Pozzolanic and cementitious reactions of fly ash in blended cement pastes

Results of thermogravimetric tests on hardened cement pastes containing fly ash are reported. Ashes derived from subbituminous and lignite coal were used at replacement levels of 30% and 50% by weight. Thermal analysis was performed on samples which were water cured for various ages from 3 days to one year. An analytical technique was used to split the non-evaporable water content of cement pastes into two components: water held by calcium hydroxide, and water held in other reaction products. The technique is used to identify and monitor the progress of reaction of the different types of fly ash. There are distinct differences in the manner in which the various ash/cement pastes gydrate. The subbituminous ash relies more upon the “pozzolanic reaction” between calcium silicate and calcium hydroxide to provide the strength-giving hydration product. Conversely, the lignite ash produces a substantial amount of hydrate by direct reaction between compounds of the ash and water. At early and intermediate ages the hydrate produced from the reaction of fly ash may combine substantially more water per unit weight than the hydrates normally produced through the reaction of cement.