There are many experimental techniques used to measure the degree of hydration of portland cement, such as measuring the heat of hydration, the non-evaporable water content, and the amount of calcium hydroxide produced in the hydration reactions. All of these methods are based on comparisons of the measured parameters with the predicted or measured parameters for a fully hydrated paste. When reactive mineral admixtures (MA) are incorporated into portland cement, the system becomes more complex. It is almost impossible to determine the degree of the cement hydration by the techniques used for plain cement paste. Although some researchers [1-5] have measured the degree of reaction of mineral admixtures by a selective dissolution procedure, little information can be obtained in this manner about the cement hydration in a blended system. Thus, it would be valuable to develop a technique that can simultaneously and directly study the degree of reaction of both mineral admixtures and the portland cement in blended cement pastes.
In recent years, scanning electron microscopy (SEM) quantitative techniques have been used in cement systems to directly estimate the degree of hydration of plain portland cement pastes [6-8]. These researchers collected backscattered electron images (BEI) of cement pastes, and then quantitatively analyzed them using system software based on the different gray levels of the different cement phases. For example, in Ref. [6], for cement pastes with water to cement mass ratio (w/c) = 0.45 cured at 20 ºC, the degree of hydration was about 70 % at 1 d, and about 80 % at 3 d. These results were much higher than those obtained from calcium hydroxide measurements and density determinations. The large disagreement may have been caused by the selection of inappropriate thresholds in the gray scale images or by limiting image resolution so as to be able to discriminate between different gray levels. In blended cement pastes, discriminating between phases is even harder due to the similar gray levels of some of the fly ash particles, the slag particles, and the cement clinker particles. Another SEM imaging technique, X-ray imaging, may be used to estimate the degree of hydration of blended cement pastes, since it gives elemental composition information that makes it easier to discriminate between phases. However, because this technique can only examine a very small field of view, it requires several hours of processing time.
In the current study, a SEM-based point-counting technique was used to directly study the degree of hydration of cement and the degree of reaction of the MA in blended cement pastes containing fly ash or slag. A similar optical-based point-counting technique is the basis for a number of ASTM standard test methods [9-10]. The technique accesses the same field of view as does X-ray imaging but in a fraction of the time, so that enough fields of view for statistical purposes can be examined in a reasonable length of time. This is an important consideration considering that the point-counting technique requires a human operator. Loss-on-ignition (LOI) measurements of non-evaporable water content for the plain portland cement pastes were also performed to compare with the results from the SEM point-counting technique. The accuracy of the LOI measurements are not known, but they could be a precise measure, since they are based on mass measurements, which can be performed with great precision.