Hydration products of slag in blended cement

Many XRD analysis results have shown the main hydration products of slag blended cement are essentially similar with that of pure Portland cement, except the amounts of CH found by this method or other are in varying degrees and less than those that should be given by the pure Portland cement constituent if the slag part did not participate the reaction.

The main hydration products of the slag-cement are C-S-H gel, Ca(OH)₂, the sulpho-aluminate hydrate phases AFt and AFm and a Mg, Al-rich hydroxide phase.

Fig. 1. Transmission electron micrograph showing foil-like Op C-S-H in a water-activated slag paste hydrated for 3 1/2 years at 40°C (W/S = 0.4) (by I.G Richardson: The nature of C-S-H in hardened cements).

Fig. 2. A TEM micrograph that illustrates fine, dense Op C–S–H in the paste containing 75% slag. (by I.G Richardson: Composition and microstructure of 20-year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag).

In the case of C-S-H, its morphology and composition may be modified by partial accommodation of M and A within the micro- or nanostructure, its Ca/Si ratio is then lowered (e.g., 1.55) than that formed from alite and belite (e.g. 1.7). Hydrotalcite-like phase with approximate composition Mg₆Al₂(OH)₁₆(CO₃)·4H₂O is formed from the MgO content of BFS, typically 5-9%.

As stated above, the hydration products can be classified into inner product and outer product. Inner product C-S-H from cement grains has a Ca/(Si+Al) ratio similar to outer product C-S-H, Meanwhile, Katoite (C₃AS_αH_β, α < 1.5) has also been suggested as a slag product, but it is less documented than the former mentioned species.