Advances in near-neutral salts activation of blast furnace slags

Abstract

The utilisation of near-neutral salts as activators to produce alkali-activated slag cements offers several technical advantages, including reduced alkalinity of the binders, minimising the risk associated with handling of highly alkaline materials, and better workability of the fresh paste compared to that of sodium silicate-activated slag cements. Despite these evident advantages, the delayed setting and slow early-age mechanical strength development of these cements have limited their adoption and commercialisation. Recent studies have demonstrated that these limitations can be overcome by selecting slags with chemistry which is more prone to react with near-neutral salts, or by adding mineral additives. A brief overview of the most recent advances in alkali-activation of slags using either sodium carbonate or sodium sulfate as activators is reported, highlighting the role of material design parameters in the kinetics of reaction and phase evolution of these cements, as well as the perspectives for research and development of these materials.

Alkali-activated materials

“…For activation of high-calcium precursors such as blast furnace slags, or for production of hybrid cements with a significant Portland cement content, Na2CO3 and Na2SO4 can also be effective [6,10]. Some definitions of alkali-activation also include MgO, CaO and/or Ca(OH)2 as potential activator compounds.…”

“…Each precursor will need the correct activator (or combination of activators) to be identified, specified and sourced. There is currently little systematic or predictive information available regarding methodologies or heuristic procedures by which activators can be optimally matched to precursors, so this is currently achieved largely through parametric testing, but the need for improvements in this area has been identified [10].…”

Phase modification induced drying shrinkage reduction on Na2CO3 activated slag by incorporating Na2SO4

“…When a slag with a high MgO content is used, the secondary reaction products can be: hydrotalcite if activated by waterglass [14]; calcite, gaylussite and hydrotalcite when activated by sodium carbonate [15]; ettringite and monosulfoaluminate or no crystalline structures (depending on the Al 2 O 3 content) if activated by sodium sulfate [16]. On the other hand, it is also known that the mechanical properties of AAS are highly depending on the applied activators when using the same solid precursors, exhibiting generally an order of Na 2-SiO 3 [ Na 2 CO 3 [ Na 2 SO 4 [ NaOH at the same curing regimes and ages [17].…”

“…Mobasher et al [27] evaluated the structure of an alkali sulfate activated slag cement, and found that ettringite, C-A-S-H, and a hydrotalcite-like MgAl layered double hydroxide (LDH) are the main reaction products. However, Bernal [16] also mentioned that the secondary reaction products are highly depending on the content of MgO and Al 2 O 3 . A high content of MgO and/or low content of Al 2 O 3 favors the formation of poorly crystalline LDHs and AFm type phases, while a reversed condition leads to the generation of monosulfoaluminate and ettringite.…”

“…A high content of MgO and/or low content of Al 2 O 3 favors the formation of poorly crystalline LDHs and AFm type phases, while a reversed condition leads to the generation of monosulfoaluminate and ettringite. It should be noted that both hydrotalcite and ettringite basically belong to the group of layered double hydroxides, while their synthesis conditions are different [16,28]. In this case, by combining the alkaline solutions of sodium carbonate and sodium sulfate as the activator, it is possible to enhance the crystallinity by forming new LDHs and avoiding the formation of gaylussite, which consequently will reduce the shrinkage.…”