Thermodynamic Modeling and Experimental Study of Carbonation of Alkali-Activated Slag Cements

Abstract

The present study investigates the carbonation of slags activated by NaOH, Na2SiO3, Na2CO3, and Na2SO4. The main hydrates in those activated with NaOH or Na2SiO3 were C-(N-)A-S-H, strätlingite, and MgAl-layered double hydroxide phases as observed using different experimental characterization techniques and thermodynamic modeling. The use of a Na2CO3 activator led to the formation of calcium carbonate and monocarbonate instead of strätlingite, and Na2SO4 to ettringite formation. Carbonation of the alkali-activated slags in a powdered form at 1 vol % concentration of the CO2 environment proceeded rapidly, while little change was observed after 7 d. The main carbonation products commonly identified were calcium carbonate, carbonate containing-MgAl-layered double hydroxide, and N-A-S-H. In Na2SiO3- and Na2SO4-activated slags, M-S-H formation was predicted, which destabilized the MgAl-layered double hydroxide phases. Strätlingite and monocarbonate, which were initially present or formed as a transient phase, were transformed to N-A-S-H during carbonation, while C-(N-)A-S-H was only partially decalcified and destabilized.

How does the pore solution chemistry influence the passivation of reinforced alkali‐ and salt‐activated slag materials?

“…Although Na 2 CO 3 ‐activated slags continue the previous trend decreasing pore solution conductivities by 27% ( μ DOR = 60% = 0.17 S/cm) as the slag degree of reaction increases (10%–60%), the Na 2 SO 4 ‐activated slags pore solution conductivities are increased by 40% ( μ DOR = 60% = 0.15 S/cm), see Figure 1C,D. While here we consider degrees of reaction of slag up to 60%, lower degrees of reaction of 39% and 33% have been observed for 10 wt.% Na 2 CO 3 and Na 2 SO 4 ‐activated slags at 50 days 29 . Hence, the results here remain theoretical and useful to understand the relative complexity of the pore solution chemistry between alkali‐ and salt‐activated slags.…”

“…While here we consider degrees of reaction of slag up to 60%, lower degrees of reaction of 39% and 33% have been observed for 10 wt.% Na 2 CO 3 and Na 2 SO 4 -activated slags at 50 days. 29 Hence, the results here remain theoretical and useful to understand the relative complexity of the pore solution chemistry between alkali-and salt-activated slags. In contrast with alkali-activated slag materials, the influence of Al 2 O 3 content of slags yields more significant differences in pore solution conductivity-for example, a difference of 7 wt.% Al 2 O 3 in the Ecocem slag composition can result in differences between 0.05 and 0.10 S/cm when analyzing Na 2 CO 3 -activated slags at 60% degree of reaction.…”

Impact of Ca, Al, and Mg on reaction kinetics, pore structure, and performance of Fe‐rich alkali‐activated slag

“…Activating solutions usually include sodium and/or potassium hydroxides or combinations of sodium/potassium silicate, sulfates, 43 or carbonate 44 . A study 45 on alkali‐activated GGBFS explored different activation solutions, such as NaOH, Na 2 SiO 3 , Na 2 CO 3 , and Na 2 SO 4 , and it was found that the slag exhibited a more extensive reaction when activated with NaOH and Na 2 SiO 3 solutions. The chemical composition of the silicate solution can be defined by the SiO 2 /M 2 O (M = Na or/and K) molar ratio and the H 2 O content.…”