Thermodynamic modelling of alkali-activated slag cements

Abstract

Please cite this article as: Myers, R.J., Lothenbach, B., Bernal, S.A., Provis, J.L., Thermodynamic modelling of alkali-activated slag cements, Applied Geochemistry (2015), doi: http://dx. AbstractThis paper presents a thermodynamic modelling analysis of alkali-activated slag-based cements, which are high performance and potentially low-CO 2 binders relative to Portland cement. The has been applied to AAS cements in the past (Lothenbach and Gruskovnjak, 2007), however the calcium silicate hydrate (C-S-H) thermodynamic model (Kulik and Kersten, 2001) used in that study does not explicitly define the uptake of Al and Na which is needed to fully describe C-(N-)A-S-H gel. Chemically complete definitions of Al chemistry in the thermodynamic models used to simulate the phases formed in AAS-based cements are important in enabling accurate prediction of the chemistry of these cements. The inclusion of alkalis as a key component in thermodynamic models for C-(N-)A-S-H gel is also important to enable correct description of the solubility relationships of this phase under the high pH conditions (>12) and alkali concentrations (tens to hundreds of mmol/L) relevant to the majority of cementitious materials (Myers et al., 2014). The CNASH_ss thermodynamic model used in the current paper was recently developed (Myers et al., 2014) to formally account for Na and tetrahedral Al incorporated in Ca/Si < 1.3 C-(N-)A-S-H gel. Here, this thermodynamic model is used to simulate the chemistry of AAS cements activated by aqueous solutions of NaOH ((NH) 0.5 ), Na 2 SiO 3 (NS), Na 2 Si 2 O 5 (NS 2 ) and Nc. This thermodynamic model can describe a large set of solubility data for the CaO-(Na 2 O,Al 2 O 3 )-SiO 2 -H 2 O and AAS cement systems, and closely matches the published chemical compositions of calcium aluminosilicate hydrate (C-A-S-H) gel, and the volumetric properties of C-(N-)A-S-H gel measured in a sodium silicate-activated slag cement (Myers et al., 2014). The CNASH_ss thermodynamic model is assessed here in terms of the prediction of solid phase assemblages and the Al content of C-(N-)A-S-H gel over the bulk slag chemical composition range which is most relevant to AAS cement-based materials. These simulations are performed using the Gibbs energy minimisation software GEM-Selektor v.3 an updated definition of Mg-Al LDH intercalated with OH -(MA-OH-LDH), and including some zeolites and alkali carbonates. The results are discussed in terms of implications for the design of high performance AAS-based cements.

The Interfacial Transition Zone in Alkali-Activated Slag Mortars

“…described as N-C-A-S-H, either as a single phase or as intimately intermixed C-A-S-H and N-A-S-H products (Myers et al, 2013(Myers et al, , 2015. This observation is confirmed through EDX analysis of several points around the aggregate, as shown in Figure 6, which confirms quantitatively that the Na/Ca ratio is on average higher in the region close to the aggregate than in the bulk paste.…”

Quantitative Analysis of Phase Assemblage and Chemical Shrinkage of Alkali-Activated Slag

“…In general, blast-furnace slag can be described as a CaO-SiO 2 -Al 2 O 3 -MgO glass. The destination of Mg in AAS has been reported to participate into the formation of hydrotalcite-type phases, regardless of activator (Ben Haha et al 2011b;Myers et al 2015). While in CaO-SiO 2 -Al 2 O 3 -H 2 O system, several different hydrated products have been identified in AAS by previous studies.…”

Chloride binding and mobility in sodium carbonate-activated slag pastes and mortars

“…Mortar discs ([100 9 50 mm) were used, and duplicate samples were prepared per formulation per curing age. Prior to the accelerated chloride migration test, each of the discs was immersed in 1.0 M NaOH solution under vacuum to obtain pore water-saturated samples; the immersion fluid was selected to have a comparable (but simplified) composition to those predicted by thermodynamic modelling for the cements assessed [43]. At the end of the test, the disc sample was split, and immediately sprayed with 0.1 M silver nitrate (AgNO 3 ) to obtain the chloride ingress profile [44].…”