Durability of concrete containing siderite-bearing microsyenite and trachyte aggregates

The durability of concrete containing siderite-bearing microsyenite/trachyte aggregates has been investigated. The aggregates were from a disused Mt Gibraltar quarry and the Nattai River Cut 5 in the Bowral/Mittagong districts, 100 km south-west of Sydney in New South Wales, Australia. Laboratory and field exposures of embedded rock discs in mortar, and concrete and rock samples, conducted under controlled and measured environmental conditions, have shown the alteration of siderite secondary mineral in the Mt Gibraltar rock to iron oxides/hydroxides through oxidation processes. The rate of such alteration was generally found to be slow to moderate, but it was more profound in an alkaline environment. These findings are consistent with the theoretical desktop study of Eh–pH diagrams. Although the in situ oxidation of the siderite was visually observable, as pinhole-sized rust spots, the mobility of the iron and subsequent staining on concrete surfaces was negligible. The field inspection and subsequent petrographic study of existing concrete structures, made from siderite-bearing microsyenite/trachyte aggregates, have shown some degrees of in situ oxidation of the siderite to iron oxides/hydroxides, but no associated iron staining on concrete surfaces. No visual defects were observed in concrete structures in service for up to 80 years. Furthermore, it has been shown that the accelerated weathering techniques, developed in this study, can be used in future assessments of quarried rock during the various phases of any investigation.