Chloride Binding Capacity Of Gap-Graded Blended Cement And Consequent Corrosion Resistance

According to?Belt and Road Initiative?policy and ocean economic development,more and more infrastructures are being built in marine environment,in which much higher concentrations of Cl^-and SO₄^2-are available compared to inland environment.In this case,the porous nature of cement paste,in terms of cracks and connected pore-network,provides migration channels for aggressive ions,leading to destructive expansion due to corrosion of reinforcement and salt crystals when the aggressive ions exceed the threshold level.As a result,the service life of infrastructures reduces sharply due to the deterioration of concrete.In laboratory researches,practicable countermeasures,such as reducing water to cement ratio,adding supplementary cementitious materials or ultrafine particles and filler,are used to densify the microstructure of concrete,then the diffusion rate of chloride reduces significantly and thereby the durability is improved.However,the effectiveness of these methods is limited in actual application due to the requirements of workability,mechanical properties and economic consideration.Recently,gap-graded particle size distribution?PSD?has been employed to increase the packing density of cement paste and to decrease the initial pore size.The gap-graded blended cement has dense microstructure and excellent crack resistance,indicating a great potential application in marine environment.To prepare blended cement with high corrosion resistance,the characteristics of hydration products and chloride binding capacity were investigated in this thesis.The species of chloride in cement pastes,in terms of chemically bound and physically absorbed chloride,were identified to discuss the chloride binding stability.Then gap-graded PSD was introduced to increase the packing density of blended cement pastes,the chemical components,hydraulic activity and addition of cementitious materials were optimized and metakaolin and modified hydrotalcite were added,to obtain target hydration products and consequently to enhance the chloride binding capacity and stability.Finally,the mechanical properties,corrosion-resistance as well as the deterioration process of microstructure of gap-graded blended cements exposed to chloride-sulfate solution were investigated.The main approaches are summarized as follows:To minimize the influence of un-hydrated particles,ultrafine granulated blast furnace slag?GBFS?was added into ultrafine Portland cement?PC?pastes and the relationship between characteristics and chloride binding ability of hydration products was investigated.Cement pastes with Ca/Si ratio of 1.24¹.62 and Al/Si ratio of 0.35⁰.45 perform better chloride binding capacity and stability.Within the range cement pastes can chemically bind chloride 12.26¹3.47 mg/g and physically adsorb chloride 5.03⁵.73 mg/g,lastly have2.15².66 times non-water-soluble chloride than PC paste.Based on PSD method,the chemical components,activity and addition of cementitious materials were optimized and chloride-bound functional components were added to obtain gap-graded blended cement with high packing density and chloride binding ability.The chloride binding capacity of new gap-graded blended cement was up to 19.59 mg/g with10.38 mg/g non-water-soluble chloride.The chloride binding ability and stability of gap-graded blended cement improved obviously compared to ordinary Portland cement.To characterize the chloride migration in concrete,rapid chloride migration?RCM?method was adopted.The chloride diffusion coefficient of gap-graded blended cement?BBCFF?concrete is 0.17×10^-1212 m²/s,which is 16%of ordinary concrete?PC?.The chloride diffusion coefficient is not only connected with porosity of concrete but also influenced by chloride binding ability of cement pastes.Compared to PC concrete,BCF gap-graded cement concrete has no significant different porosity but its chloride diffusion coefficient is much less because of better chloride binding ability.The corrosion-resistance of gap-graded blended concrete exposed to chloride-sulfate solution was investigated by characterizing the microstructural deterioration process.Because of dense structure,the cracking of gap-graded blended concrete due to sulfate attack occurred earlier than PC concrete but the chloride diffusion coefficient was much less.GCM concrete performs the best chloride penetration resistance with chloride diffusion coefficient of3.7×10^-1212 m²/s after 6 months corrosion.The mutual competition and promotion of sulfate and chloride was found in corrosion process.The expansion products due to sulfate attack will fill the pore space first which can slow down the diffusion of chloride.As the expansion products grown enough,the concrete will expand to generate cracks and accelerate chloride corrosion.Gap-graded blended cement with high initial packing density and low porosity can delay chloride penetration effectively.Meanwhile adjusting cement components and adding Al-rich cementitious materials can improve chloride binding ability of cement pastes and reduce the concentration of free chloride in concrete.For gap-graded blended cement,fine Al-rich functional components which could bind chloride effectively were added.The porosity and pore size of concrete were decreased and the chloride binding ability of hydration products was improved and that made the ion corrosion resistance of concrete promoted significantly.The results provide theoretical foundation for the relation of chloride binding with characteristics of hydration products and technical support for high durability performance cementitious materials in marine environment,all of which will be helpful to the development of?Belt and Road Initiative?and Guangdong-Hong Kong-Macao Greater Bay Area,and engender great society,ecology and economic benefits.