Alkaline Hydration Of C₂S and C₃S

Abstract

This study analyzed the behavior of two laboratory-synthesized calcium silicates, C 3 S and C 2 S, after hydration in 8-M NaOH and in water as a control. Two-and 28-d mechanical strength values were determined and the products were characterized with XRD, TEM, and 29 Si and 23 Na MAS NMR. The results showed that hydrating C 3 S in a highly alkaline medium had no significant effect on the mechanical development of the material, whereas in C 2 S hydration, that medium hastened hydration substantially, impacting setting and hardening times. This finding has technological implications, given the low early-age reactivity of dicalcium silicate under normal hydration conditions.

Alkali-activated binary concrete based on a natural pozzolan: physical, mechanical and microstructural characterization

“…The points associated with the C-A-S-H and C-S-H gels, which are rich in Ca, have Ca/Si values in the range of 0.7-1.8. Theoretically, sodium aluminosilicate hydrate (N-A-S-H) gels do not contain Ca, unlike the (N,C)-A-S-H gels that have Ca/Si values in the range of 0-0.4, which is consistent with the Ca/Si ranges reported in the literature (38,(66)(67)(68)(69)(70). The EDS elemental mapping results shown in Figure 8 provide a visual representation (colours) of the elementary composition of the AABC.…”

Studies About the Hydration of Hybrid “Alkaline-Belite” Cement

“…The 29 Si MAS NMR spectrum for the anhydrous cement exhibited a narrow signal in the clinker fraction at around −71 ppm attributed to belite, and a series of wider signals associated with the C 2 S and C 3 S Q 0 units in alite (Sánchez-Herrero et al, 2016, 2017. The area of the spectrum between −81 ppm and −120/−130 ppm was associated with the fly ash fraction in the cement.…”

C3S and C2S hydration in the presence of Na2CO3 and Na2SO4

“…C–S–H gel is generally acknowledged to interact with the alkaline oxides present in cement, spurring research on the details of that interaction . In a 1978 review of the effect of alkalis on cement, Jawed et al observed that the presence of ‘soluble’ alkalis hastened the initial C 3 S and C 2 S hydration reactions, favoring the precipitation of portlandite and C–S–H gel.…”

“…More recent research has analyzed the behavior of the two calcium silicates in the presence of moderate and high concentrations of alkalis (0.1 M‐8 M NaOH). The findings revealed that the presence of alkalis does indeed expedite C 2 S and C 3 S hydration kinetics, much more intensely in the former .…”

“…More recent research has analyzed the behavior of the two calcium silicates in the presence of moderate and high concentrations of alkalis (0.1 M‐8 M NaOH). The findings revealed that the presence of alkalis does indeed expedite C 2 S and C 3 S hydration kinetics, much more intensely in the former . An analysis of the gel precipitating in such conditions revealed the existence of C–(N)–S–H‐like gels in which Na + replaces the calcium in the structure, offsetting the charge on the OH − groups .…”