Carbonation Curing Mechanism Of Cement-based Materials And Its Improvement On Pervious Concrete

Carbonation curing is a new type of CO₂capture-storage and cement-based materials strengthen technique.It can sequestrate CO₂into stable and insoluble Ca CO₃ during early age of hydration,and improve early strength,density and long-term properties in the same time.Carbonation curing can also reduce energy comsuption in manufacture of concrete by replacing traditional accelerating curing method such as steam curing.Considering the increasingly serious problem of energy and environment,carbonation curing has been widely concerned and studied.This study aims to investigate the influence of different carbonation curing regime and mineral composites on cement-based materials,durability of carbonation-cured cement paste,strengthen mechism of carbonation curing and carbonation strengthening effect of pervious concrete.The experimental studies and mechanism analysis are conducted to optimize carbonation curing technology,expand the application scope of pervious concrete and provide theoretical basis of carbonation curing of cement-based materials.The main research contents of this study are as follow:A mathematical model for one-dimensional early carbonation is established to predict important factors during carbonation curing such as porosity,sat urability,and carbonation degree.The predicted results are verified by MIP and TGA anaylze.In addition,the transfer and reaction meachisms of reactants in carbonation curing are researched by dimensionless analysis.It is found that early carbonation is a diffusion limiting process initally and then turns ino a reaction rate limiting process.It was also demonstrated that carbonation depth and rate of early age Portland cement paste are mainly controlled by contents of tricalcium silicate and calcium hydroxide while water to binder ratio is kept constant.The influence of different pre-curing and carbonation curing regimes on compressive strength and micro-structure are researched.The results show that carbonation curing can improve early compressive strength of cement paste significantly,while the effect of carbonation on compressive strength decreases for long-term strength.The optimal pre-curing duration are corrsponding to 40%relative water loss of cement paste.The results of TGA,MIP and EDS measurements indicate carbonation curing can modified pore structure and decrease porosity of cement paste by transforming a part of capilary pores to gel pores.In the process of carbonation curing,hydration reactions could be inhibited by carbonation and excessive carbonation can lead to decalcification of C-S-H.The influence of mineral composites on carbonation curing of cement paste.Experimental results suggest that carbonation rate of C₃S is the fastest within 8 h carbonation,then slow down gradually.When the carbonation duration exceeds 24 h,the compressive strength ofβ-C₂S andγ-C₂S get close to C₃S and the carbonation degree is higher than C₃S obviously.Carbonation curing increases 3 d compressvie strength of LPC paste to simular level as common Portland cement paste,and the optimal carbonation duration of LPC paste is longer then common Portland cemetn paste.In the process of early carbonation,the main crystal Ca CO ₃generated by C₃S is Calcite.However,forβ-C₂S andγ-C₂S,both Calcite and Aragonite could be observed clearly in XRD patterns.It could be also found a small amount of Vaterite in carbonatedβ-C₂S andγ-C₂S initially,while as the carbonation proceeds,Vaterite transforms into other phases.Calcium leaching resistance of carbonation-cured cmenet paste is studied.The results show that carbonation curing can increase surface density of cement paste and sequestrate calcium in stable Ca CO₃.Therefore,carbonation curing can improve calcium leaching resistance of cement paste effectively.Calcium solid-liquid equilibrium curves for carbonation-cured cement paste leaching in 6 M NH₄NO₃are experimental determined by simulated pore solution method.It could be concluded that carbonation curing increases solubility of C-S-H according to this equilibrium curve,so excessive carbonation may result in potential risks for long-term property of cement paste in leaching environment.Baed on experimental determinterd calcium soli-liquid equilibrium relationship and impact of carbonation curing on pore structures,a modified PNP model is developed to simulate calcium leaching of carbonation-cued cemnet paste and verified by EDS measurements.The mechanical strength and properties evolution of carbonation-cured pervious concrete in coupled conditions of freeze-thaw and calcium leaching are investigated.The results indicate that proper carbonation curing can improve early compressive strength of pervious concrete effectively.Compared with conventional concrete,the carbonation degree of pervious concrete are more homogeneous and the negative influence of exccessive carbonation is weeker.Carbonation curing can also improve freeze-thaw and calcium leaching resistance,and retard the degradation of pore structure induced by leaching damage.Moreover,binding property between aggregates and cement paste in carbonation-cured pervious concrete could be strengthen by improving aggregate-binder interfaces and decreasing the porosity of ITZ.