Durability enhancement mechanism of mortar using blast furnace slag fine aggregate against combined deterioration of frost and salt damage

Abstract

The effect of BFS (Blast Furnace Slag Fine Aggregate) on the freezing and thawing resistance under chloride environment was evaluated, and the freezing and thawing resistance was proven to be improved by using BFS with fine grain size and by increasing the mixing ratio of BFS to natural sand. The electrical resistivity and effective chloride ion diffusion coefficient were measured in order to evaluate the effect of BFS on the resistance of the penetration of chloride ions into concrete, and the mortar using BFS had higher electrical resistivity and lower effective chloride ion diffusion coefficient than the mortar using mountain sand. In order to elucidate the reaction products at the interface between cement paste and BFS, which cause densification of the BFS interface when BFS is used as fine aggregate in concrete, an investigation and analysis of the reaction products were conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As a result, it was found that a reaction phase is formed in the BFS-cement paste interfacial transition zone (ITZ), and its thickness is 0.45 to 1.4 μm. TEM observation and selected-area diffraction (SAD) analysis revealed the existence of hydrotalcite-like compounds in the BFS-cement paste ITZ. The chloride enrichment in hydrotalcite-like compounds at ITZ was confirmed by TEM observation and TEM-EDS of mortar with BFS after freeze–thaw tests under chloride environment. Therefore, it was considered to contribute to the improvement of the resistance of the penetration of chloride ions into concrete mixed with BFS.