Study On Modification Of Rheological Properties And Shrinkage Properties Of Alkali-activated Self-leveling Mortar

The CO₂ emissions caused by cement production and use in China in 2020 reached 1.48billion tons,accounting for 14.3%of the total CO₂emissions.With the continuous development of civil engineering,the demand for cement will increase,and the CO₂emissions will continue to increase.Therefore,the development and use of new low-carbon cementitious materials is an important way to promote the low-carbon and sustainable development of the construction industry.Alkali-activated material is a new green cementitious material with hydraulic properties,which is produced by using silicon aluminate as the precursor under the action of alkaline activator.It has the characteristics of low CO₂emission,low energy consumption,fast hardening and early strength,high temperature corrosion resistance,etc.However,if the ratio is not appropriate,alkali-activated material will appear flash coagulation,poor fluidity,shrinkage cracking and other problems.In order to improve the early performance of alkali-activated materials,the setting time,rheological properties and early shrinkage properties of alkali-activated materials were studied.Using slag and fly ash microspheres as precursor materials,the effects of the amount of fly ash microspheres,the modulus of activator,the type and content of retarder on the setting time,rheological properties and mechanical properties of alkali-activated material were studied.Calcium oxide was used as expansion agent to reduce the early shrinkage of alkali-activated materials.In addition,based on the research theory of two-part alkali-activated self-leveling mortar,the preparation of one-part alkali-activated self-leveling mortar provided a theoretical basis for its application in practical engineering.The results show that:（1）When the content of fly ash microspheres is within the range of 0-20%,the"ball-bearing"effect of fly ash microspheres can significantly improve the early fluidity of alkali-activated mortar,reduce the yield stress and plastic viscosity of the slurry.Meanwhile,due to the small particle size of its microspheres,it can further fill the pores and improve its mechanical properties.When the dosage exceeds 20%,the water demand of fly ash microspheres increases,resulting in an increase in the yield stress and plastic viscosity of the slurry,a decrease in flowability,and a decrease in strength.（2）With the increase of the modulus of activator,the yield stress and plastic viscosity of slurry gradually decrease,and the fluidity increases.The mechanical properties of mortar increase with the increase of the modulus of activator in the range of 0-0.9.When the modulus of activator is greater than 0.9,the mechanical properties of mortar will be decreased by increasing the modulus of activator.Therefore,it is necessary to select the appropriate modulus of activator to control the fluidity and mechanical properties of mortar.（3）The three types of retarders all delayed the reaction process of alkali-activated materials,with the retarding effects ranging from strong to weak being LB,zinc sulfate,and borax.The early reaction heat results showed that LB type composite inorganic retarders delayed the occurrence of the second exothermic peak,but did not affect its cumulative exothermic heat;Zinc sulfate increases the cumulative heat release;Borax not only delayed the occurrence time of the second exothermic peak,but also significantly reduced the cumulative exothermic energy,delaying the formation of C-A-S-H gel.The Zeta potential results indicate that the effect of retarders on the rheological properties of alkali-activated slurries mainly changes the absolute value of Zeta potential,affects the electrostatic repulsion between particles,and determines the flocculation or dispersion state of particles.Among them,borax has a significant impact on the surface potential of particles and can effectively improve the rheological properties of mortar.（4）Adding calcium oxide expansion agent can effectively reduce the autogenous shrinkage and drying shrinkage of alkali-activated materials.For autogenous shrinkage,the sample doped with calcium oxide experienced expansion in the first 24 hours,generating calcium hydroxide crystals to compensate for early autogenous shrinkage.When the calcium oxide content was 5%,it exhibited volume expansion.The inhibitory effect on drying shrinkage becomes more pronounced with the increase of calcium oxide content.Due to the addition of calcium oxide,the pore size inside the slurry becomes larger,increasing the number of pores and reducing the internal capillary pressure,thereby reducing the drying shrinkage of alkali-activated materials.（5）Based on the above research,two types of geopolymer based self-leveling mortars with two-part（gray body+activator）and one-part（directly mixed with water）were developed.The setting time,flow performance,mechanical properties,and dimensional change rate of the mortar were studied.The prepared self-leveling mortar has good comprehensive performance,with the highest fluidity up to 132 mm,the setting time controllable over 120min,the 28-day compressive strength up to 79.01 MPa,and the 28-day shrinkage rate-0.2945%.Compared to cement self-leveling mortar,it has advantages of longer operation time and higher mechanical properties.