| Accelerated carbonation used as curing technology can realize the absorption and storage of CO2,while enhancing early strength,improving durability,and shortening the curing time of cement-based materials.Therefore,accelerated carbonation curing is of great advantage to reduce carbon footprint and important for the development for the implication of concrete materials.Accelerated carbonation curing is a diffusion reaction process,thus the penetration depth is usually low and usually only part of the whole sample will be fully carbonated,resulting in the formation of the carbonated zone in the surface layer and the uncarbonated zone in the internal part.At present,many scholars use a variety of test methods to study the microstructure of the carbonated area,but the study on microstructure of the internal uncarbonated area is still limited.In view of this,it is important to compare the compressive strength,carbonation depth,as well as its water absorption,mineral phase composition and content,and microhardness of cement paste of both internal and surface layer with different water-cement ratios（w/c,0.25,0.35 and 0.45）,that cured under accelerated carbonation curing（65%of RH、20%of CO2）and other curing regimes（water curing and 65%humidity curing）.The progress change of microstructure and mechanical properties of the internal and surface layer of cement pastes at different curing ages（1～28d）with varying w/c were studied,which aim to provide a theoretical basis for the development technology of accelerated carbonation curing.The results show that:（1）For a given age,the compressive strength of cement paste samples are:water curing group>accelerated carbonation curing group>65%humidity curing group.This is because the curing humidity in the water curing group is much higher than that in other two groups,which facilitates a more complete hydration.Compared with the65%humidity curing,the carbonation products（Ca CO3）formed during the accelerated carbonation curing process creates a dense and hard"shell"on the surface of the cement paste,which promotes a slight improvement in strength.Therefore,the strength of cement paste in accelerated carbonation curing is greater than the same without CO2injection（65%humidity curing）.At the same time,in different curing regimes,the strength of cement paste is negatively correlated with the initial w/c.While carbonation depth of cement paste is positively related to the initial w/c.Carbonation depth increases with the carbonation time and w/c,and carbonation depth of 0.45 w/c cement paste reaches 10.17mm at 28d.While the growth rate of carbonation depth is faster in the early stage compared with the later ages.The turning points of growth rate of carbonation depth of 0.35 w/c and 0.45 w/c cement paste are 3-day and 14-day respectively.According to the water absorption of the internal and surface layers,the water absorption of surface layer is larger than that of the internal layer for the case of water curing group.While the water absorption of the surface layer in the accelerated carbonation curing group,is smaller than that of the internal layer in the accelerated carbonation curing group and the surface layer in the water curing group.（2）Through thermogravimetric analysis,X-ray diffraction and infrared spectroscopy analysis,the types and contents of mineral components in the internal and surface layers are studied.The results show that for a given age,the hydration product content of surface layer is lower than that of internal layer.Regardless of w/c,the components of the internal and surface layer in water curing group are the same as that of the internal layer in accelerated carbonation curing group,which mainly include hydrated products and unhydrated cement clinker,but the surface layer in accelerated carbonation curing group also found carbonation product,Ca CO3.In addition,the degree of polymerization of hydration products（calcium silicate hydrate,C-S-H）of the surface layer in the accelerated carbonation curing group varying with w/c.According to the quantitative analysis results of hydration and carbonation products of the surface and internal layer in accelerated carbonation curing group,the carbon fixation rate of cement paste increases with age and w/c,the carbon-fixing components are the reaction from both unhydrated cement particles and hydrated products.It is found that at 28 days,more than 50%of the total Ca CO3 in the sample are derived from the carbonation of hydration products.（3）According to the results of microhardness,the significant increase in the surface layer microhardness is mainly due to the presence of Ca CO3.For the internal part of cement paste,the difference in microhardness between the accelerated carbonation curing and the water curing group at 28d is not obvious,indicating that the accelerated carbonation curing has little effect on the internal microstructure even after28 days of curing.In addition,based on the linear fitting results between compressive strength and microhardness values of the uncarbonated internal area,there is a good linear relationship is established.Microhardness can be used to predict the compressive strength of cement paste.However,when using microhardness for strength prediction,special attention should be paid to avoid simultaneously analysis of the surface and internal microhardness.Also,the influence of factors of curing environment impact on the microhardness should not be neglected. |
