Cleanliness of Sand

Impurities and contaminants in sand can cause staining within the finished mortar, or weaken it. The most simple test is to squeeze the sand between the fingers; if a stain remains, the sand should not be used.

The ‘Field Settling Test’ is a more scientific assessment that can be carried out on site to give a guide to the amount of silt in natural sand. Reasonable accuracy can be achieved with a straight-sided glass, vase or a 500g jam jar, plus a measuring tape to calculate the results:

1. Fill the glass or jar up to a height of about 50mm with a salt-water solution (one teaspoonful of salt to 750ml of water).

2. Pour in sand up to a level of 100mm.

3. Add more salt solution until the level rises to 150mm.

4. Shake the sand and salt solution well.

5. Stand the glass, vase, or jar on a level surface and tap it until the top of the sand is level.

6. Leave it to stand for three hours, during which a layer of silt will be clearly seen to form on top of the sand.

7. Measure the height of the silt layer and the height of the sand layer.

The silt content percentage is calculated by taking the height of the silt layer and dividing by the height of the sand, then multiplying the result by 100.

The silt content should be not more than 8 per cent. If it is more than 8 per cent, the sand should not be used, as excess silt or clay will inhibit a good bond between the cement and aggregates, causing weaknesses in the finished mix. The example in Fig 22 has a huge silt content, clearly more than the 8 per cent minimum, making this sand unsuitable for use in mortar.

For an even more accurate assessment, use a 250ml measuring cylinder and substitute volume for height when measuring out the ingredients for the test and for calculating the silt content.

Cleanliness of Sand

Fig. 22 Result of a field settling test carried out on soft sand.