Shrinkage Control And Mechanism Of Cement-based Self-leveling Mortar

The shrinkage of cementitious self-leveling mortar(CSLM)caused by high water-cement ratio,high cement-sand ratio and large-scale construction has a negative influence on its application.Existing studies have proved that super absorbent polymers(SAP)and MgO expansive agent(MEA)can alleviate the shrinkage of CSLM effectively.But there is hardly research on the effect on shrinkage of CSLM with SAP and MEA.In this paper,SAP and MEA are used to compensate the shrinkage of CSLM.The effect on fluidity,strength,shrinkage and pore structure of CSLM are studied and analyzed.The main conclusions as follow:The early strength of CSLM is lower than that or ordinary mortar.After 3 days,the strength of CSLM increases rapidly and catches up with that of ordinary mortar gradually.Hydration heat test results that in 72 hours,the hydration reaction of ordinary mortar is faster than CSLM,the second hydration heat peak appears earlier,and the heat release is larger than CSLM,which may be one of the reasons for the difference of mechanical properties between ordinary mortar and CSLM.Meanwhile,at 28 days,the porosity of CSLM is relatively small,and the proportion of pore number below 50 nm pore diameter increases.The strength of CSLM is enhanced in the macroscopic performance.The drying shrinkage of CSLM is larger than that of ordinary cement mortar,and the development trend of autogenous shrinkage is just the opposite.There are some differences in hydration reaction process,pore structure and early mechanical properties between ordinary mortar and CSLM,which are closely related to the large number additives in CSLM.SAP has less influence on the initial fluidity and 20 min fluidity loss of CSLM with the water absorption ratio 20.The compressive strength are decreased significantly caused by SAP,content 0~0.6%,on mass of cement.At 28 days,the compressive strength of CSLM with SAP,content is 0.6%,is 75.9% of the reference.When the dosage of SAP is 0.4%,the shrinkage compensation effect is the best.The proportion of diameter in the range under 50 nm increases and the number of diameter in the range of 100~1000nm decreases.When MEA is added to CSLM,the fluidity and early strength are reduced.At 28 days,the strength is increased.The effect of shrinkage compensation increases with the rise of the dosages.The drying shrinkage and autogenous shrinkage,content is 6%,are 79.5% and 81.1% of the reference respectively.At 28 days,the shrinkage of CSLM with 120s-MEA is-0.099%,which is better than-0.101% of CSLM with 170s-MEA.when temperature rise from 20。C to 40。C,the shrinkage of CSLM with 120s-MEA is-0.092% and the shrinkage of CSLM with 170s-MEA is-0.068%,at 28 days.It can be obtained that low-chemical activity MEA is more sensitive to change of temperature and has a large expansion value than high-chemical activity MEA.