Macro- and micro- characteristics of cement binders containing high volume fly ash subject to electrochemical accelerated leaching

Although high volume fly ash (HVFA) concrete has shown benefits for mass concrete such as dams, the calcium leaching in these hydraulic structures becomes a concern. This paper presents a study on leaching behavior of HVFA concrete on both macroscopic and microscopic scales. The fly ash was added to replace cement at weight ratios ranging from 30% to 70%. Two curing ages, 28 days and 90 days, were tested. An accelerated electrochemical leaching test was developed to investigate leaching characteristics of HVFA mixtures. Leaching calcium contents, compressive strengths and pore structures were examined. Microstructural changes of HVFA concretes after leaching tests were analyzed through FT-IR, 29Si NMR and SEM/EDX techniques. It was found that addition of fly ash into cement could effectively enhance the leaching resistance of cement based materials, but the contents of fly ash should be limited not to exceed 50% of cementitious materials. Fly ash replacement ranging from 40% to 60% produced optimal pore structure that is beneficial to leaching resistance. The fly ash addition also increased condensation polymerization of C-S-H and increased Q1/Q2 ratio, leading to a more stable and durable C-S-H gel structure.