Pozzolans

Abstract

It is well understood that partial replacement of portland cement with pozzolans generated as industrial by-products, such as fly ash and silica fume, have advantageous implications on the long-term performance of concrete. However, the production of the most widely available and exploited pozzolan, fly ash, is predicted to substantially decrease in the future due to the termination of coal-fired power stations around the globe. Therefore, it is crucial to explore pozzolans such as natural pozzolans and ground glasses to meet the burgeoning demand for sustainable cementitious materials. It is of a great significance to precisely assess the reactivity of pozzolans before using in major applications in the construction industry. The existing reactivity tests: strength activity index (ASTM C311/618) and lime-reactivity (CSA A30004 E1) are unreliable in evaluating pozzolans because of either inconsistent or inadequate mixture proportions. The Canadian standard test based on lime-reactivity, CSA A30004-E1, was modified by investigating four mixture designs comprising of combinations of pozzolan, portlandite (hydrated lime), calcite, and either alkaline or sulphate solution. Thermogravimetric analysis (TGA) and compressive strength tests were performed to determine the optimum portlandite proportion. The mixture design resulting in the highest compressive strength in mortars and bound water in pastes was used to establish the highest degree of the reactivity of pozzolans. This mixture design was further tested by modifying the curing regime: solution, temperature and duration. A broad-range of cementitious materials including industrial by-products, natural pozzolans and ground glasses were investigated in concretes for compressive strength, chloride permeability and migration coefficient, electrical surface and bulk resistivity, and bulk electrical conductivity. Mortar prisms were prepared for periodic monitoring of expansion due to alkali-silica reaction and sulphate attack. The results demonstrated that a larger number of natural pozzolans and ground glasses tested have considerable potential for use in concrete considering their comparable or better performance than industrial by-products (e.g., fly ash and silica fume).