Geoscience Reference
In-Depth Information
Fine aggregate for use in Portland cement mortars,
renders, and screeds should have rounded particles ('soft
sand' or 'building sand') to improve the workability of
the mix (see
132
). Fine aggregates with angular particles
('sharp sand') are suitable for the fine aggregate fraction
of concrete or road aggregate (see
131
).
'Silver sand' is sand with a very low iron content that
is favoured for the manufacture of high-quality optical
glass. Its light colour ('silver') is a result of consisting
almost entirely of quartz and there being very little
contamination with other minerals (
134
). It has limited
uses in the construction industry.
Road aggregate is used to make up the various layers of
road pavements (see Chapter 10). The sub-base is a
drainage layer with coarse, uniformly graded aggregate
particles with large interparticle spaces, to prevent capillary
rise of water. The roadbase bears and spreads the traffic
load and ideally needs strong, rounded, continuously
graded particles (<40 mm maximum sized) that give good
compaction. The upper two layers of the pavement, the
basecourse and wearing course, are always bitumen bound.
This means that for the aggregate (<20 mm maximum
sized), a rough surface texture and angular shape to the
particles are important as well as strength. Crushed igneous
rocks and coarse-grained, indurated sandstones make the
best wearing course aggregates.
134
Natural aggregates exhibit considerable variability. For
example, a single granite quarry can contain
compositional variants, textural variants, hydrothermally
altered zones, weathered zones, mineralized zones, dykes,
metamorphic enclaves, sheared lithologies, cataclasite,
and tuffisite (French, 1994). This has important
consequences for the potential occurrence of deleterious
materials and for the investigation of these materials.
The soundness of a rock may be decreased through
natural weathering (
135
) or alteration (
136
). Geological
weathering typically results in an increase in porosity and
water absorption. Depending on the degree of weathering,
134
'Silver sand' consisting almost entirely of fine
quartz grains. As the sand was derived from the
weathering of granite there are traces of
contamination with tourmaline (orange); XPT, ×35.
135
136
135
Slightly weathered granite (weathering grade II)
exhibiting slight discolouration and slight weakening;
PPT, ×35.
136
Hydrothermally altered andesite (St Vincent,
Caribbean) showing chloritization of ferromagnesian
minerals. Chlorite appears green and blue; XPT, ×150.
