Alkali Activation of Blast Furnace Slag Cements

Alkali activation of blast furnace slag cements involves the reaction of ground granulated blast furnace slag (GGBFS) with highly alkaline solutions to form a hardened binder. This process transforms the amorphous calcium-silicate glass in slag into a calcium-sodium aluminosilicate hydrate (C–(N)–A–S–H) gel complemented by secondary phases such as hydrotalcite-like layered double hydroxides, AFm-type carbonate or sulphate phases and, under certain conditions, crystalline portlandite. The chemistry of both slag and activator—particularly the CaO/SiO₂ ratio, the Al₂O₃ and MgO contents of the slag, and the nature of the activator anion (silicate, carbonate, hydroxide or sulphate)—critically governs reaction kinetics, phase assemblage, pore solution pH and mechanical performance. This modifiable system offers rapid strength development, excellent durability against chloride ingress and carbonation, and the potential for encapsulation of industrial wastes. Globally, alkali-activated slag cements are championed as low-carbon alternatives to Portland cement, leveraging steel-making by-products to reduce embodied carbon and energy in the built environment.

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Alkali Activation of Blast Furnace Slag Cements publication trend

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Technical terms

Alkali activation: Chemical process in which an aluminosilicate precursor reacts with an alkaline solution to produce cementitious gels and crystalline phases.

Ground granulated blast furnace slag (GGBFS): A finely milled by-product of steel manufacture composed predominantly of amorphous calcium-silicate glass used as the main precursor in alkali-activated binders.

C–(N)–A–S–H gel: Calcium-sodium aluminosilicate hydrate, the principal binding phase formed during alkali activation of blast furnace slag.

Hydrotalcite: A magnesium-aluminium layered double hydroxide that precipitates as a secondary, anion-retaining phase in alkali-activated slag systems.

Activator anion: The negatively charged species (for example silicate, carbonate or sulphate) in the alkaline solution that directs gel structure, kinetics and phase assemblage.

References

1. Modifications to reaction mechanisms, phase assemblages and mechanical properties of alkali-activated slags induced by gypsum addition. Cement and Concrete Research (2023).
2. Activator Anion Influences the Nanostructure of Alkali-Activated Slag Cements. The Journal of Physical Chemistry C (2021).
3. Effect of slag chemistry on the hydration of alkali-activated blast-furnace slag. Materials and Structures (2014).
4. Stability of hydrotalcite (Mg-Al layered double hydroxide) in presence of different anions. Cement and Concrete Research (2022).
5. Chloride binding and mobility in sodium carbonate-activated slag pastes and mortars. Materials and Structures (2017).
6. Thermodynamic modelling of phase evolution in alkali-activated slag cements exposed to carbon dioxide. Cement and Concrete Research (2020).
7. Slag and Activator Chemistry Control the Reaction Kinetics of Sodium Metasilicate-Activated Slag Cements. Sustainability (2018).