Study On Shrinkage Of Alkali Activated Slag-Metakaolin Binder

Alkali-activated slag binder（AAS）and metakaolin-based geopolymer（MKGP）,which are green and low carbon binders,meet the requirement of energy saving and omission reducing strategy.AAS has the characteristics of high early strength,fast hardening and excellent resistance to chemical attack,but the large shrinkage limited its application.Although the MKGP has good volume stability,it has the disadvantage of long setting time and low early strength.The combination of slag and metakaolin is a possible technical way to achieve their performance complementation.In this paper,slag and metakaolin were combined to prepare alkali-activated binder,mechanical properties and shrinkage properties of this binder were studied to prepare a kind of binder with high compressive strength and good volume stability.The effects of alkali concentration,curing temperature and high temperature curing time on drying shrinkage,autogenous shrinkage and compressive strength of alkali-activated slag/metakaolin binder（AASM）were studied.Moisture loss,internal relative humidity,chemical shrinkage,thermogravimetric analysis,back scattering image,pore structure and other tests were used to analyze the shrinkage mechanism.Activating the AASM by a combination of Na₂CO₃/Na₂SiO₃ was a effective way to reduce the chemical shrinkage of AASM.The main results can be summarized as follows:（1）Relacing 50%of slag with metakaolin could reduce the drying shrinkage of AAS mortar.When alkali concentration was 6%and temperature was 20°C,the compressive shrength of AASM mortar was comparable to AAS mortar,but the drying shrinkage of AASM mortar was smaller than AAS mortar by 47.6%.Compared to AAS mortar,AASM mortar had higher free water content,lower mesopore ratio,lower autogenous shrinkage and less sensitive to moisture loss.（2）Alkali concentration had effect on drying shrinkage of AASM mortar.The180d drying shrinkage of the AASM with 6%alkali concentration was less than the AASM with 10%alkali concentration by 17%under 20℃curing.Compared to AASM with 10%alkali concentration,AASM with 6%alkali concentration had higher moisture loss and its drying shrinkage and internal relative humidity changes were less sensitive to moisture loss.（3）Compared with standard curing,drying shrinkage of AASM mortar with 6%alkali concentration reduced by 74%and with 10%alkali concentration reduced by 20%under high temperature curing（60℃,24h）.High temperature curing increased the moisture loss and reduced the sensitivity of drying shrinkage to moisture loss.The microcracks and unreacted slag particles content of AASM cruing at high temperature increased at 28 days.（4）Longer high temperature curing time had harmful influence on drying shrinkage of AASM mortar.When the alkali concentration is 6%,the 180 days drying shrinkage of the AASM mortar under 60°C curing at 72h increased by 243%and the moisture loss reduced.Its sensitivity of drying shrinkage to moisture loss increased.However,Prolonging the high temperature curing time,the compressive strength reduced at 28 days.（5）Alkali concentration and high temperature curing had significant influence of autogenous shrinkage of AASM mortar.Under standard temperature curing,when the alkali concentration of AASM mortar reduced from 10%to 6%,the autogenous shrinkage reduced by 31%and the internal relative humidity increased.Under 60°C curing for 24h,autogenous shrinkage of AASM mortar with 6%alkali concentration reduced by 75%.The internal relative humidity of the AASM mortar changed lower than 3%from 3d to 91d,suggesting that the shrinkage caused by self-desiccation was relatively small.