Opinions are divided about using siderite-bearing rocks in concrete. The main concern is the potential oxidation of the secondary mineral siderite (FeCO3), causing unsightly iron stains on the surface of concrete, and/or the development of weaknesses in bond strength at the paste-aggregate interface.
Previous studies (on siderite-bearing porous sedimentary rocks) have concluded that siderite (FeCO3) is not stable under atmospheric conditions and can rapidly oxidise to iron oxides/hydroxides with resultant staining problems.
The occurrence of siderite in igneous rocks is rare. However, in the Bowral/Mittagong district of NSW, Australia, there are several Jurassic microsyenite intrusions that contain siderite as a secondary mineral. The siderite is generally anhedral to subhedral in shape, ranges in size from 0.4 to 2.2mm, and forms interstitially adjacent to feldspar crystals. The siderite, (up to 10% of the stone) is commonly found replacing pyroxene due to deuteric alteration. In these rocks the oxidation of the siderite to iron oxides/hydroxides is variable from minimal alteration to complete oxidation. It is more common for siderite to display not only peripheral alteration but internal alteration along relict (pyroxene) cleavage planes.
This study details results of a number of laboratory experiments to evaluate the potential for staining from dense siderite-bearing microsyenites sourced from the disused Mt Gibraltar quarries (and other undisclosed localities) in the Bowral/Mittagong district in NSW. Testing involved a range of accelerated exposures: UV/IR radiation; water wetting and drying cycles; salt wetting and drying cycles; alkaline solution immersion; and long term outdoor exposure. Polished rock slices and thin sections were examined using petrographic light microscopy at regular intervals. The findings clearly demonstrate that the siderite secondary mineral is susceptible to alteration through oxidation processes to iron oxides/hydroxides. However, there is no evidence that it causes any adverse staining when used as an aggregate in concrete.