Among the variant durability indexes concerning the service life of reinforced concrete, one important index is the resistance toward carbonation. Carbonation incurs gradual reduction of pH inside concrete, which will further depassivate the reinforcement and accelerate the steel corrosion, leading to failure of the whole construction ahead of expected service life. With accelerated industrialization process and drastic population increase started from last century, both global temperature and the CO2 concentration have gained significant increase. The increase will further accelerate the carbonation speed.
To understand the carbonation mechanism as well as to control the carbonation issue, one new material characterization technique (extended X-ray attenuation method, XRAM) was introduced in the present research for the characterization of cement-based materials. After confirming the reliability and applicable scope of XRAM, the method was applied to investigate the evolution patterns of chemical reaction, microstructure and moisture for cement-based materials during carbonation after different preconditioning schemes, and the most suitable preconditioning scheme for laboratory was determined then. By employing this extended XRAM, the evolved microstructure of cement-based materials blended with fly ash during carbonation was studied as well. Besides, through accelerated carbonation tests, the influence of cellulose fibre on carbonation was investigated. Moreover, combining results from XRAM and results from accelerated carbonation tests, a model which predicts carbonation depth in the context of global climate change was refined. The refined model was further applied to predict the carbonation depth of infrastructures in four China cities (Beijing, Nanjing, Guangzhou and Ji’nan) in year 2100. 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 results were compared with literature, and the reliability of extended XRAM in characterization of cement-based materials was therefore, attested.
2) Staining technique, which is commonly used in medical science, was applied first time for characterization of cement-based materials, and phases of mortar were identified through the technique. Besides, potassium iodide (KI) was used first time as the staining agent in mortar, the chemical can selectively strengthen the attenuation coefficient of hydrated cement paste, while leave the quartz sand unaffected.
3) A refined X-ray attenuation method (XRAM) was introduced. During calibration of porosity, the drying procedure, which is indispensable in traditional attenuation method, was substituted with the staining procedure. Since drying is no longer necessary, the refined method can avoid additional, unspecified damage due to drying, so the revealed microstructure should be closer to that of the intact specimens.
4) Experimental validation of representative elementary volume (REV) was applied. The work can help determine the applicable scope of extended XRAM in cement-based materials, and can help validate the REV determined from modelling. For pastes, both REVs determined from experiments or from HYMOSTRUC model are of the order of hundreds of microns. For mortars, the REV is determined to be 3 to 4 times greater than the maximum sand size based on experiment, which is also in good agreement with that determined from modelling, where a REV is suggested to be 3 to 5 times greater than the maximum sand size.
5) The influence of preconditioning schemes on carbonation was evaluated. This research reveals that all preconditioning schemes would incur damage, and the damage extent would be lower for standard-cured specimens as compared to water-cured specimens. After oven drying, the specimen surface suffers from excessive dryness, which will further lead to an exaggeration of early-age carbonation speed. After mass balancing, the inner moisture distribution would be more uniform, and less fluctuation of humidity would occur during accelerated carbonation. In addition, with desorption isotherm curves, the profiles of local humidity were built in this research, and based on the humidity profiles, the local humidity where carbonation performed, was confirmed to be higher than the humidity level in the carbonation chamber.
6) The evolved microstructure of fly ash blended paste specimens subjected to accelerated carbonation was investigated. The results reveal that carbonation leads to an increase on the number of pores with radius smaller than 3 nm, and that part of increase can be quantified by XRAM. Besides, based on instant porosity profiles, most accelerated carbonations were proved to be diffusion-controlling processes. Moreover, exploratory work was carried out in this research to evaluate the content of released water by C-S-H carbonation.
7) The influence of cellulose fibre on carbonation speed subjected to different humidity levels was studied. This study reveals that, with the incorporation of cellulose fibre, carbonation processes can still be regarded as diffusion-controlling processes. With 0.3% fibre content, carbonation behaviours, microstructure and chemical compositions of both pastes, mortars and concretes evolved in a pattern similar to that of campaigns containing no fibre. However, with 0.6% fibre content, carbonation speed gained a significant increase, and clear microstructure deterioration was observed. Under the humidity level of 80% or 65%, the carbonation behaviours between no-fibre campaign and 0.3%-fibre campaign showed no clear difference; however, under humidity level of 50%, no-fibre campaign carbonated faster. Combining the irregular-shaped carbonation front in no-fibre campaign, this study attests, suitable fibre content could increase the resistance toward cracking in cement-based materials without altering their pore structure, the carbonation speed was therefore, slower. In addition, according to fitted results based on experimental data, this research refers 50% to be the optimal humidity level for no-fibre campaign, while 57% to be the optimal humidity level for 0.3%-fibre campaign regarding carbonation.
8) In the context of global climate change, one refined carbonation model was used to predict the carbonation depths of some mainland cities in year 2100. This study reveals that, due to booming population and large consumption of mineral fuels, urban CO2 concentrations are significantly higher than the average CO2 concentration worldwide, and that leads to faster carbonation speed. Without fibre incorporation, the carbonation depths of infrastructures in Nanjing, Ji’nan and Guangzhou will be 50 mm, 57 mm and 40 mm, respectively in year 2100 considering no environmental governance, and Beijing is expected to attain 60 mm carbonation depth already in year 2080. If suitable amount of fibre was incorporated to resist cracking, the year in which carbonation depth of Beijing attains 60 mm will be postponed by 8 years, and accordingly, the carbonation depth of Ji’nan would be reduced from 57 mm to 52 mm in year 2100 (considering no environmental governance).