Making building products by extrusion and cement stabilization: limits of the process with montmorillonite clay

The addition of cement to a montmorillonite clay does not result in an adequate stabilization of end products which are extruded. This can be explained as follows: firstly, Ca2+ ions of the binder are adsorbed by montmorillonite preventing the hydration of the binder and secondly, montmorillonite is very susceptible in contact with water because of its expansive property. Other mixtures have been tested by substituting progressively montmorillonite by kaolinite which has been successfully used in some previous research works. A `stabilization thresholdʹ—corresponding to the minimum amount of montmorillonite and defined by a water resistance criterion—allows to determine under which conditions extrusion is possible. It appears that clay masses containing more than 5% montmorillonite cannot be stabilized with economically justified amounts of cement. Silico–aluminous fly ash has been used in such cases in order to benefit from the filler and possibly pozzolanic effects of this additive. The use of fly ash allows also to contribute to the problems of waste elimination. The results show that the products containing fly ash have a good dimensional stability, water resistance and mechanical properties. A small quantity of montmorillonite (5%) acts in such a case as a plasticizer.