Alkali activation of a high MgO GGBS – fresh and hardened properties

Abstract

In this study, concretes and pastes were produced from a high MgO ground granulated blast furnace slag (MgO content 16.1 wt.%) by alkali activation with various amounts and combinations of sodium carbonate and sodium silicate. Sodium carbonate activators tended to reduce slump compared to sodium silicate at the same dose, and, in contrast to the literature for many blast furnace slags with more moderate MgO, to shorten the initial and final setting times in comparison with concretes activated by sodium silicate for dosages less than 10 wt.%. Higher heat curing temperatures and the use of larger dosages of alkali activators resulted in higher early age cube compressive strength values. The XRD analysis of 7 and 28 days old pastes activated with sodium carbonates revealed formation of gaylussite, calcite, nahcolite and C-(A)-S-H gel. Curing at 20˚C appeared to promote dissolution of gaylussite and calcite, while heat curing promoted their replacement with C-(A)-S-H, which also resulted in higher ultimate cube compressive strength values. Conversely, mixes activated with sodium silicate contained less crystalline phases and more disordered gel which strengthened the binder matrix.

Geopolymer Based on Mechanically Activated Air-cooled Blast Furnace Slag

“…Both effects were observed also by others and were related to a higher pH values which accelerated dissolution of the slag [24,27,[35][36][37]. Higher alkali modulus of the used SS activator decreased the compressive strength values in both slags (Figure 9).…”

Effects of Curing Conditions on Shrinkage of Alkali-Activated High-MgO Swedish Slag Concrete

“…The high MgO content of the slag tends to affect the early-age properties of the binder matrix. Earlier studies, also performed by the authors, indicated formation of hydrotalcite-like Mg-Al layered double hydroxides (Bernal et al, 2016;Humad et al, 2018;Abdollahnejad et al, 2019). The consumption of Al may also contribute to the strength development.…”

“…Although, not verified experimentally, it is very likely that the dissolution rate of GBFS was higher in the heat-cured samples thus leading to a more intensive formation of calcite and gaylussite. The formation of gaylussite was shown earlier to be predominant at an early age, in high-MgO AAS binders (Humad et al, 2018). The high sodium carbonate content in the mix SC10 provided an excess of Na ions, which supported the formation of gaylussite Na 2 Ca(CO 3 ) 2 •5H 2 O (Allahverdi et al, 2017).…”

“…The longest final setting times were observed for mixes activated with 10 wt.% SC and the combination of 5 wt.% of SC + 5 wt.% of SS (SC10, SC5 + SS5), Figure 2. These results can be explained by a high concentration of calcium carbonate and high MgO content which resulted in formation of gaylussite, hydrotalcite and calcite (Humad et al, 2018). Earlier studies showed that a presence of calcite CaCO 3 tends to shorten the initial setting time rapidly and decreases the viscosity of the fresh mix.…”

“…In general AAS develop higher shrinkage in comparison with PC concretes (Allahverdi et al, 2017;Humad et al, 2018;Orosz et al, 2019). In the case of the SS activated GGBFS concretes, the high shrinkage was linked with the presence of more mesopores (Shi, 1996) which led to the increased tensile stresses and thus to a greater drying shrinkage (Häkkinen, 1993).…”