Suitability of a high-calcium slag for alkali activation, and strength and microstructure of the resultant materials.

Abstract

We study the feasibility of producing building materials by activating an Irish slag with alkali-metal activators. The slag is highly amorphous and basic (CaO+ MgO/SiO2=1.56), with CaO/SiO2=1.41 and Al2O3/SiO2=0.34 which evidence high reactivity and potential for alkali activation-AA-. Its chemical composition complies with standard requirements for slags in concretes/mortars/grouts. Devitrification, supported by calorimetry, evidenced that the slag consists of a melilitegehlenite isomorphous solution which denotes high reactivity. The slag was activated with NaOH and Na2SiO3, both combined and separately, to produce mortars. The mortars achieved reasonable setting times and workability, and strenght significantly increased between 28 and 270 days. Crack ing by drying shrinkage, a challenge in AA materials, is hindered by the slag’s high calcium. The Na2SiO3 + NaOH slag mortars gained the greatest compressive and flexural strengths. Rising the curing temperature to 60°C, enhanced their strength and microstructure (mainly up to 28 days), and develops hydrogarnet–gehlenite hydrate cements that may be responsible for the high 270-day strength (96MPa). Generally, curing at 60°C enhances early strength (3-7 day) but the increase between 28 and 270 days is not significant, with sometimes lower values than ambient-cured material (Na2SiO3-activator). An excessive %Na2O by mass of slag is blamed for the low strength generated by the Na2SiO3 activator (the slag’s high reactivity requires lower %Na2O). The NaOH-activated slags tend to perform best when ambient-cured. An undue high molarity of the NaOH solution has generated an excessive alkalinity for the highly reactive slag, lowering the strength of the NaOH-activated slag mortars. The results indicate that the best activator is a combination of Na2SiO3 and a low molarity (<6M)NaOH.