Microstructure,Carbonation Of Cement-Based Materials And Models Predicting Carbonation Depths In The Context Of Global Climate Change

During the evaluation of durability performances of concrete,one important index that should be taken into consideration is the capability to resist carbonation.In general,carbonation incurs gradual reduction of pH inside concrete,which will depassivate the reinforcement and accelerate the steel corrosion,leading to failure of the whole infrastructure ahead of designed service life.With increased CO₂ concentration in atmosphere associated with global warming,the current carbonation speeds are faster than ever.Accordingly,there is a renewed interest in understanding the effect and mechanism of carbonation.In this thesis,a novel method?extended X-ray attenuation method,XRAM?was introduced.After verifying the reliability and the applicable scope of XRAM,the method was used to understand the influence of preconditioning schemes on cement-based materials subjected to accelerated carbonation.Through investigation on the chemical composition,microstructure and moisture distribution,the optimal preconditioning scheme for lab carbonation was evaluated.Meanwhile,the microstructure of paste specimens blended with fly ash and subjected to accelerated carbonation was studied.Moreover,this thesis studied the carbonation mechanism for cement-based specimens strengthened with cellulose fibre.Based on experimental results from XRAM and from traditional testing techniques,the research refined a carbonation model in the context of global climate change,and the carbonation depths of infrastructures in four mainland cities?Beijing,Nanjing,Guangzhou and Ji'nan?at Year2100 were predicted.The main conclusions from this research are listed as follows,1)The width and the average porosity of interfaces from a two-phase?hydrated cement paste-aggregate?material were measured by extended XRAM.The width of the interface from this study is 60?m,and its average porosity is 0.423,which is 36%higher than that of the hydrated bulk paste.2)Staining technique,which was previously used in medical science,was applied for phase recognition in mortar.Meanwhile,this research refined traditional XRAM,and the spatial distribution of porosity for a mortar was obtained.Either depending on local porosity or depending on local sand volume fraction,the representative elementary volume?REV?of a mortar was measured to be approximately 3 to 4 times of the maximum size of sand grain.Besides,this study verifies that the mix design has a significant effect on the REV.3)The influence of preconditioning schemes on carbonation was evaluated.The results reveal that the damage due to drying would be smaller for standard-cured specimens as compared to water-cured specimens.Also,the inner moisture distribution would be more uniform after mass balancing,and less fluctuation of humidity would occur during following carbonation process.4)The evolved microstructure of paste specimens blended with fly ash and subjected to accelerated carbonation was investigated.The results reveal that carbonation leaded to an increase on the number of pores with radius smaller than 3 nm.Besides,based on instant porosity profiles,carbonation process for paste specimens blended with fly ash was proved to be a diffusion-controlling procedure.5)The relationship between carbonation speed and humidity level for cellulose fibre reinforced mortar was studied.The study reveals that,even with the incorporation of cellulose fibre,carbonation process can still be regarded as a diffusion-controlling process.The microstructure did not significantly altered in 0.3%fibre campaign;however,in 0.6%fibre campaign,due to clear deterioration of microstructure,carbonation speed was faster compared with no-fibre campaign.Under the humidity level of 50%,due to the fact that fibre addition can improve the resistivity toward cracking,the carbonation speed was faster in no-fibre campaign.Furthermore,the study shows that the optimal carbonation humidity is approximately 50%for no-fibre campaign,but the humidity changed to 57%for 0.3%-fibre campaign.6)A carbonation model in the context of global climate change was refined in this research,and the carbonation depths of infrastructures in four mainland cities at Year 2100 were predicted.This study reveals that,due to booming population and drastic use of fossil fuel,the CO₂ concentrations in urban areas will be significantly higher than the global average CO₂concentration,and that leads to faster carbonation speed in urban areas.Without fibre incorporation,the carbonation depths of infrastructures in Nanjing,Ji'nan and Guangzhou will be 50 mm,57 mm and 40 mm,respectively at Year 2100,and Beijing is expected to attain 60mm carbonation depth already in Year 2080?considering no environmental governance?.If moderate amount of fibre was incorporated to resist cracking,the year in which carbonation depth of Beijing attains 60 mm will be postponed by approximately 8 years,and the carbonation depth of Ji'nan will be reduced from 57 mm to 52 mm in Year 2100?considering the occasion of no environmental governance?.