Low cost building materials prepared with ash of burnt agriculture waste and lime represent an alternative binder/construction system. Pure lime and Portland cement (OPC) are energy intensive to manufacture, and are expensive and scarce in developing countries. However, pozzolanic binders prepared by burning agriculture waste can be used as partial or complete substitutes for pure lime or OPC. These agricultural wastes, such as rice husks, wheat straw, sugar cane bagasse and sugar cane straw are widely available in many developing countries. Some types of clay are also pozzolanic after thermal treatment. The reactivity of the ash depends on its composition and on several factors involved in the burning process such as temperature, time, environment and cooling rate as well as chemical activation. This paper presents research focused on building materials produced by using lime combined with a pozzolanic mixture of thermally treated clay and ash from agricultural wastes. The study assesses the accelerating effect of sodium sulfate in strength development of building materials.