The use of red tropical soils as pozzolanas: reactions, products and properties

Nineteen red tropical soils from the States of Paraiba and Pernambuco which are located in the North East of Brazil were treated with high calcium lime, they reacted showing a wide range of variation of maximum unconfined compressive strengths with - 4.5% or 6.0% lime generally required to produce maximum strength. The reaction between the soil and lime is mainly a diffusion controlled process. Most of the reaction takes place within five to seven days and the reaction within this period obeys Jander's diffusion equation for solid state reactions. Based on the constant rate of reaction obtained from this equation and on the quantity of the reactive components of the soil, an activity index, Ai, is proposed and its validity tested against the unconfined compressive strength of the soil-lime system. A valid statistical correlation is presented linking Ai value with the empirical reactivity parameter of Thompson. Identification of the types of the reaction products by x-ray diffraction which is greatly aided by the use of a semiautomated computer search program reveals that the soils form cementitious reaction products varying from dense and close textured calcium silicates of the tobermorite-like type through alumino-silicates to calcium alumino-ferri-silicates with relatively larger voids when the soils are treated with lime and cured for two years at 22°C. The increase in strength of the soil-lime system not only depends on the volume of the reaction products but also on their types and purity. Other constituents of the soil-lime system affect the strength to varying degrees. While the increase in the amount of unreacted material at the expense of cementitious reaction products is not desirable, an amount of unreacted material is essential for other engineering requirements other than strength to be satisfied. Unlike other cemented composites, the strength of the soil-lime system is not related to the overall porosity of the system.