The flocculation of soils. II
Authors: Comber Norman M. 
Published: March 2009

Three types of mechanism whereby clay is flocculated by electrolytes, are discussed.

1. Normal or direct flocculation the mechanism of which is precisely that by which electrolytes coagulate electro-negative suspensoids. The isoelectric point theory of such flocculation is assumed in this paper. Experimental evidence is brought forward to show that iron and aluminium salts behave in this “normal” way.

2. Indirect flocculation which takes place whenever the electrolyte reacts with the clay and thereby brings into solution from the clay other substances which cause flocculation. The action of some neutral salts and of acids is regarded as being largely indirect.

3. Abnormal flocculation which is the result of a reaction between the added flocculant and the emulsoid surface of the clay particle. The action of calcium hydroxide is placed under this heading for reasons set forth in the earlier paper.

Because of its obvious bearing on important agricultural operations, the action of calcium hydroxide on clay is considered more fully. The following are the salient points of that consideration:

(i) Although it is reversible to carbonic acid the flocculation of clay by calcium hydroxide is not due to the formation of calcium carbonate. Several experimental results are set forth to show this, the chief is that barium and calcium hydroxides behave quite differently to carbonate and carbonic acid, and alike to clay.

(ii) The action of calcium hydroxide on clay is an action on an “emulsoid gel surface” and not an action on a sol present in gross amount.

(iii) The hydroxyl ion may perform at least two functions in its cooperation with the calcium ion in the flocculation of clay: (a) when added with or after the calcium ion it produces the alkalinity necessary for the reaction between calcium compounds and silica, etc., as discussed in the earlier paper; (b) when added before the calcium ion it not only produces this alkalinity but also peptizes the clay with the production of a greater amount of the emulsoid surface and a consequent increase in the rapidity of flocculation and the volume of the coagulum.

(iv) Critical mixtures of clay and silt—in which neither wholly dominates the system—can be obtained by decantation at an appropriate time during the sedimentation of soil suspensions. It is shown experimentally that when the clay of such mixtures is flocculated more rapidly than the silt, the silt then becomes dominant; and that when the silt is flocculated, more rapidly than the clay, the clay then becomes dominant.

(v) The different behaviour of “clay” and of “silt” to calcium hydroxide is not regarded as due to any essential difference in the structure cf these particles but as being determined by the ratio of the emulsoid surface to the core of the particle. In clay the surface dominates the system, in silt the core dominates the system.

(vi) In very low concentrations the hydroxyl ion appears to behave normally and to militate against the flocculation of clay by calcium salts.

(vii) Organic emulsoids may, in ordinary soils, be partly responsible for the anomalous action of calcium hydroxide on clay, but it is shown that calcium hydroxide exhibits that anomalous action on a deep subsoil clay containing no organic matter.