Wall Putty

Hello Ddp,

The reaction of lime and clays involves two distinct phases, modification and stabilisation. Modification occurs very quickly after mixing and involves cation exchange between the calcium ions from the lime and the ions on the surface of the clay particles. This causes a change in the overal surface charge on the particles which changes their electrostatic attraction and causes flocculation, resulting in improvement in (among other things) workability and permability.

The second phase, which is the most important phase for an application such as wall putty, is called stabilisation. Stabilisation proceeds over a much longer period and is very similar to the pozzolanic reaction that occurs between some types of flyash and lime. In this reaction lime combines with the silica and alumina components at the plate edges of the clays to form calcium silicate hydrates (CSH), calcium aluminate hydrates (CAH), and calcium alumino-silicate hydrates (CASH) which form cementitious gels similar to those produced by hydrating cement.

The amount of lime added to the clay must therefore be sufficient to satisfy the lime demands of both phases for full strength potential of the mixture to be achieved. In addition, enough lime must be present to keep the pH of the mixure above 12.4, because this improves the rate of reaction by speeding up the dissolution of the silica and alumina from the clays.

The most reactive clays are those with the smallest particle size and highest content of alumina and silica. Clays such as kaolinite or montmorillonite are therefore ideal.  The cementitious reaction is also temperature sensitive, proceeding faster and more completely as the ambient temperature increases.

Regards,

Ted.