Effect Of Polyether Shrinkage Reducing Admixture On Properties Of Alkali-activated Cementitious Materials And Its Mechanism

Alkali-activated cementitious materials have the advantages of low hydration heat release,high strength and good durability.In recent years,the preparation technology and hydration reaction of alkali-activated cementitious materials have been widely studied,and positive progress has been made.The most difficult problem hindering the engineering application of alkali-activated cementitious materials is its high shrinkage.If the stress caused by shrinkage exceeds the tensile strength of the material,cracks can initiate and further develop into deeper cracks,resulting in a decrease in the strength and durability of the material.Shrinkage reducing admixture(SRA)is a kind of nonionic surfactant,which can reduce shrinkage and crack resistance by reducing the interfacial tension of liquid phase in pores.The application of shrinkage reducing agent in alkaliactivated cementitious materials can help to promote the application,promote the resource utilization of solid waste,and realize the beautiful vision of energy saving,emission reduction and sustainable development.In this paper,the effect of polyether SRA on the properties of alkali-activated slag(AAS)/alkali-activated coal gangue-slag cementitious material(AACGS)was studied.The shrinkage reduction effect was evaluated by constant humidity and gradient drying tests,and the effect of SRA on the hydration products of alkali-activated cementitious materials was studied by means of XRD,TG-DTG and FT-IR.The chemical composition and concentration of alkali-activated cementitious material pore solution were determined by inductively coupled plasma emission spectrometer(ICP-OES),and its action mechanism was further analyzed by other analysis and testing methods.Finally,the effect of SRA on the durability of alkali-activated cementitious materials is discussed.The main results are as follows:(1)With the increase of SRA content,the setting time of AAS paste increases at first and then decreases,while the setting time of AACGS paste increases with the increase of SRA content,which accords with the influence law of hydration exothermic peak.(2)SRA has an adverse effect on the mechanical properties of alkali-activated cementitious materials,especially on the early mechanical properties.The addition of SRA significantly reduced the drying shrinkage of alkali-activated cementitious materials.The gradient drying results showed that the drying shrinkage of AAS mainly occurred in< 85%RH,and the drying shrinkage increased in proportion with the decrease of humidity,and most of the length changes could not be recovered after a complete dry-wet cycle,indicating that irreversible shrinkage occurred in the drying process of AAS accounted for a large proportion,while the addition of SRA could better inhibit the occurrence of drying shrinkage.The addition of more than 2% SRA can increase the humidity range to85%RH,which can more effectively restrain the occurrence of drying shrinkage.Most of the shrinkage of AACGS occurs above 50% RH,most of which is irreversible.The addition of SRA can inhibit the drying shrinkage of AACGS,especially at high relative humidity.In addition to the decrease of surface tension,the beneficial effect of SRA on irreversible shrinkage may be that it hinders the polymerization of silicate chains of C-AS-H in the drying process and alleviates some capillary tension after binding with the gel.(3)According to the analysis of XRD,FT-IR and TG-DTG,the addition of SRA did not change the type of hydration products,but the amount of hydration products changed,which caused the change of bulk density of hydration products and affected the micromechanical properties.Pore structure analysis shows that the pore size of SRA is thickened due to its hydration delay,which is beneficial to the relief of drying shrinkage,but the existence of too many harmful pores at high content is not conducive to the development of strength.(4)The change of element concentration in pore solution shows that the change of alkali metal ion concentration has no obvious correlation with the decrease of shrinkage,but the change of calcium plasma consumption can reflect the degree of hydration reaction.(5)When the content of SRA is less than 2%,it has little effect on the resistance to chloride ion penetration of alkali-activated cementitious materials,but when the content is high,it will reduce the resistance to chloride ion penetration;the addition of SRA reduces the carbonation depth and decreases obviously with the increase of SRA content;the addition of SRA has almost no effect on the sulfate resistance of alkali-activated cementitious materials.