| Boiling Point No. 31 - August 1993 |
Summary of a manual on Clay/Non Clay Ratio Measurement Techniques (approx 40 pp.) by Dr Messer, Dr Gaspe and Peter Young which will shortly be published by ITDG
All clay minerals show characteristic weight losses due to water loss when heated in the temperature range of 400°C to 700°C. This dehydroxylation weight loss for a given mixture of clay minerals should be constant and is a function of the amounts of the individual clays present and their characteristic weight losses.
Based on this, a simple test method can be established to determine the clay/non clay content of natural clays. The clay minerals, which are sub 2pm fraction, can be separated from the raw clay and can be used as a reference mixture to obtain the characteristic dehydroxylation weight loss for the particular mixture of clays. This requires the mixture of the clay minerals in the separated fraction to be the same as that in the raw clay material. The ratio of the dehydroxylation weight loss for a raw clay to that of the separated clay fraction gives the clay mineral content of the given clay.
The test procedure consists of:
1. Separating the sub 2pm particles ie. the clay fraction, by a sedimentation process based on Stokes Law*.
2. Samples of raw clay and the sub 2pm particles are air dried and fired separately at 375°C for 8 hours to burn off any organic matter.
3. About 200gm of each sample is weighed and then fired 700°C for 3 hours. The samples will loose weight due to dehydroxylation.
4. The dehydroxylation loss for each sample is then calculated by subtracting their initial and fired weights.
5. The clay content can then be calculated from the percentage ratio of the dehydroxylation weight losses from the raw clay divided by the dehydroxylation losses from the clay fraction.
* Stokes Law deals with the settling of individual spherical particles falling freely at a steady velocity under the influence of gravity, resisted only by the viscous drag of the medium. It provides the basis for a formula to calculate the time required for particles of different sizes to settle.
C/NC Ratio Calculation
- weight of the raw clay after heat treatment at 375°C - W1
- weight of the clay fraction after heat treatment at 375° - W2
- weight of the raw clay after heat treatment at 700°C - W3
- weight of the clay fraction after heat treatment at 700°C - W4
- calculation dehydroxylation weight loss of raw clay - (W1-W3)
- dehydroxylation weight loss of clay fraction - (W2-W4)
- clay mineral content of raw clay (natural clay body) X% = (W1-W3)/(W2-W4) x 100
- the non-clay mineral content = (100-X)%
- the clay/non clay ratio = X:(100-X)