| Low-carbon alkali-activated cementitious materials with Si-Mn slag as precursor is regarded as an efficient way of resource utilization in large-scale and green mode.In this paper,the reaction kinetics characteristics of alkali-activated Si-Mn slag were systematically studied,the microstructure and phase evolution of alkali-activated Si-Mn slag reaction process were explored,and the process mechanism of alkali-activated reaction is clarified.The macroscopic properties of Si-Mn slag activated by Na OH and water glass systems were investigated respectively,and their properties were explained from the perspective of microstructure and phase evolution.In view of the low activity and poor mechanical properties of alkali-activated Si-Mn slag at early stage,the granulated blast furnace slag mixed with Si-Mn slag was used to improve its early mechanical properties,and the characteristics of the reaction process were analyzed from the microstructure and phase evolution process.The rheological properties of Na OH and water glass activated Si-Mn slag fresh pastes were investigated.The variation of plastic viscosity of fresh pastes were analyzed from the perspective of microstructure and particle geometry.The viscoelastic properties and transformation process of fresh pastes were studied.The main research contents and results are as follows:(1)The alkali-activated Si-Mn slag reaction process can be divided into three steps.First step,slight Si-rich gels were formed in the initial reaction period,corresponding to the first main exothermic peak of the isothermal calorimetric curve.Second step,massive Al-rich gels were formed in the acceleration and deceleration period,corresponding to the second main exothermic peak of the isothermal calorimetric curve,at which time the cementitious material had a considerable strength.Last step,the degree of gel polymerization continued to increase and gel became more mature in the stable period,at which time the isothermal calorimetric curve also tended to be flat.The modulus of alkaline activator determined its alkalinity and existence form of polymeric silicate,which determined the intensity of AAR and the characteristics of AAR process respectively.The compressive strength of alkali-activated Si-Mn slag was up to 66.5 MPa at 24 h,which was a kind of alkali-activated material with quite high mechanical property.The high curing temperature was an important factor in its high performance.(2)The increase in the dosage of Na OH and water glass and the decrease in the modulus of water glass would promote the activation reaction of alkali,resulting in the increase of heat release in the early reaction,and the formation of more gel products at the same age,enhancing the mechanical properties of the cementing material.However,too high content of activator would lead to efflorescence phenomenon,damage the original pore structure of the cementing material,and lead to the deterioration of its mechanical properties.When the content of Na OH was 6%,the mechanical properties of Na OH system were the best,and the compressive strength of Na OH system can reach 47.6MPa at 28d and 59.6MPa at 90d,respectively.The decrease of water glass modulus would lead to the decrease of active component content in alkali activation system,which was not conducive to long-term reaction.When water glass was mixed with 1.2 modulus and 12%dosage,the activation effect were the best,and the compressive strength of water glass can reach 86.0MPa and 100.1MPa at 28d and 90d,respectively.(3)The addition of granulated blast furnace slag improved the early reaction intensity of alkali-activated Si-Mn slag,the acceleration peak was enhanced and advanced,the total amount of heat release in early reaction(5d)was significantly increased,the setting time was significantly shortened,and the early mechanical properties of cementitious materials were greatly improved.When the content of granulated blast furnace slag was 30%,the 7d strength of the composite cementing material in Na OH system was 53.2 MPa,and the 3d strength of the composite cementing material in water glass system was 71.4 MPa,which was 197.2%and189.3%higher than that of the pure silica-manganese slag cementing material,respectively.However,the addition of granulated blast furnace slag has a low contribution to the later strength of the cementing material,and will also lead to the cracking of the cementing material due to shrinkage.Considering the strength growth and cracking comprehensively,10%is the best dosage of slag.(4)The rheological behavior of the Na OH activated Si-Mn slag fresh pastes is in accordance with the Herschel-Bulkey model,and its plastic viscosity increased with the increase of Na OH content at low water-binder ratio(0.35 and 0.40),which was mainly attributed to the increase of the viscosity of the activator.At high water-binder ratio(0.45 and 0.50),the plastic viscosity first decreased and then increased with the increase of Na OH content.With the increase of OH~-concentration,more active siloaluminate dissolved,the absolute value of Zeta potential of pastes increased,the charge force between particles increased,the volume and shape coefficient of flocculation structure in pastes decreased,the dispersion of particles increased,so the plastic viscosity of fresh pastes decreased.The increase of OH~-concentration exceeded the threshold value,and the increase of Na~+concentration reduced the absolute value of Zeta potential of pastes,the charge force between particles decreased,the volume and shape coefficient of flocculation structure increased,so the plastic viscosity of new pastes increased.The viscoelastic and viscoelastic transformation characteristics of fresh pastes were closely related to the content of Na OH.When the content of Na OH increased above the threshold value(9%),the active components dissolved faster,the increase of electrostatic repulsion between particles affected the formation of network structure,and the elastic index(EI)and macroscopic viscosity index(MVI)of pastes decreased first and then increased.Meanwhile,the acceleration peak of the exothermic reaction was greatly advanced,and a large number of gel products were generated in the early stage,which causd the intersection of G’and G’’curves to move to the left,and the visco-elastic transition time node to advanced.(5)The rheological behavior of the waterglass activated Si-Mn slag fresh pastes is in accordance with the Herschel-Bulkey model.The correlation coefficients were high(0.999),and the yield stress was mostly low or 0.The plastic viscosity of fresh pastes decreased first and then increased with the increase of water glass modulus.When the modulus was from 0.9 to 1.8,the alkalinity of water glass decreased,the proportion of active silicate decreased,and the number of flocculating structures formed in pastes decreased.The absolute value of Zeta of pastes increased,the charge force between particles increased,the volume and shape coefficient of flocculating structure decreased,the free water wrapped in it also decreased,so the plastic viscosity of fresh pastes decreased.When the modulus of water glass was from 1.8 to 2.4,the large size of water glass with multi-chain polysilicate increased,and the connected particles formed a large volume of flocculating structure.At the same time,more free water was wrapped,so the plastic viscosity of the fresh pastes increased.With the increase of water glass content,the alkalinity of the activator increased and the activator contained more active silicates.Meanwhile,the increase of Na~+concentration resulted in the decrease of the absolute value of pastes Zeta and the weakening of the charge force between particles.Therefore,more flocculating structures with large volume and large shape coefficient were formed and more free water was wrapped.So,the plastic viscosity of fresh pastes increased with the increase of water glass content.(6)The MSD curves of particle movement of fresh pastes from water glass–Si-Mn slag can be divided into three stages,and the MSD curve in the second stage moves to the left and upward(reverse).Accordingly,the elastic index(EI)and the macroscopic viscosity index(MVI)of t fresh pastes increased first,then decreased,and finally increased to a fixed value.With the increase of water glass modulus,the second-stage reaction of pastes gradually advanced and lasted for a shorter time.With the increase of water glass modulus,the second-stage reaction of fresh pastes gradually advanced and lasted for a shorter time.Correspondingly,the period of increase and then decrease of EI and MVI values of pastes became shorter,and the time node of viscoelastic transition was also delayed,which was attributed to the low basicity and low activity silicate content in high modulus water glass.With the increase of water glass content,the second-stage reaction of pastes was gradually delayed and lasted longer,and the stage of EI and MVI values of pastes increased first and then decreased obviously became longer,while the time node of viscoelastic transition was delayed,which was attributed to the increase of activator basicity and active silicate content. |
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