GEOLOGIC INVENTORY OF NORTH ISLAND AGGREGATE RESOURCES:

1
GEOLOGIC INVENTORY OF NORTH ISLAND AGGREGATE
RESOURCES

• Influences on Engineering Materials Properties

Philippa Black, Geology, School of Environment,
University of Auckland
Funded by FRST contracts UOAX 402 407 Mineral
Wealth of North Island
2
TWO PARTS to the Report

• PART 1
• Reviews physical and engineering properties of
  rocks and component minerals used as aggregates
  and
• Tests used to specify aggregates for roading in
  NZ what are these tests actually measuring and
  what do they really tell us?
• PART 2
• Geological inventory of North Island Aggregates
  criteria used are strength (CR) and
  durability/soundness (SE, CI, PI).

Report can be downloaded from the following
websites http//www.aqa.org.nz/documents/NI20GE
OL20INVENTORY20.pdf http//www.sges.auckland.ac.
nz/about_us/our_people/black_philippa/ni_geol_inve
ntory.pdf
3

• AGGREGATE TESTS GENERAL COMMENT 1
• Many tests used to specify aggregates in NZ were
  developed overseas and adapted for NZ conditions.
• Overseas aggregate resources (eg Australia, UK,
  Continental Europe, USA-Canada, Southern Africa)
  are all old continental-type rocks. They are
  dominantly limestones
• plutonic rocks (granites, gabbros etc) or
• high grade (ie high Temp) metamorphic rocks
• These rocks and the minerals they contain are
• generally anhydrous and
• they do not contain swelling clay minerals

4

• AGGREGATE TESTS GENERAL COMMENT 2
• In contrast .. NZ aggregates are sourced
  (approximately)
• 75 from young weakly metamorphosed greywackes
  
• 25 from young volcanic rocks often altered
• NZ aggregate resources are young hydrous rocks
  and many of them contain swelling clays which
  result either from
• low levels of lithification / metamorphism
  (greywackes) or
• alteration in the eruption environment
  (volcanic rocks)
• In NZ, aggregate durability (soundness) is
  directly related to the presence and the nature
  of the clays in the rock.
• Thus it is unwise to use any tests to predict of
  aggregate durability / soundness if they are
  carried out only in the dry state.
• (US Example of WASDOT tests versus ASTM or
  AASHTO tests)

5
The two most important properties of any material
being considered for use as an aggregate are
STRENGTH and DURABILITY Strength (crushing
resistance) related to nature of the
constituent minerals and how they are bound
together. Durability mechanical and
physical-chemical changes while in service. The
most commonly used tests to predict the
durability of both fine and coarse aggregate are
sand equivalent (broadly related to rock
strength) and clay index (amount and nature of
clay content).
I will make comments on 5 aggregate tests used
in NZ
6

• CRUSHING RESISTANCE measures toughness
  hardness of aggregate
• (toughness and hardness are not the same
  property !)

Corundum
Quartz
Feldspar
Calcite
Relationship between Mohs (scratch) number and
values of hardness as measured by the Vickers
microhardness (indentation) and Depth Sensing
Indentation (DSI) methods (left) and relationship
between absolute values of toughness and
microhardness (right). Minerals representing
the points on the Mohs scale are 1 talc 2
gypsum 3 calcite 4 fluorite 5 apatite 6
orthoclase 7 quartz 8 topaz 9
corundum. Diagrams modified from Bronz et al
(2006). American Mineralogist 91, 135-142.
7
CRUSHING RESISTANCE (CR) Percentage of
fines produced either by a specified load (NZ
4407) or the load which produces 10 fines (NZS
3111).
corundum
Quartz hardest / toughest common mineral in
aggregates Is a major component of sand grains
in greywackes but rather rare in volcanic
rocks. CR distinguishes between volcanic rocks
and greywackes (also types of greywackes and
volcanic rocks) CR is a first order (source )
property Material Strength (CR) has a flow-on
affect on other aggregate test properties such as
particle size distribution, compaction
capability, SE, aggregate cleanness, weathering
quality Index etc.
Quartz
feldspar
pyroxene amphibole
clays
Rocks are natural materials and as such have a
range of properties. Aggregate is a processed
material Processing / crushing methodology is
used to concentrate the toughest / hardest
particles in the product.
Data for olivine from Evans and Goëtze, 1979 J
geophysical research 84 (B10) 5505-5524
magnetite Wen and Liu (1987) J Materials Science
Letters 6, 1057 volcanic glasses from Ben
Abdelounis et al, 2009. Wear 266, 621.
8
Greywackes (c. 75 of NZ aggregates) - Clastic
grains are physically and chemically the most
stable / strongest component of the rock The clay
matrix is physically and chemically the weakest
component of the rock. Volcanic rocks (c. 25 of
NZ aggregates) - Olivine, magnetite and
volcanic glass are physically the strongest
components. Volcanic glass and olivine
chemically the weakest components (both readily
hydrate to form clays). Major constituents of
volcanic rocks (feldspars, pyroxenes and
amphiboles) have similar toughness/ strength
(which is much less than quartz).
9
In the case of sediments such as greywackes the
most important component of the rock as far as
providing strength and durability is the
matrix. The matrix cements clastic grains
together The nature of the matrix (clay versus
mineral cement) and the type of clay minerals (ie
level of diagenesis /metamorphism) determines
rock strength and durability.
10

