Experimental Study On Autogenous Shrinkage Characteristics Of MgO Based Alkali-activated Slag Cementitious Material

Alkali-activated cementitious materials have early hardening and high strength,good frost resistance and chemical corrosion resistance,and the main raw materials are industrial wastes.Compared with traditional cement,they have the advantages of low energy consumption and low carbon emission.Therefore,as a new green cementitious material that can replace Portland cement,alkali-activated material has broad application prospects.However,alkali-activated cementitious materials have large shrinkage deformation,and the cracks caused by shrinkage greatly limit its application in engineering practice.Hence,this paper studies the effects of different alkali dosages,activator types and MgO/NaOH mole ratio on the setting time,autogenous shrinkage,compressive strength,physical properties,hydration products and pore structure of alkali-activated slag,so as to provide some theoretical research results for the popularization and application of alkali-activated cementitious materials.The results show that:(1)The setting time of alkali-activated slag decreases with the increase of alkali dosages and the decrease of MgO/NaOH mole ratio.Generally,the promoting effect of compound activator on alkali-activated slag cementitious materials is much greater than that of a single group of activators.The compressive strength of alkali-activated slag prepared by NaOH and water glass/NaOH increases first and then decreases with the increase of alkali dosages,and the maximum strength is obtained at 7%alkali dosage,which are 44.62Mpa and 87.67Mpa,respectively.The compressive strength of alkali-activated slag prepared by MgO/NaOH increases with the increase of alkali dosages and the decrease of MgO/NaOH mole ratio.When the alkali dosage is 9%and MgO/NaOH mole ratio is 6/4,the maximum strength is 27.16Mpa.(2)With the increase of alkali dosages,the autogenous shrinkage of alkali-activated slag excited by NaOH increases first and then decreases,and reaches the maximum at 7%alkali dosage;The autogenous shrinkage of alkali-activated slag prepared by water glass/NaOH increases with the increase of alkali dosages,and reaches the maximum at 10%alkali dasage;The autogenous shrinkage of alkali-activated slag prepared by MgO/NaOH increases first and then decreases with the decrease of MgO/NaOH mole ratio,and reaches the maximum shrinkage value when the MgO/NaOH mole ratio is 8/2.The addition of three alkaline activators makes the mortar sample shrink significantly in the early hydration stage.The maximum group is M9-2 group excited by MgO/NaOH,which is 84.9%of the total shrinkage value in 28 days.When the activator is MgO,the alkali-activated slag sample even has micro expansion in the early-age of hydration reaction,which may be one of the potential effective ways to reduce the shrinkage of alkali-activated cementitious materials.(3)For alkali-activated slag mortar samples prepared by NaOH and water glass/NaOH,with the increase of alkali dosages,the number of mesopores which is less than 10nm gradually increases,and the pore structure of mortar samples becomes more dense.For the mortar samples prepared by MgO/NaOH,with the decrease of MgO/NaOH mole ratio,the pore structure becomes thicker,most of which are middle capillary pores between 10-100nm,so the strength is lower than that of alkali-activated slag mortar excited by the other two groups of activators.Figure 34 Table 11 Reference 74...