Investigation On The Saturated Carbonation Process And Simultaneous Shrinkage Of Cement Mortars

The carbonation of cement-based materials can reduce the alkalinity of the materials,leading to the depassivation and corrosion of steel bars,which is a common harm to buildings.At the same time,with continuous development of carbon capture and storage,carbonation has become a new carbon sequestration technology with environmental and economic benefits.However,whatever the advantages and disadvantages,carbonation process and carbonation shrinkage need to be paid attention to.Since C-S-H gels are sensitive to water,water loss can lead to the collapse of interlayer pores and make pore structure coarsen.In conventional carbonation studies,drying is inevitable and it is difficult to decouple carbonation shrinkage and drying shrinkage.However,many scholars paid less attention to the effect of drying on pore structure and carbonation shrinkage.The carbonation process and the mechanism of carbonation shrinkage are quite complicated.Although there have been a lot of experimental studies,there is no unified conclusion at present.Low field Nuclear Magnetic Resonance（LF-NMR）is used to quantitatively analyze the evolution process of pore structure.It is found that the porosity increases with the increase of water to cement ratio,and the porosity can be effectively reduced by adding silica fume.The larger the porosity,the more pores below 20nm,and the larger the critical pore size.After carbonation,the change of pore structure was dominated by the inner pores of C-S-H gel,and the porosity appears 4 stages of evolution.The content of the pores below 20nm increased,while the pores larger than 20nm showed no obvious change rules.Through testing length,it is found that the length also appears 4 stages of evolution,which corresponds to the change of pore structure and solid volume.Theoretically,the solid volume will expand and the porosity will decrease after pure carbonation,but dissolution occurs in the experiment,which affects the development of pore structure and length.By comparing the measured data to the theoretical data,the main factors of each stage of carbonation are summarized:In the first stage,the surface Ca（OH）₂carbonation generates Ca CO₃,which is rapidly transformed into soluble Ca（HCO₃）₂.Meanwhile,AFt and AFm are also carbonated with a small increase in porosity.In the second stage,Ca（OH）₂continues rapid carbonation and the porosity decreased.In the third stage,the dissolution of AFt,AFm and Ca（HCO₃）₂dominated again,and the porosity continued to increase.The fourth stage is dominated by C-S-H gel,so the porosity decreases slightly.By testing the water permeability before and after pure carbonation,it is found that the water permeability decreases by about one order of magnitude,which is caused by the refinement of pore structure.The water permeability is proportional to the square of the characteristic radius,which further confirms the existence of dissolution.By optimizing the coefficients of Katz-Thompson equation and Kozeny-Carman equation,the prediction error of water permeability is controlled between-80.9%and 25.3%,it shows that the water permeability can be accurately calculated even according to the simple prediction model bsaed on the saturated pore structure obtained by LF-NMR testing,and the calculation accuracy is much higher than the calculation error of 1～2 orders of magnitude in the relevant literature.