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Wollastonite is a calcium inosilicate mineral (CaSiO3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium.

Wollastonite is a calcium inosilicate mineral (CaSiO3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. The mineral is usually white and is formed when limestone and dolomite are subjected to high pressure and temperatures. It is available in two powder grades of fineness.

Some of the properties that make wollastonite so useful are its high brightness and white colouration, low moisture and oil absorption, and low volatile content. It is used primarily in ceramics, friction products, metalmaking, paint filler and plastics. Despite its chemical similarity to the compositional spectrum of the pyroxene group of minerals where magnesium and iron substitution for calcium ends with diopside and hedenbergite respectively — it is structurally very different, with a third SiO4 tetrahedron in the linked chain (as opposed to two in the pyroxenes).

The mineral is used in many industries and is commonly used to improve the performance of properties in ceramic tiles. It is a preferred additive due to its fluxing properties, freedom from volatile constituents, whiteness, and acicular particle shape. In ceramics, wollastonite decreases shrinkage and gas evolution during firing, increases green and fired strength, maintains brightness during firing, permits fast firing, and reduces crazing, cracking and glaze defects.

The mineral is also used in cement, serves as a flux for welding, a source for calcium oxide, a slag conditioner, and to protect the surface of molten metal during the continuous casting of steel.

As an additive in paint, it improves the durability of the paint film, acts as a pH buffer, improves its resistance to weathering, reduces gloss, reduces pigment consumption, and acts as a flatting and suspending agent. Its high pH is useful in maintaining the desired alkaline level of water-based coatings and it is an excellent extended pigment for powder coatings as well as other coatings.

Pigment volume concentrations (PVCs) in the range of 38% to 45%, and a PVC/CPVC ratio of 0.6 to 0.9, showed the best overall coating performance when the extender package included 67,5kg per 378 litres of treated wollastonite and the anti-corrosive pigment level was 4,5kg per 378 litres.

Wollastonite has a brightness value between 85% and 90% when compared to magnesium dioxide and barium sulphate. This extreme whiteness has minimal effect on a product’s colour and it can be used as a white pigment.

In plastics, wollastonite improves tensile and flexural strength, reduces resin consumption, and improves thermal and dimensional stability at elevated temperatures. Surface treatments are used to improve the adhesion between the wollastonite and the polymers to which it is added.

It is important for Wollastonite to be processed, as a range of particle sizes and rations are required. Garnet and diopside are removed by highintensity magnetic separators and the mineral can be chemically treated to improve binding in resin-based products. The removal can be tested in a lab by adding dilute hydrochloric acid and seeing if it foams. c

WOLLASTONITE PROPERTIES

Chemical Formula: CaSiO3

Composition: Calcium silicate

Colour: White, beige, grey, light yellow

Hardness: 4.5-5

• Wollastonite is slightly alkaline in nature which can help to assist in PH stabilisation of aqueous system

• It can be offered as a generalpurpose filler with a clean bright colour

• The acicular shape of wollastonite recommends it as a low-chalking reinforcing extender in exterior latex paints

• It has also been proposed as a replacement for asbestos in some applications such as asphalt-based coatings

• Synthetic calcium silicates are also produced for use as pigment extender. Exhibiting a high oil absorption of 70 – 80 ml/100 g, the fine particles contain enclosed voids, which are claimed to contribute to opacity in waterbased emulsion paints.

PROPERTIES

Chemical name Calcium metasilicate

Specific gravity 2.9 Kg/L

Refractive index 1.63

Oil absorption 25-30 ml/100 g

Average particle size 1 – 30 µm

Mohr hardness 4 - 5

Particle shape Acicular

Main Source Canada, India and USA

Image Credits: Omega Fine Products

Sources: https://en.wikipedia.org/wiki/ Wollastonite

https://www.naturalpigments.com/ wollastonite.html

https://pubs.usgs.gov/fs/fs-0002-01/fs- 0002-01textonly.pdf