The Effect of Partial Replacement of Cement with Alkali-Activated Electric Arc Furnace Slag on the Compressive Strength of Mortar: A Comparison between Alkaline Curing and Early Thermal Curing
Abstract
This study aimed to evaluate the effect of the partial replacement of cement with alkali-activated electric arc furnace slag (EAFS) on the compressive strength of cementitious mortar, while also examining the influence of different curing regimes on strength development. Mortar specimens were prepared with a sand-to-binder ratio of 1:2.75. Five levels of cement replacement with slag were investigated: 4, 8, 12, 16, and 20%. A sodium hydroxide solution was used as the sole alkaline activator at a concentration of 10 M, and a superplasticiser was incorporated to improve workability. After casting the mortar into 50 × 50 × 50 mm cubes, different curing regimes were applied. The reference specimen was cured by immersion in water, whereas the other specimens were either immersed in a 10 M sodium hydroxide solution for 28 days or subjected to early thermal curing immediately after demoulding at 65 °C for 24 hours, followed by sealing in plastic wrapping for 27 days until the testing age. The results showed that the reference specimen cured in water achieved the highest compressive strength among all specimens (41 MPa). In contrast, curing by immersion in the sodium hydroxide solution reduced the strength to 22 MPa, while early thermal curing resulted in the lowest compressive strength (18 MPa). Furthermore, the alkali-activated slag mixtures exhibited a noticeable reduction in strength compared with the reference specimen, although early thermal curing produced a relative improvement at certain replacement levels. These findings indicate the limited effectiveness of a single alkaline activator in activating slag within the investigated replacement ratios. A comparison with previous studies that utilised the same type of slag but activated it using a combination of sodium silicate and sodium hydroxide, together with curing at higher temperatures, suggests that the combined presence of a soluble silica source and intensive heat treatment enhances the microstructural development and densification of slag-based systems, thereby resulting in higher compressive strength.
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