Comparison of tuff and fly ash in blended cement

The effect of tuff and two types of fly ash on the hydration of cement was studied. Compressive strength of cement mortars, pozzolanic activity index of tuff and fly ashes, rate of heat evolution of cement, nonevaporable water and calcium hydroxide content in the hardened cement pastes were determined. It was found that the mechanical strength of mortars containing fly ash, especially high-lime fly ash, was higher than that of mortars containing tuff. The pozzolanic activity index of fly ash, especially high-lime fly ash, was greater than that of tuff. The rate of heat evolution at the second exothermic peak was diminished with the addition of tuff or fly ash to the cement, but the second peak position and the length of induction period was almost the same for cements with and without tuff. Nevertheless, the addition of fly ash to cement had a retarding action on the time to reach the maximum thermal output at the second peak. The rate of development of hydration products, as monitored by nonevaporable water measurement, was higher for pastes with tuff than for pastes with fly ash during the early period of hydration, and this difference tends to disappear with time. The contentmore » of calcium hydroxide in the cement pastes was diminished due to the addition of tuff or fly ashes to cement, especially when tuff or the high-lime fly ash was used.« less