- Previous research has shown that using fine lightweight aggregate (FLWA) can be a promising strategy to mitigate alkali-silica reaction (ASR) in mortar and concrete. While several possible theories were proposed by researchers, discrepancies in testing methods, materials, and supporting evidence still exist. Therefore, this study was initiated. There are two primary focuses: (1) use current ASTM standards to evaluate the efficacy of ASR mitigation when FLWAs were used; (2) elucidate mechanisms by which FLWAs may mitigate ASR. This study assessed the efficacy of ASR mitigation when expanded clay, shale, and slate were used. ASTM test methods including ASTM C289, ASTM C1260 and ASTM C1293 were completed. The fine normal weight aggregates were replaced by the FLWAs at 25% and 50% by volume in concrete mixtures, and 25%, 50% and 100% by volume in mortar mixtures. Results showed that ASTM C1260 and ASTM C1293 can be used to evaluate the mitigation efficacy when FLWAs were used. All three FLWAs were effective in reducing the ASR-induced expansion in ASTM C1260 and ASTM C1293, except the mortar mixture with 25% in ASTM C1260. The investigated FLWAs were especially effective for the concrete with a moderate reactive aggregate. For concrete with a highly reactive aggregate or very highly reactive aggregate, other mitigation strategies may need to be combined with FLWAs to effectively mitigate ASR.
The pore solution analysis and scanning electron microscopy (SEM) analysis were completed to elucidate how expanded shale, clay, and slate can mitigate ASR. A reduction in alkalinity was observed when FLWA was incorporated, especially for expanded shale and expanded clay. A high aluminum content was also found in the pore solution of the concrete mixtures with 50% expanded shale and clay through the pore solution analysis. SEM analysis revealed that infilling reaction products formed in the pores of the FLWAs. From energy-dispersive X-ray (EDX) analysis, the chemical composition of the infilling reaction product was found to be close to C-A-S-H, rather than alkali-silica reaction product. The Ca/Si of the reaction product can vary from 0.8~1.1, which is lower than the Ca/Si of ASR reaction product in the paste, but higher than that of the ASR reaction product in aggregates. This finding also indicated that the FLWAs were pozzolanically active.
Thermogravimetric analysis (TGA) and simulated pore solution testing were completed to further confirm the potential pozzolanic activity of the expanded shale, clay, and slate of three different sizes: 1.19~2.18 mm, 0.60~0.30 mm, and 0.15~0.30 mm. The reduction of CH and increase of non-evaporable water in the mixtures showed that the expanded shale and clay of 0.15~0.30 mm and 0.30~0.60 mm, as well as the expanded slate of 0.15~0.30 mm, were pozzolanically active. Compared to the reference aggregate, FLWAs showed less dissolved silica content in the simulated pore solution at 38 °C at 84 days. The consumption of Si, Al, Na, and K in the simulated pore solution at 84 days indicated that pozzolanic activity occurred when FLWA is present.