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Volcanic rocks and greywackes are the two sources of aggregates in the North Island, and whether the volcanics were thrown up by a marine or a terrestrial eruption is an important factor determining their commercial properties, the University of Auckland’s Philippa Black told the Aus-IMM conference. Black is working on a survey to classify the North Island’s aggregate resources in terms of their source properties (see story on page ...), and to gain a further understanding of the causes of their properties and performance variations. The way the operator handles the material has a significant effect on aggregate properties: that produced by simple scalping, crushing and screening will differ from the same rock put through multi-stage processing. “The toughness of the individual mineral constituents and the cohesiveness of the rock are the most important controls on rock strength and its crushing resistance.” There are five types of North Island greywacke: Murihiku, Waipapa, Torlesse, eastern North Island and Caples. Volcanic aggregates generally have a lower crushing resistance than the best greywackes, with those from a marine environment being glass-rich. New Zealand uses 46 million tonnes of aggregates a year, 53 percent of which is used for roading, 20 percent for building construction, about eight percent for concrete and the rest as fill or environmental protection and restoration.
Q&M Vol.6 No.5 October-November 2009 All articles on this website are copyright to Contrafed Publishing Co. Ltd. |