Study Of Preparation And Properties On Alkali-activated Materials For Repair And Protection

The researches were carried out from the aspects of composition design,modification by nano-materials,hydration dynamics,and optimization of rheological properties in order to optimize the properties of alkali-activated materials for repair/protection,reduce shrinkage and inhibit effloresence and other urgent problems.The purpose is to prepare alkali-activated materials with high mechanical properties and good working performance for repair/protection,so as to provide theoretical basis for engineering application and extend the service life of structural engineering.The main research contents are as follows:（1）Study on composition design of alkali-activated materials for repair/protectiononThe feasibility of using a variety of silicon aluminum raw materials and activators as alkali-activated materials for repair/protection were explored and the composite system composed of steel slag and slag was screened out.The best mix ratio for preparing and designing alkali-activated slag mortar for repair/protection was determined:the steel slag content is no more than 30%,the sodium silicate solution modulus is 1.0-1.5,the Na₂O equivalent is 6.0%,the water-cement ratio is 0.40-0.45,the curing temperature is no higher than 40^oC,and the compressive strength of mortar at 1d and 28d is 54.6 MPa and 75.6MPa respectively,the bonding strength at 1d and 28d are 8.1MPa and 9.4MPa,respectively.Effects of various nanomaterials on the hydration properties,microstructures,and mechanical properties of alkali-activated slag were studied,and the action mechanism of nanomaterials was analysised.Nano-silica particles are easily consumed in strong alkaline solutions,providing siliceous raw materials without micro-aggregate effects,but providing nucleation sites,which accelerates early hydration asd improves the microstructure and optimizes the pore structure of the hardened paste.Nano-alumina reacts with alkaline solution slowly.Unhydrated nano-alumina particles act as micro-aggregates to fill pores and make harden paste densely.Moreover,nano-alumina accelerates the hydration process and induces to generate more gel-like hydration products,which optimization the pore structure and improving the mechanical properties of the hardened paste.Nano-calcium carbonate and nano-titanium oxide have crystal nucleus effect and micro-aggregate effect,which promoting hydration,improving microstructure and optimizing pore structure.Nano-graphene oxide accelerates early hydration,but introduces more harmful pores.The compressive strength at3d and 28d decreased and the flexural strength at 28d increases by 20.62%compared to the control.（2）Study on early hydration reaction kinetics of alkali-activated slagEffect of hydroxide ion concentration on the early hydration reaction kinetics of slag and steel slag was studied,the hydration reaction kinetic parameters were analyzed,the slag and steel slag hydration mechanism was analyzed,and the hydration process was simulated based on the Krstulovic-Dabic model.The results show that the slag is hardly hydrated when activated by alkaline solution with a pH value lower than 12,while the steel slag is partially hydrated.When 2mol/L sodium hydroxide solution was used,the hydration rate of slag and steel slag is fast and the total amount of hydration heat is the highest.The slag and steel slag hydration mechanism is NG-I-D and NG dominates hydration process,while the hydration of steel slag is affected by the hydroxide ion concentration.The high concentration of hydroxide ions is not conducive to NG the nucleation crystals,the crystal growth process and I boundary reaction of slag and steel slag.Effects of nano-silica and nano-alumina on the early hydration reaction kinetics of alkali-activated slag were studied.The results show that with increasing of nano-silica amount,the alkali-activated slag hydration mechanism converts NG-I-D to NG-D,while the nano-alumina modified alkali-activated slag hydration mechanism is NG-I-D,and the phase I boundary reaction is suppressed.As the amount of nanomaterials increased,the nucleation and growth process of NG crystals was prolonged,confirming that nanomaterials have obvious nucleation effect.（3）Study on rheological properties of alkali-activated materialsEffects of water-cement ratio,sodium hydroxide content,sodium silicate modulus,steel slag content and nano-materials on the rheological properties of alkali-activated slag were explored.The Binghanm model and Herschel-Bulkley model were used to fit and resolve the the yield stress of paste.The results show that,with increasing of water-cement ratio,the apparent viscosity,yield stress,plastic viscosity and thixotropy of the alkali-activated paste decreased.The sodium hydroxide content affects the hydration process of the alkali-activated slag,thus,with increasing the sodium hydroxide content,the apparent viscosity,yield stress and plastic viscosity of the alkali-activated slag paste increased,and the thixotropic properties of the paste was optimized.As the modulus of the sodium silicate solution increases,the apparent viscosity and the thixotropy of the paste increase.The alkali-activated slag paste mixed with steel slag or prepared by sodium silicate solution belongs to swelling plastic fluid,which has the characteristics of shear thickening.The Herschel-Bulkley model is more suitable than the Binghanm model for the alkali-activated paste activated by the sodium silicate solution.With increasing of the steel slag amount,the apparent viscosity,yield stress and thixotropy of the alkali-activated slag paste increased,and the fluidity of the paste decreased.The addition of nanomaterials to the alkali-activated slag paste increases the apparent viscosity and yield stress and reduces the fluidity.The pastes are all non-Newtonian fluids,where the alkali-activated slag paste with nano-silica,nano-alumina,nano-calcium carbonate,and nano-titanium oxide is a pseudoplastic fluid,while the paste with nano-graphene oxide is swell plastic fluid.Nano-alumina reduces the slurry’s thixotropy,while nano-silica,nano-alumina,nano-calcium carbonate,and nano-titanium oxide improves the thixotropy of alkali-activated paste.（4）Research on shrinkage and effloresence of alkali-activated slagEffects of steel slag,nano-calcium carbonate and nano-titanium oxide on the shrinkage of alkali-activated slag paste were studied.The results show that,the incorporation of steel slag,nano-calcium carbonate and nano-titanium oxide can effectively reduce the chemical shrinkage of the paste and the drying shrinkage of the mortar.The improvement effect depends on the amount and fineness of steel slag.Nano-calcium carbonate and nano-titanium oxide have obvious micro-aggregate effect and skeleton support effect,which is the main reason for the reduction of chemical shrinkage and drying shrinkage.Effects of 5A zeolite and nano-silica on efflorescence of alkali-activated slag were studied,and the inhibition mechanism of effloresence was discussed.The results show that 5A zeolite has micro-aggregate filling effect and ion exchange properties,which can optimize the pore structure of hardened paste and effectively inhibit efflorescence of alkali-activated material.The concentration of bicarbonate ions of the mortar mixed with 15%zeolitewas reduced by44.18%.The inhibition effect for efflorescence for nano-silica is affected by the particle size and content of nano-silica.Nano-dioxide induces to generate more hydration products,which improves the pore size distribution of the hardened paste,neutralizes excess hydroxide ions,and effectively suppresses effloresence.