same as that at 20℃,and the flexural strength continues to decrease.At 800℃,the solid phase reaction of the slurry produced a large number of calcium feldspar,and the internal structure of the specimen was destroyed.High temperature also has an obvious effect on the pore structure of AUHPC.After high temperature at 200℃,the pore size of the slurry is refined,and the number of harmless pores increases in the range of 2～10nm,with the most compact structure.With the increase of temperature from 400℃to 600℃,there are less harmful holes,the number of holes increases,the holes begin to move towards the big holes,and the internal structure of the specimen becomes loose.Therefore,AUHPC should not be used in the environment above 600℃.