Comparison of different methods for enhancing reactivity of pozzolans

This paper compares three methods for the activation of pozzolanic reactivity of a natural pozzolan: (1) prolonged grinding of natural pozzolans (mechanical method), (2) elevated temperature curing of cement pastes containing a pozzolan (thermal method), and (3) use of chemical activators such as Na2SO4 and CaCl2 (chemical method). The pozzolanic reactivity of natural pozzolans was evaluated using a mixture of 80% natural pozzolan and 20% hydrated lime. Several parameters such as Ca(OH)2 consumption rate, strength development, ultimate compressive strength and cost were compared. All three activation methods accelerated both the pozzolanic reaction and strength development rates of the lime-pozzolan pastes. However, mechanical activation did not show a significant effect on the ultimate strength, while thermal activation decreased the ultimate strength and chemical activation (addition of 4% Na2SO4 or CaCl2.2H2O based on the mass of lime-pozzolan blends) increased the ultimate strength of lime-pozzolan mixtures significantly. The application of mechanical or thermal activation techniques needs extra equipment and energy, and thus increases materials costs significantly. Chemical activators can be added during the grinding of the natural pozzolan or during the mixing of concrete containing a pozzolan, so the requirement of extra equipment is minimal. Although the addition of chemical activator(s) increases the total material costs, the cost per unit strength development decreases. Based on current test results, chemical activation is the most efficient and feasible method for the activation of natural pozzolans.