The calcium carbonate content of limestone rocks has been used from the earliest civilisations, dating back to 14,000 BCE, to its extensive use in modern times. It is a valuable resource that services the needs of a multitude of industries.
Annual New Zealand usage of limestone in the year 2010 was:
- marl and cement – 2 million tonnes
- agriculture – 1.9 million tonnes
- industry and roading – 874,460 tonnes.
Building and construction
Depending on the quality of the deposit, limestone can be quarried to service the needs of the building and construction industries. For example, certain types of limestone such as that found in North Otago can be cut into blocks suitable for direct use as building stone (Oamaru stone). In road construction, limestone aggregate direct from the quarry can be used as base material.
Lower-grade limestones, particularly ones with some clay content, can be used in the production of cement for use in the building and construction industry. Cement is a key ingredient of concrete, huge volumes of which are poured each day to serve as roading, foundations, structural beams and columns.
Concrete is a hardened material that forms when a mixture of cement, sand, crushed stone and water is poured into moulds or formwork and allowed to cure or harden. Cement is made by strongly heating a mixture of limestone, silica and clay in a kiln until small pea-sized lumps called ‘clinker’ form. This is then ground to a powder, the key ingredients of which are dicalcium silicate, tricalcium silicate, tricalcium aluminate and calcium oxide.
Kiln reactions include:
| CaCO3(s) in limestone | → | CaO(s) | + | CO2(g) |
| CaO(s) | + | silica | → | calcium silicates |
| CaO(s) | + | clay | → | calcium aluminates |
When water is added to dry cement powder to make a stiff mix, a complex series of chemical reactions (hydration) occurs, which begins the hardening process. Over time, the hardening process continues until full strength is obtained.
| 2(3CaO.SiO2)(s) | + | 6H2O(l) | → | 3CaO.2SiO2.3H2O(s) | + | 3Ca(OH)2(aq) |
| tricalcium silicate | water | hydrated tricalcium silicate | calcium hydroxide |
New Zealand’s total cement production in 2010 was approximately 1.1 million tonnes.
Agricultural use
Limestone deposits with a calcium carbonate content around 90% can be quarried to be used by the farming sector. By crushing the limestone, various grades (based on particle size) can be produced. These products are marketed as ‘agricultural lime.
Soil acidity is one of the factors that can influence plant growth and can seriously limit crop production. By spreading agricultural lime onto the paddock or soil, the calcium carbonate content of the limestone is capable of neutralising some of the acid in the soil. This also has the effect of freeing up some of the soil minerals, such as phosphates, and making them available for absorption into the plant.
| CaCO3(s) in limestone | + | acidic soil→ |
| CO2(g) | + | soluble calcium salts | + | higher soil pH |
Heavy industries
In the steel production process carried out at the Glenbrook plant south of Auckland, large amounts of limestone and lime are needed to remove impurities during the iron ore reduction phase and the steel production phase. The Ōtorohanga region supplies these much-needed ingredients.
Calcium oxide, obtained from limestone, acts as a pH regulator when extracting gold from quartz. The chemical extraction of gold from powdered quartz rock uses a solution of sodium cyanide. In this cyanidation process, the pH of the solution needs to be maintained between 10–11 to prevent the production of deadly hydrogen cyanide gas. The addition of calcium oxide to the solution maintains the pH 10–11 level.
90% of manufactured glass is called soda-lime glass. Its production requires the use of calcium oxide (lime) obtained from limestone.
Fillers in manufactured goods
Very high-grade limestone deposits with a calcium carbonate content over 95% are found in the Te Kuīti-Ōtorohanga region. This limestone is quarried and ground to a variety of sizes with diameters ranging from a fraction of a micron to several microns (1 micron = 10-6 m). These fine powders find a multitude of uses as fillers in manufactured goods such as paints, plastics, cleansers, paper and carpets.