• 2. Repeat Load Triaxial Testing
• Test is used to estimate stiffness and resilient
  modulus of compacted aggregate sample.
• Aim of test is to place aggregates into
  performance bands.
• These bands will largely reflect aggregate
  type
• Results will be affected by rock strength /
  toughness (ie mechanical properties of
  material), grainsize distribution and shape (of
  sample and its fatigue products).
• Some components of rocks (particularly of
  greywackes) are not brittle
• Layer silicates (clays micas etc) are the weakest
  component of aggregate
• Layer silicates, when stressed deform by the
  individual layers sliding over each other - ie
  react as semi ductile material
• Some layer silicates are semi-elastic
• While test may inform about resistance to wear
  (mechanical breakdown) it will not inform about
  any physical-chemical changes that may occur with
  time i.e. provides no information about
  soundness.

11
3. WEATHERING QUALITY INDEX (WQI) Test
essentially takes a sized sample (19mm to 4.75mm)
and subjects it to cycles of wetting, drying and
rolling (abrasion under a light load) followed
by boiling, drying and weighing to determine the
percentage reduction in size (ie the weight of
material that as a result of the test passes
4.75mm sieve). Uses the rolling force and
expansion / contraction forces of any expanding
clay minerals to physically disaggregate the
material. The WQI test has nothing at all to do
with weathering! Many unweathered greywackes
contain expanding clay minerals because they have
not been metamorphosed to a sufficiently high
Temp to destroy clays ie the matrix of the
sediment is poorly lithified. While
weathering initially increases the amount of
smectite in most rocks, the ultimate product of
weathering is kaolin (which is non-swelling). Mul
ti stage processing can change an aggregate WQI
from CB to AB!!!!!! WQI IS NOT AN AGGREGATE
SOURCE PROPERTY.
12

• SAND EQUIVALENT
• Test developed in the USA to detect deleterious
  minerals (clays) in aggregate fines.
• Where rocks are anhydrous the presence of clay
  minerals indicates weathering.
• In a standing column (test cylinder) settling
  velocity is a function of size, shape and mass
  (volume x density) of the settling particles.
• The fine fraction remaining in suspension (after
  20 min) is actually the clay size range it may
  contain clay minerals or may contain only rock
  flour.
• Clays density c. 2.7 2.9 platy shape and
  very high surface area
• The common components of rock flour tend to be
  equidimensional.
• quartz and feldspar density 2.65
• tridymite / cristobalite density 2.3 (common
  component of andesites/dacites)
• zeolites density 2.1 2.3, (occur in
  greywackes and volcanic rocks)
• In some NZ aggregates rock flour may contain
  material that is very low density and thus will
  remain in suspension for long periods.
• If the ratio of rock flour deleterious minerals
  in fines is wanted - this the test is not doing
  it for NZ aggregates perhaps the test should
  be modified!

13

• CLAY INDEX (CI) AND PLASTICITY INDEX (PI)
• Measure complex structural and chemical
  properties of clays and some other common
  minerals. CI and PI are not measuring the same
  properties.
• CI is measuring surface area (methylene blue is
  held onto surface by a cation exchange process)
  swelling clays and zeolites both have very high
  cation exchange capacity and surface areas (but
  most zeolites are non swelling). The presence of
  zeolites (common in some types of greywackes and
  volcanics) will elevate CI values
• PI (Atterberg limits) kaolins and some micas are
  highly plastic but non swelling zeolites are
  non plastic.

An Omission in the test portfolio? Swelling no
test specifically measuring swelling / shrinkage
of aggregate chips although shrinkage is very
important in the concrete industry. J.
Mackechnie Shrinkage of concrete containing
greywacke aggregate ACI Materials Journal,
Sept/Oct 2006 pp 390-393.
14
SUMMARY / CONCLUSIONS

• Tests should be carried out wherever possible on
  aggregate in the moist or saturated state.
• Most aggregate test results are strongly affected
  by processing / production methodology.
• Thus it is quite difficult to use test results to
  predict the long-term performance durability /
  soundness of an aggregate if the processing
  methodology is unknown.
• There are between-type differences
• eg volcanic versus sedimentary rocks) which have
  quite different sets of properties and
• and within-type differences
• e.g. greywackes with different degrees of
  lithification, and different types of volcanic
  rocks
• which suggests caution in using universal terms
  to describe aggregates.
• There is scope for review of some tests, for
  example
• Weathering Quality Index has nothing to do with
  weathering!!
• The Sand Equivalent test needs modification to be
  useful.