The sand to be used for ferro-cement is the same as is used for waterproof cement plaster applied to bricktanks. The sand has to be clean and well graded. This means having grains of many sizes, but 90% should still pass through a mosquito wire sieve. Sand must be clean, because like water it may have some impurities that weaken the cement bond, such as clay, silt and organic matter. Dirty sand can be washed by repeatedly rinsing with water. This should be done on a well-prepared sloping ground by pouring water on the sand while turning it with shovels. The dirt must run off, otherwise the effect is minimized. If sand has to be washed afterwards, it is too wet for ferro-cement plaster and should be given time to dry partly while turning it with shovels three times a day. There are two easy field tests for determining if the sand needs to be washed:
1. Rub a moist handful of sand between your palms. Suitable sand will leave hands only slightly dirty.
2. Fill a clear glass container 100 mm high with sand. Then fill with water. Shake the glass vigorously, place it on level ground and leave undisturbed for one hour. The sand settles immediately and any silt and clay settle as a dense layer on top. This layer is of another colour than the sand, often darker, and should not be more than 6% of the entire thickness of sand (Fig. 3.1). If you have had 100 mm sand, a 6-mm upper layer of silt or clay is acceptable; if it is more, the sand has to be washed.
If sand is not taken at the site but supplied, so-called river sand is likely to meet the quality demanded. Sand has to be stored close to the mixing area on cleared ground. Before the sand can be used for mixing, it has to be sifted. For this purpose a special sieve has to be made out of galvanized gauze wire supported by chicken wire (see Fig. 3.2). The frame of the sieve must be of boards about 100 mm high so that all material which does not pass the sieve remains on top and cannot fall onto the sifted sand. The sieve is to be used by shaking it. Two men hold the sieve, while another shovels sand onto it, not more than three or four shovel fuls at a time. The two men then shake the sieve backwards and forwards. The clean material will fall through the sieve. The rest has to be put aside in such a way that it cannot accidentally be mixed with either of the two piles of sand, the sifted or the unsifted one. Since shaking the sieve is hard work, it has been observed that people start to make mistakes after some time. It should therefore be made clear in advance that the sifting crews are to be changed. Sifting sand and preparing the mixture are just as important as plastering. The final product, the ferro-cement tank or the plaster of the bricktank, depends very much on the care taken by the staff preparing the mixture and in charge of the material.