Strengths developed in fly ash concretes usually equal or exceed that of similar plain concrete after a few months, with much of the response usually attributed to “pozzolanic” reaction between ash and secondary calcium hydroxide (CH). The CH contents of pastes made with five different fly ashes were determined by DTA for periods up to six months. The CH contents found did not decrease notably over the period, and were substantially identical to that expected for plain cement pastes diluted with the same amount of inert material as the amount of fly ash used. Scanning electron microscope examination of the pastes showed only minimal evidence of reaction even up to 1 year of age, although many fly ash grains were in intimate contact with CH. Non-evaporable water contents of the fly ash pastes were substantially higher than expected at each age, suggesting that the fly ash promoted more complete cement hydration or that the hydration products formed bound substantially greater amounts of water than plain cement paste ordinarily does.