Quarrying & Mining Magazine

Thank you dead diatoms

This article first appeared in Q&M‘s February- March issue.

ALAN TITCHALL reviews the historic beginnings of the silica extraction industry in the Rotorua district on the Taupo Volcanic Plateau.

In February 1972 Road Transport & Contracting magazine, the precursor to Contractor magazine, featured an article on a quarry operation at Ngakuru, near Rotorua.

It built the Colosseum

Pozzolanic concrete was used by the ancient Greeks and Romans long before the birth of Christ and both the Roman Pantheon and the Colosseum were built of it.
Pozzolan derives its name from the town of Pozzuoli in Italy where the ancient Romans found a volcanic ash that formed cement with hydraulic properties when mixed with lime putty.
Instead of just evaporating slowly off, the water would turn this sand/lime mix into a mortar strong enough to bind lumps of aggregate into a load-bearing unit. This made possible the cupola of the Pantheon, which is still the world’s largest unreinforced concrete dome.
Today other natural or man-made siliceous or aluminous materials, such as fly-ash (sourced from coal burning at the Huntly power station), give concrete the same attributes and are still known as ‘pozzolans’.

The diatomite at Ngakuru, near Rotorua, was mined and made into what was called ‘pozzolan’, which is used in concrete manufacturing. When added to concrete, pozzolan produces an acid-resistant product that is ideal for cow sheds and fruit processing plants (plus it makes concrete easier to pump).
The fine graded pozzolan particles also fill the microscopic voids between the cement particles, improving “packing” and creating a less permeable concrete microstructure (and the reason why pozzolan concrete is popular for making boat hulls). The fine particle size also reduces concrete bleed and helps reduce the interface layer between aggregate and cement paste, improving general strength and durability.
In 1972, 1300 tons of pozzolan was railed to New Plymouth for the construction of the chimney at the power station built on the edge of the harbour. The power station has since been demolished but the chimney has been left standing as a local ‘icon’.
The Ngakuru quarry near Rotorua is still operating, as are a few other quarries (see Alice and the Tikitere Quarry). The difference is that the silica is made from single-celled plants related to algae, whereas other geo-silica deposits in the region have been produced by volcanic activity.
These days Kiwi ‘pozzolan’ from the Rotorua region is marketed under different ‘brands’, such as Microsilica 600 and is joined by a number of other natural, or man-made, siliceous or aluminous materials, such as fly-ash, to provide concrete with the same ‘pozzolan’ attributes.

The beginnings of Ngakuru

The old government Ministry of Works department carried out investigations in the 1950s to find a suitable material to counter the problem of alkali reactivity in concrete aggregates used in major hydro dam construction along the Waikato River.
Experimental work initiated by the Ministry of Works began in 1955 and the Ngakuru diatomite resource was found to be the best in the region.
In 1957 the Nelson mine and quarrying company – Lime & Marble – in partnership with Wilkins and Davies Construction set up a joint operating company called Pozzolan Products to operate the original Ngakuru quarry and its resource covering 40 acres.

Water challenge

An inherent characteristic of all diatomite deposits is the high free water content (about 300 percent), which makes mining almost impossible in wet weather.
The deposit was partly dried by rotary hoeing or discing the upper three inches to let air in and make the material workable. It was advantageous to mine the diatomite in summer and stockpile it under cover in big storage areas to keep the plant going through the winter.
The material was then put through a pneumatic dryer that was designed and built with the help of the chemical engineering section of the old DSIR government agency. The material, with less than five percent moisture, was then fed into a ball mill to produce a material as fine as talcum powder (the pozzolanic properties of diatomite are dependent to a large extent on the particle size – in general, the finer the particle – the more effective it is).
The pozzolan was then either bagged in five-ply multiwall paper bags, or blown into bulk silos, for loading into a fleet of three tankers that distributed the material throughout the central North Island.

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