Construction Of Synergistic Carbonation Regime Of Cement Paste And Application In Mortar

Concrete carbonation curing was developed in late 1970s and promoted by lots of subsequent researches.From now on,it happens to get the newly driving force originated from lowering CO₂ emissions and increasing carbon storage required by China strategy related with the achievement of carbon peaking and carbon neutrality goals,and obtains special opportunity for its further development.In order to employ more accurate carbonation curing to meet different requirements for diverse performances and properties,it is necessary to further improve the fundamental theories and practical technology of carbonation curing.Therefore,in this study,based on the present process of carbonation curing,the influences of synergistic carbonation constructed by carbonation intervention point（Cα）and degree of carbonation（DOC）as well as the post-hydration on the composition（especially calcium carbonate phase）of cement-based materials were successively investigated.On this basis,the synergistic carbonation regime of cement paste composed by the two key parameters,that is,Cαand DOC was constructed and applied into cement mortar.The achievements from this study provide additional theoretical foundations for mastering the carbonation reaction regularity under different carbonation regimes,improving the understanding on carbonation curing and implementing more accurate carbonation curing on cement-based materials.Firstly,the Cαrequired for synergistic carbonation regime was constructed and its influence on the composition of cement-based materials was systematically studied.The target Cαwas determined by better consistency with the characteristics of cement hydration exothermic curve and GEMS thermodynamic simulation.The composition and carbonation kinetics of cement-based materials under different Cαconditions were diversely studied by XRD-Rietveld analysis,thermogravimetric analysis（TG）,²⁹Si NMR analysis and carbonate quantitative analysis.The results showed that the increase rate of cement reaction degree decreased with the increase of Cα.When the Cαwas lower（0.03,0.15）,the bound water content of C-S-H in the samples increased with carbonation,while the Cαwas higher（0.38,0.90）,the bound water content of C-S-H in the samples decreased with carbonation.The percentage contents of calcium carbonate with high thermal stability（corresponding to decomposition mode I）and vaterite increased with the increase of Cα.When Cαwas extremly small（0.03）or extremly large（0.90）,carbonation produced C-S-H with heavy decalcification.The process of carbonation kinetics under different Cαconditions was mainly controlled by the diffusion through the product layer,and the process can be further divided into two stages:a former stage with faster reaction rate and later stage with slower reaction rate.Then,the DOC required for synergistic carbonation regime was constructed and its influence on the composition of cement-based materials was systematically studied.The target DOC was constructed by the GEMS thermodynamic simulation.The composition and carbonation kinetics of cement-based materials under different DOC conditions were diversely studied by XRD-Rietveld analysis,TG/DTG,²⁹Si NMR analysis and carbonate quantitative analysis.The results showed that the reaction degree of cement,the percentage content of calcium carbonate with moderate thermal stability（corresponding to decomposition mode II）and the decalcification degree of C-S-H increased gradually with the increase of DOC.The total percentage content of metastable calcium carbonate（aragonite and vaterite）increased and the DOC required for the appearance of the aragonite phase increased gradually as the Cαincreases.Afterwards,the effect of synergistic carbonation on the composition of cement paste under post-hydration was systematically studied.Based on comprehensive consideration,the synergistic carbonation regime with Cα≤0.15 and 8%≤DOC≤20%was recommended.The effect of different synergistic carbonation regimes on the composition of cement paste under post-hydration was systematically studied by XRD-Rietveld analysis,TG analysis and ²⁹Si NMR analysis.The results showed that increasing the Cαappropriately was conducive to increase the reaction degree of cement and improve the content of C-S-H bound water in the samples.The silica gel produced by the earlier carbonation of cement-based materials underwent a chemical reaction similar to the“pozzolanic reaction”in the post-hydration process,resulting in a decrease in CH content,at the same time,an increase in C-S-H content,and the disappearance of resonance absorption peaks at Q³（OH）and Q⁴ positions.Different thermal stability（corresponding to different decomposition modes）and different crystal forms of calcium carbonate generated by early carbonation underwent phase transformation during the post-hydration process,resulting in higher thermal stability and improved crystal form stability.Finally,the obtained synergistic carbonation regularity was applied to cement mortar.The synergistic carbonation regime of cement mortar was determined and its influence regularity was investigated through the immediate compressive strength and28 d compressive strength of cement mortar,XRD-Rietveld analysis,TG analysis and pore structure analysis.The results showed that the synergistic carbonation regime had well applicability in cement mortar.The synergistic carbonation regime（especially the carbonation regime of pre-hydration for 6 h followed by carbonation for 1 h）significantly promoted the strength development and improved the pore structure of cement mortar.In the synergistic carbonation regime,with the increase of pre-hydration time or carbonation time,the effect of carbonation on the improvement of cement reaction degree was gradually weakened,and the content of C-S-H bound water in cement mortar was gradually increased.With the increase of pre-hydration time,the content of calcium carbonate with high thermal stability（corresponding to decomposition modeⅠ）increased gradually.And the percentage content of calcium carbonate with low thermal stability（corresponding to decomposition modeⅢ）decreased with the increase of carbonation time in cement mortar.