Does particle size of slag influence its reactivity?

It is mentioned previously that the reactivity of slag is influenced by its fineness of grinding (specific surface area) that is determined by the sizes of slag particles. The finer the slag particles, the larger the surface area, which means higher hydration degree of slag.

However, if disregarding the fineness, in other words, take two slag particles with significantly different sizes (diameters of “spherical” particles) as the study target, do the reacting front layers of the two particles grow at same rate? The growing rate of reacting layer can be a good tool to calculate the hydration degree of slag at particle level.

Simulated results conducted by Wei Chen showed the thickness of the hydrated layer is not significantly influenced by the particles size of slags, which is in good agreement with experiment result in which Na(OH)2 solution was used as activator [Ref. Three-dimensional computer modeling of slag cement hydration].

thickness of the hydrated layer vs particles size of slags

Another research carried out in 1986 reported similar result: “the thickness of hydrated layer of particle did not depend on the particle size, that is, the hydration reactivity had nothing to do with particle size and the rate of hydration was actually in proportion to the total surface area.” Detailed information is quoted as follows [Ref. Hydration of blast furnace slag particle, from 8th International Congress on the Chemistry of Cement Vol 4 (1986) Pages: 98-103],

It is most practical to evaluate the hydraulic reactivity of water granulated blast furnace slag in terras of the strength of hardened body. In order to make the extensive use of slag, however, it is important to investigate the hydration properties of individual particles. Hydration properties mainly investigated here were the change in the amount of hydration and the thickness of hydrated layer with time, and the effect of alkaline activator.

We examined the hydration of ground commercial slag with various particle sizes. Experiment was carried out as follows: samples with five different particle sizes (3.27, 4.05, 5.86, 8.66 and 13.36 μm by volume mean diameter) were prepared. They were hydrated in suspension with the water-slag ratio of 10 with NaOH, Ca(OH)2 or ordinary Portland cement as alkaline activator.

The amount of hydration of the slag under such an alkali-condition was measured by determining the soluble part of the hydration product using salicylic acid-acetone-methanol solution, and also by loss on ignition.

The following results were obtained. The thickness of hydrated layer of particle did not depend on the particle size, that is, the hydration reactivity had nothing to do with particle size and the rate of hydration was actually in proportion to the total surface area. It was suggested, on the other hand, that each alkaline activator show the characteristic effect on the hydration of each sample. Some of these results were supported by further experiments.

According to these research results, it is concluded that the reactivity of slag has nothing to do with particle size at particle level, while the surface area (fineness) determines hydration degree of slag.

It is should be noted that the two research results mentioned here only cover a narrow particle size distribution (0-15 micrometers). To apply this conclusion to wide size range, further research work is needed.