|The Use of Selected Indigenous Building Materials with Potential for Wide Application in Developing Countries (HABITAT, 1985, 80 p.)|
|V. THE PRODUCTION AND USE OF SELECTED INDIGENOUS BUILDING MATERIALS5/|
97. Natural pozzolanas are basically of volcanic origin and are usually found in areas which have experienced volcanic activities. For example, in Africa, natural pozzolana deposits can be found in six countries -Burundi, Cameroon, Caper Verde, Ethiopia, Rwanda and the United Republic of Tanzania (see annex III for a brief description). Pozzolanas of this type occur either in a pulverized state or in the form of compact layers, and this, in turn, determines the production process which the pozzolana has to undergo before being mixed with lime to produce a binder.
98. Where volcanic tuff occurs as a naturally fine-grained material, it requires no preparation apart from ensuring that it is sufficiently dry prior to mixing with lime. Sun-drying is feasible, even though a small-scale, locally fabricated kiln can be used for this purpose. For example, the Arusha-Moshi area of the northern part of the United Republic of Tanzania is volcanic, and large deposits of fine-grained pozzolanas are widely available. These deposits which require no grinding after quarrying can be mixed with lime to prepare mortars, plasters and building blocks.
99. Where the natural pozzolana occurs in a coarse-grained form, it is desirable to dry the material, either in the sun or a kiln, and, thereafter, grind it in a ball-mill to the desired fineness, ready for mixing with lime. In some instances, the grinding of coarse-grained pozzolanas is restricted to the preparation of mortars and plasters, while the preparation of blocks is feasible without any grinding. For instance, in Lembang, Indonesia, unground coarse-grained pozzolana is mixed with 20 per cent lime and sufficient quantities of water to produce solid blocks for building construction.