Subsequent Hydration And Resistance To Chloride Ions Corrode Of Early CO₂-cured Cement Pastes And Mortars

Early CO₂ curing has attracted increasing attentions all over the world due to improved early mechanical strength for cement-based materials and sequestration of carbon dioxide.A large number of literatures had reported the principle of early CO₂ curing and mechanical strength and long-term durability of the resulted cement-based materials.These reports all focused on the plain concrete such as concrete blocks.However,early CO₂-cured reinforced concrete was rarely reported.This thesis compared the chloride binding and transport as well as steel bars corrosion properties of cement pastes or mortars with and without early CO₂ curing.Then,the effects of fly ash and ground granulated blast-furnace slag（GGBS）on the chloride binding and transport as well as steel bars corrosion properties of early CO₂-cured cement paste or mortar were studied.Lastly,the service life of early CO₂-cured reinforced concrete exposure to sea water was predicted based on the aforementioned fundermental test results.Since some uncarbonated cement particles remain in early CO₂-cured cement-based materials,the subsequent hydration of them has strong influence on the long-term durability.Thus,before aformentioned experiments,the subsequent hydration properties of early CO₂-cured cement paste were examined,which will be helpful to understand the steel bars corrosion in early CO₂-cured cement mortars.Based on the experimental results in this study,the following findings can be drawn:（1）The uncarbonated cement particles in early CO₂-cured cement paste will hydrate once contact with water,and the hydration products Ca（OH）2 will react with silica gel produced during early CO₂ curing to form C-S-H gel.However,early CO₂ curing hindered the secondary hydratioin of fly ash and GGBS.As a result,the reaction degrees of fly ash and GGBS after 27 and 89 d subsequent lime water curing were near zero.The subsequent hydration degrees of early CO₂-cured cement pastes gradually decreased with the increasing replacement level of fly ash and GGBS,inducing greater gap of compressive strength between the cement pastes with and without SCMs with the extending of water curing period.After subsequent water curing,the p H value of pore solutions for early CO₂-cured plain Portland cement and SCMs-blended cement pastes could recover to higher than 12.4 and 11.8,respectively,which were higher than the threshold value for depassivation of steel bars.Subsequent water curing decreased the porosity and modified the pore size distribution of early CO₂-cured cement pates.At the curing ages of 28 d and 90 d,the most probable pore size of early CO₂-cured cement pastes with water to cement ratio of 0.4 by mass was 16.39 and 8.63 nm,which were about 22.0%and 11.4% as larger as that of water cured cement pastes in the reference group,respectively.（2）Early CO₂ curing reduced the chloride binding capacity of cement paste,in particular the chemical chloride binding capacity,leading to the chloride binding of early CO₂-cured cement pastes mainly depended on the physical adsorption by C-S-H gel.The chloride binding of early CO₂-cured plain Portland cement paste could be described by Langmuir isotherm.Since the pozzolanic reactions of fly ash and GGBS were hindered in early CO₂-cured cement pastes,the addition of them decreased the chloride binding capacity of cement pastes.Moreover,the fly ash blends and GGBS blends showed similar chloride bingding capacity when the replacement level of them were identical.Extending the subsequent water curing period after CO₂ curing improved the chloride binding capacity of early CO₂-cured cement pastes.The CO₂-cured cement paste showed higher chloride binding capacity exposure to Ca Cl2 solution compared to Na Cl solution.（3）Compared to the traditional water cured cement mortars,the chloride migration and diffusion coefficients of early CO₂-cured cement mortars decreased by 20～50%,and the capillary absorption coefficients decreased by 17～55%.However,the chloride permeability of early CO₂-cured cement mortars gradually enchanced with the increasing replacement level of SCMs including fly ash and GGBS.When the replacement level of SCMs was 50%,the chloride migration coefficients of cement mortars increased by170～230%.The fly ash-blended cement mortars showed larger chloride migration and diffusion coefficients than that of GGBS-blended cement mortars.It could be ascribed to that GGBS was finer than fly ash,resulting in better filler effects for GGBS.（4）The results in simulated pore solutions of concrete suggested that stable oxide passive films could form on the surface of steel bars embedded in early CO₂-cured concrete.However,early CO₂ curing decreased the chloride threshold level for depassivation.The chloride threshold level of water and CO₂-cured conrete were ranged from 0.45 to 0.5M and 0.2 to 0.25 M,respectively,while the chloride threshold level of concretes contaning 50% fly ash or/and GGBS was ranged from 0.05 to 0.08 M.（5）Although early CO₂ curing decreased the chloride binding of cement paste and chloride threshold level for depassivation,the reinforcement corrosion resistance of cement mortars were enhanced due to lower chloride permeability.However,the addition of fly ash and GGBS reduced the reinforcement corrosion resistance of early CO₂-cured cement mortars.The service life prediction model predicted that the service life of early CO₂-cured plain Portland cement concrete was 42.5% higher than that of traditional water cured ones exposure to sea water.When 50% fly ash and GGBS were incorporated into cement concrete,the service life decreased by 88.6% and 86.0% separately.This investigation clarified the subsequent hydration and chloride resistance of Portland cement-fly ash-slag ternary pastes and mortars.It provided theoretical guidance for the application of early CO₂ curing in reinforced concrete.The expected objectives of this study had achieved.There are 115 figures,25 tables and 225 references in this dissertation.