Andesites as Resources for Roading and Concrete Industries, North Island Of New Zealand

Andesites are a major source of aggregate for much of the North Island of New Zealand. There are regional differences with respect to some important properties of the andesites. Some are reactive with cement and are designated as potentially deleterious or deleterious by the concrete industry. Other andesites, particularly those that are slightly altered or weathered, have rather high Plasticity or Clay Indices and perform poorly in road construction.   The performance of the andesites has been investigated and can be shown to be determined by: +        the chemistry of the rock which in turn determines the chemistry of the glass in the rock; +        the presence of large amounts of groundmass glass in the rock; and +        the nature of the superimposed alteration processes. Environment of extrusion and how rapidly the rock has been cooled or chilled are also important as they determine the fabric of the rock and how much glass will be present in the rock.   There are three main types of alteration that affect the quality of the aggregate: 1.   hydrothermal alteration, 2.   late magmatic alteration associated with the devitrification of glass, and 3.   weathering.   Each of these alteration types produces different alteration minerals. In fresh andesites the devitrification of glass has an important affect on the rockÆs properties. The Taranaki andesites are shown to have unusually (in comparison with other North Island andesites) high Na2O contents, held in the groundmass glass, which is the fundamental reason for the high alkali û silica reactivity of these andesites.