Although it is somewhat difficult to measure the reaction degree of slag in slag blended cement comparing with the case in Portland cement, many researchers performed experiments on this field. A large body of literature can be found on the kinetics of Portland cement/slag blends.
Based on the modification of Takashima’s salicylic acid extraction method, Taylor collected some researchers’ data on the reaction degree of slag, as is showed in Table 1.
Table 1. Data on reaction degree of slag in blended cement from literature.
Other researchers using EDTA or DTA also reported data on this issue under different hydration conditions; Table 2 is an uncompleted collection of these data.
Table 2. Uncompleted data collection of reaction degree of slag blended cement.
By means of EDTA extraction method, the work of Lumley et al. focused on the effect of water to solid ratios and the proportions of slag on the degrees of reaction of slag blending with Portland cement. It is reported that at water to solids ratios of 0.4-0.6 and 20 °C 30-55% of the slag reacts in 28 days and 45-75% in 1-2 years, for blends containing 92% slag and w/s=0.3, the hydration degree of slag is probably limited to 30%.
They also found that at water/solid ratio 0.3 the composition of Portland cement, even with an inclusion of 5% gypsum, has no significant effect on the degrees of slag reaction in the blend at ages of 28 days to 1 year.
Luke and Glasser reported that approximate 40% of the slag in a 30%/70% ratio of slag to Portland cement blend reacted in one month with little change up to 6 months, but over 60% of slag was reacted at the age of 12 months. Their latter study showed similar results with data at 25 °C, 40 °C and 50 °C for times of 0.5 to 24 months.
The reaction degree of slag in blended cement is difficult to measure using those methods which are successfully applied for Portland cement. Demoulian et al. described a method based on extraction of the constituents other than unreacted slag with a reagent based on Ethylene Diamine Tetraacetic Acid (EDTA), finding it effective for determining reaction degree of slag in unhydrated blended cements.
EDTA has long been the most used and well-known method to measure the reaction degree of slag in blended cement; however, as stated by Luke and Glasser, special care should be taken when applying this method, e.g. systematic error may occur due to the residue of hydrotalcite in the extraction of unreacted slag.
Lumley et al. reported that EDTA extraction method can be used to determine the degree of reaction of slag under certain corrections, but the accuracy is poorer at degrees of reaction above 70%.
Besides EDTA method, SEM-BSE image analysis could be applied to directly estimate the hydration degree of both plain cement and blended cement paste. This method is based on the fact that cement and slag grains can be distinguished from unhydrated clinkers and paste matrix, though there is some overlap due to the similar gray levels of cement and slag. However, it should be bore in mind that using this method may produces an over-estimated result on the reaction degree of slag due to its fine particles (G. Ye).
To improve the precise of ESEM image analysis determining the reaction degree of slag, V. Kocaba studied SEM-IA-Mg-mapping method to clearly distinguish slag phase from clinkers. The methodology is based on the Mg element of slag does move during the hydration process, thus the presence of Mg can mark the unreacted slag phase. The disadvantage of this method is extremely time consuming, which is the drawback of SEM-BSE image analysis as well.
As a conclusion,there is no perfect method so far to determine the reaction degree of slag in blended cements. The best solution may be using more than one method to validate each other and get reliable result.
The hydration kinetics of slag is generally divided into three stages: (1) a nucleation period during which product growth is accelerating, (2) a phase boundary controlled stage, and (3) a diffusion controlled stage.
The reaction degree of slag in blended cement is influenced by many factors. The main factors affecting the reaction of slag in blended system include the reactivity of slag that could be defined as (C+A+M)/S, the fineness of grinding (specific surface area), the vitreous fraction of slag, the replacement level of slag in blended system, the hydration temperature and the water/solids ratio.
Precisely, the rate of reaction of slag decreases with the decreasing water/solids ratio and with increasing proportion of slag in the blend. Higher hydration temperatures increase the reactivity of slag. However, the composition of cement and the incorporation of additional gypsum in slag blended cement have no influence on the extent of reaction of slag at ages of 28 days to 1 year, though variations in Portland cement can affect early strength. The following figure is a representation of the effect of hydration conditions and slag characteristics on the reactivity of slag.
Representation of the effect of hydration conditions and slag characteristics on the reactivity of slag.