Study On The Rheological Mechanism And Regulation Techniques Of Alkali-activated Slag Pastes

Alkali-activated slag（AAS）,with its high mechanical property,high frost resistance and excellent chemical durability,is a kind of eco-friendly cementitious material of wide application prospect.The development and application of it both accord with the trend of sustainable economic and social development.Even though AAS has excellent properties,it is not widely used due to its high viscosity that limit the placing properties.As a result,it is of great significance to investigate the rheological properties of AAS paste to improve its workability and expand its application.For the problem of high viscosity of AAS pastes,the study explores the influence of alkali activators on its rheological properties.The surface charge of GGBS,surface energy and friction coefficient of alkali activator solutions are analyzed by electrophoresis method,thin layer penetration method,contact angle method and rotary friction method,respectively.The effects of electrostatic repulsion force,van der Waals force,Lewis acid-base force and friction on the rheological properties of AAS pastes are studied to understand its rheological mechanism.Based on that,mineral admixtures,Lewis acid and surfactant are incorporated into AAS paste to regulate its rheological properties.The FITR,XRD and TEM are conducted to test the molecular stability and morphology of surfactant in alkali activator solutions.Combined with the diffusion properties of surfactant in alkali activator solutions,the reasons for the decrease of surfactant efficiency were analyzed.Then the measures to improve the efficiency of surfactants by Lewis acid were proposed and the mechanism of the improvment was revealed by studying the effect of Lewis acid on the water structure and silicon structure of alkali activator solutions.The main conclusions are listed as follows:（1）The rheological behavior of AAS paste is significantly affected by the types and content of alkali activator.At the same alkali equivalent,the yield stress of AAS paste activated by Na OH is higher while the plastic viscosity of it is lower than that activated by sodium silicate.With the increase of alkali content,the yield stress of AAS paste decreases and the plastic viscosity increases.Reasons are mainly in two aspects:first,low friction between particles in tangential direction reduces the yield stress of AAS paste.The friction coefficient of sodium silicate solution is lower than that of Na OH solution,and it decreases with the increase of alkali equivalent.The lower friction coefficient increases the lubrication between particles and reduces the yield stress of slurry.Second,the plastic viscosity of slurry is increased by the high cohension between particles.Lewis acid-base force and Van der Waal attraction are the main factors of cohension between particles in AAS paste.The high ionic strength in alkali solution seriously compresses the double electric layers,which leads to the non-overlapping of the double electric layers and the electrostatic repulsion.The polarity of alkali solution is strong and the Lewis base component of surface energy is quite large.The van der Waals attraction and Lewis acid-base force between particles in the AAS paste are large.In addition,the large amount of micelle structure in sodium silicate solution also increases the agglomeration between particles.（2）Reducing the surface energy of binder in AAS paste is beneficial to reducing the force between particles and improving its rheological properties.The van der Waals force between particles in AAS paste can be reduced by reducing the non-polar component of surface energy of binder.And the Lewis acid-base force can be reduced by reducing the Lewis base component of the surface energy of the binder.The fluorite powder and lithium slag powder with a lower total surface energy,non-polar surface energy and Lewis base surface energy are adopted in this paper to reduce the interparticle forces in AAS paste.Reducing the interparticle forces can increase the particle dispersion and reduce the yield stress and plastic viscosity of the AAS paste.However,the addition of fluorite powder and lithium slag powder will reduce the mechanical properties of alkali slag cement mortar.（3）The high Lewis base surface energy of alkali activator solution leads to the high Lewis acid-base force between particles in AAS paste,which is an important reason for its high plastic viscosity.Decreasing the Lewis base component of surface energy of alkali activator solution can effectively reduce the interparticle force in AAS paste and improve its rheological property.The addition of appropriate Lewis acids such as barium nitrate,borax and zinc borate can reduce the Lewis base component of surface energy of alkali activators,and reduce the Lewis acid-base force between particles in the pastes which will decrease the yield stress and plastic viscosity of the pastes and improve its fluidity.Lewis acid has a retarding effect,and can reduce the 3d mechanical properties of AAS mortar,but it has little effect on the 28d mechanical properties.（4）The molecular structure of naphthalene superplasticizer（PNC）remains stable in AAS paste,and its effect on the paste fluidity decreases with the increase of alkali content,and the higher alkaline the alkali activator is,the more the effect of PNC decreases.The reason is that the ions in alkali activator solution can promote the micelle of PNC and reduce its solubility.The solution viscosity of alkali activators is higher than that of water,which reduces the diffusion performance of PNC.After adding barium nitrate solution,the Ba²⁺can react with Si O（OH）₃^-in sodium silicate solution,reducing the degree of hydrogen bonding of water in solution and the content of unsaturated silicate group,which will improve the solubility and diffusivity of PNC in AAS pastes effectively.