Story: Rock, limestone and clay
Page 5 – Cement, marble and dolomite
Limestone for cement production
Cement, used to make concrete, is a mixture of two main materials – lime and marl (a calcareous clay). Cement was imported until local cement works were established. In New Zealand it was first produced near Warkworth, and in the 1850s was used in the Queen Street sewer and other Auckland construction projects. It was not until manufacturers could produce high-quality cement that they could compete with imports.
Limestone deposits close to rail links or deep-water ports were ideal for cement manufacture. They were quarried at Whāngārei, Tarakohe in Golden Bay and Milburn in Otago. The names Golden Bay and Milburn became synonymous with cement.
In the early 2000s cement was produced at only two plants – one at Cape Foulwind, near Westport, the other at Portland, near Whāngārei. Their combined output approached 1 million tonnes per annum.
For driveways, garden paths, boxing walls, or securing fence posts, concrete is the wonder material. Many Kiwi men, living in the suburbs that sprang up in the 1950s and 1960s, owned concrete mixers. These churn a combination of cement, sand, gravel and water, producing a concrete mixture ready for use. Politician Tim Shadbolt used his mixer as a campaign mascot, towing it behind his car.
Marble is metamorphosed limestone. This means that it has been buried deep within the earth and subjected to great heat and pressure, which alters and hardens the rock.
Large marble deposits are found in north-west Nelson and Fiordland. The main uses are as a building stone and, when crushed into a powder, as a filler and surface coating in various industries.
The Ngārua quarry on Tākaka Hill in Nelson produced between 15,000 and 20,000 tonnes of marble each year until 2003. In years past, Ngārua glass-grade marble was shipped from Port Motueka on the coastal scow Mamaku at the rate of 6,000 tonnes a year, bound for Auckland glass works. With its high calcium carbonate content, Tākaka Hill marble was also ideal for agricultural use – making soils less acidic, improving soil structure and assisting nutrient uptake.
Dolomite is essentially limestone with a high magnesium content. Mt Burnett, which overlooks Collingwood in Golden Bay, is the only source of dolomite in New Zealand. In the early 2000s, 25,000–40,000 tonnes were produced per annum, mostly for use as an additive to phosphate fertilisers.
Dolomite blocks from Mt Burnett have been barged across Cook Strait. Their distinctive brown weathering sets them apart from the rocks of Wellington, and massive blocks can be seen at Wellington Harbour, where they act as breakwaters. They are also used to prevent the Hutt River overflowing its banks.