Investigation On Carbonation Of Shrinkage-compensating Concrete And Microstructure

Along with the rapid development of concrete technologies and serious situations of environment problems, it has been universally reckoned by many scientists that improving the service life of concrete is the most direct and effective way to protect environment and save resources. Therefore, to research the durability of concrete materials is increasingly becoming the important tasks for material scientists. With the extensive use of shrinkage-compensating concrete in construction projects, many scientists put attentions towards the carbonation of shrinkage-compensating concrete. Thus, it is crucially theoretical and practical meanings to carry out the researches on carbonation properties and micro-mechanisms of shrinkage-compensating concrete.The carbonation properties of shrinkage-compensating concrete, which is influenced by different addition of expansive admixture and fly ash,restraint condition and curing condition, are investigated in this paper from the aspects of the properties of materials,carbonation mechanisms and influencing factors. Further, the influence mechanisms are revealed with TG-DSC,SEM and MIP(Mercury Intrusion Porosimetry) analysis. Besides, C-S-H gel is synthesized with chemistry method and characterized, moreover, the carbonation ability and mechanism of C-S-H gel are analyzed with XRD,SEM and FTIR analysis. The research results are as follows:(1) Expansive admixture can compensate the dry shrinkage of concrete effectively, which is good for improving concrete volume stability. But it simultaneously enlarges the distributions of large size pores in concrete, resulting in the increase of total porosity of concrete.(2) Carbonation can dense concrete structure and reduce the accelerated carbonation rate of concrete further. The calcite is formed not only from the carbonation reaction with Ca(OH)2, but also from the carbonation of ettringite and C-S-H gel in carbonation process. And the carbonation reacts of Ca(OH)2 and C-S-H gel are happened at the same time, yet carbonation process of tobermorite phrase is slow due to its crystal phrase stabilization.(3) Theoretically, C/S has certain effects on the carbonation process of C-S-H gel. Generally speaking, if the sample has higher C/S, it is more difficult to be carbonated. Oppositely, the lower C/S is, the easier carbonation is accordingly. (4) In restraint condition, expansive admixture can dense concrete structure and decrease the total porosity of concrete and the distributions of large size pores. However, in non-restraint condition, expansive admixture increases the total porosity of concrete and the addition of expansive admixture is bad for development of concrete pore structure.(5) In standard curing condition, the compressive strength of concrete goes up and carbonation depth of concrete declines with the increase of addition of expansive admixture. There is an approximate linear relationship between compressive strength and carbonation depth of concrete. However, nature curing condition is the worst to the ability of resistance carbonation of concrete due to the lack of water under 70% of relative humidity. When samples are cured in water for 3 or 7days, plenty of ettringite will form in hydration process and stresses generated in expansive reactions will also increase correspondingly, causing total porosity to broaden and compressive strength to decline, which raises the probability of concrete carbonated.