Study On Activation Methods Of Recycled Concrete Powder And Its Influence Mechanism On Properties Of Alkali-Activated Slag Materials

Recycled concrete powder（RCP）comprises a fine powder with particle size less than 0.15 mm produced in the crushing process of waste concrete,accounting for approximately 20-30%of the total volume of waste concrete.Recycling and using RCP as a partial substitute for slag to produce alkali-activated slag material（AASM）is of great significance for green development of concrete industry.This can digest the storage of construction waste and achieve the reduction of carbon emissions in the cement industry.However,the main component of RCP is old,hardened cement stone with a rough and porous surface,characterized by large water absorption and low activity,which leads to a decline in the working and mechanical properties of AASM,and restricts the application of RCP in AASM.Therefore,different methods were used to stimulate the activity of RCP,the effects of activated RCP on the setting and hardening properties of AASM were studied,and the influence mechanism was revealed.The main research contents and results are as follows.（1）RCP was carbonized in aqueous solution for different duration,and the temporal evolution of RCP surface product composition and microstructure morphology was analyzed.The effects of RCP with different carbonation duration on the hydration,working,and mechanical properties of AASM were studied,and the mechanism of carbonation RCP in the hydration process of slag was explored.Results show that the particle size of RCP is significantly decreased after carbonation treatment,which promots the formation of C-（A）-S-H,and reduces the total porosity of mortars,resulting in a significant increase in the early compressive strength of AASM.The enhancement effect of carbonation RCP on the strength of AASM is positively correlated with the duration of carbonation.The surface of carbonation RCP is covered with a layer of chemically stable calcite particles,with a low solubility under alkaline conditions.This limits the leaching of Ca²⁺,delays the hydration of slag,and increases the setting time of AASM.（2）Based on the comprehensive utilization of the products in the entire carbonization process,the waste liquid for the preparation of carbonation RCP was used as the mixing water,and the carbonation RCP was used as part of the precursor,which was pre-mixed with the Na OH and applied in AASM in coordination.The reactivity between RCP suspensions with different carbonation duration and Na OH was explored,and the influences of RCP suspension with different carbonization duration after premixed with Na OH on hydration,working,and mechanical properties of AASM were studied.The synergistic interaction mechanism of carbonation RCP suspension after premixed with Na OH in the hydration process of slag was also explored.Results show that,after pre-mixing the carbonation RCP suspension with Na OH for 4 h,the alkalinity of the mixed solution decreases and the Si O₃^2-concentration increases significantly,especially when the carbonation duration is longer than 10 min.This provides a mild environment for the hydration of slag,and delays the setting of AASM.Under the action of Na OH,the porosity of carbonation RCP increased,and it has no significantly influence on the fluidity of AASM compared with uncarbonized RCP.However,the Si O₃^2-in the suspension provids additional[Si O₄]^4-for the reaction system,promotes the formation of C-（A）-S-H gel,and reduces the porosity of AASM.Therefore,the compressive strength of the mortars is significantly improved,and the optimal compressive strength can be obtained when the carbonation duration of RCP suspension is 20 min,which was 13.6%higher than that of control mortar.（3）RCP were immersed in different concentrations of tannic acid（TA）solution,and the influence of TA solution on RCP surface reaction products and the modification effect of reaction products on RCP microstructure was studied.The effects of RCP treated by TA solution on the hydration,working,and mechanical properties of AASM were evaluated and mechanism of action was analyzed.Results show that the TA-Ca complexes can be formed by the reaction between TA and the Ca（OH）₂ as well as Ca CO₃ in RCP,which are deposited on the surface of RCP.The hydration of slag is inhibited and the setting time of AASM is extended due to the chelation of phenolic hydroxyl functional groups on Ca²⁺.However,the retarding effect of TA activated RCP is weakened with increasing hydration time,and the phenolic hydroxyl functional group on the surface of RCP can combine or cross-link hydration products through a variety of interactions,which reduce the pore content（≤50 nm）and the total porosity of mortars.Therefore,the compressive strength of mortars is improved.RCP treated with TA solution has a certain dispersion effect and work well in AASM.（4）RCP was carbonized in different concentrations of TA solution for different durations,and the induction and regulation of TA on the carbonation process was analyzed.The effects of RCP under different concentrations of TA solution and carbonation duration on the hydration,working,and mechanical properties of AASM were studied.The mechanism of complex treated RCP in the setting and hardening processes of AASM was explored.Results show that TA inhibits the absorption of CO₂by RCP through lattice distortion and the formation of TA-Ca complexes.The complex treated RCP display a smaller particle size,which provides more nucleation points for the growth of hydration products.In addition,the phenolic hydroxyl functional group on the surface of RCP can cross-link hydration products.Therefore,the compressive strength of mortars is significantly improved.When the concentration of TA solution is 1%and the carbonation duration is 20 min,the compressive strength of AASM reaches the optimal value,which is 30.2%higher than that of the control mortar.The incorporation of complex treated RCP significantly improved the fluidity and setting properties of AASM,and the improvement was enhanced with increasing carbonation duration and TA solution concentration.The research results of this paper provide new methods for the activation of RCP activity and provide a theoretical basis for the application of activated RCP in AASM,which are of great significance to promote the resource utilization of RCP and reduce the carbon emissions of concrete industry.Figure 94 Table 15 Reference165...