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 rock's
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 - silica
reactivity of these andesites.