Specifications

The chemistry of gypsum

Introduction

Pure gypsum consists of hydrated calcium sulfate: CaSO₄.2H₂0.

Chemistry
% of calcium in pure gypsum:	23.3%
% of sulfur in pure gypsum:	18.6%
% of oxygen (in sulfate) in pure gypsum:	37.2%
% of fixed water in pure gypsum:	20.9%

Laboratories often assay for sulfur and convert the reading to gypsum.

Thus percent S x 5.38 = percent gypsum
and percent Gypsum x 0.186 = percent sulfur.

It is not legitimate to calculate the purity of gypsum from the calcium content, as calcium may be present also as lime, CaCO₃.

The fixed water (water of crystallization, the 2H₂O in the gypsum molecule) should not be confused with the moisture content of gypsum. Fixed water can be driven off between temperatures of 45^oC and 128^oC. When this happens, Plaster of Paris, CaSO₄.0.5H₂O is formed. On the other hand, all moisture in gypsum can be removed by heating up to 40^oC. All assays are carried out after moisture is removed. Figures are for dry gypsum.

Impurities

Gypsum is a natural material, found in a number of geological environments, for example dunes and lake beds. Some even occurs as solid rock. Therefore various impurities may occur within it, for example, lime, clay, sand and salt. It is possible to work out which impurities are present by analysing the gypsum.

Gypsum Regulations

In South Australia four grades of gypsum are recognised.

Grade
Premium	Minimum 90% gypsum or 16.7% sulfur
Grade 1	Minimum 81% gypsum or 15.0% sulfur
Grade 2	Minimum 67% gypsum or 12.5% sulfur
Grade 3	Minimum 54% gypsum or 10.0% sulfur

Properties
Salt content	NaCl less than 2%, (Na < 0.8%) otherwise warning required
Moisture	The moisture content must be less than 15%
Fineness	At least 50% of product must pass through a 2mm screen At least 80% of product must pass through a 5.6mm screen
Heavy metals	Should be less than 0.001% for cadmium Should be less than 0.01% for lead

These standards essentially conform with those of Victoria and New South Wales. However these eastern States of Australia do not define a Premium Grade.

Consistency

We have observed that some of our competitors are inconsistent in their bag labelling. Beware of bag labelling that claims 99% gypsum and Premium Grade when the sulfur analysis does not back this up. The ratio of gypsum content to sulfur content should be 5.38 to 1 and no higher.

The high purity and fine grain size of MENINGIE GYPSUM sets it apart from other gypsums, whether supplied in bags or bulk.