For blended cements containing fly ash or slag, LOI measurements cannot at present be used to determine the degree of hydration. Therefore, a SEM point-counting procedure was adopted to determine the amount of unreacted cement and mineral admixture left at a given time, which gives the degree of reaction if the original amount is known.
This procedure was first checked by comparing SEM point-counting with LOI measurements on two plain portland cements. It was found that the degree of hydration of plain portland cement pastes obtained from SEM point-counting techniques were consistent with the results from the traditional LOI method, with differences between the two methods no more than ± 10 %. The SEM point-counting results were systematically lower than the LOI measurements. The standard deviation in the point-counting determination of the degree of hydration of plain portland cement ranged from ± 1.5 % to ± 1.8 %. This is for a single operator, on a single small sample cut from a larger sample. Once this technique was established, it was used to measure the degrees of hydration of cement in blended cement systems for the first time. The standard deviations for the degree of hydration of cement in the blended cement pastes were within ± 1.4 % to ± 2.2 %. The same SEM point-counting technique was also used to measure the degree of reaction of the mineral admixtures in the blended cements. The results were comparable with previous results obtained using a selective dissolution method. The standard deviations for the degrees of fly ash reaction ranged from ± 4.6 % to ± 5.0 % and from ± 3.6 % to ± 4.3 % for the slag blended cements.
Results of the present study indicate that the SEM point-counting technique can be a reliable and effective analysis tool to measure the degree of hydration of the portland cement and mineral admixtures in blended cement pastes. These measurements will allow models like the Virtual Cement and Concrete Testing Laboratory [15] to be validated on these materials. With the growing use of blended cements utilizing waste stream materials, this combined measurement and prediction capability will allow better and more controlled use of these materials.