Alkaline Hydration of Tricalcium Aluminate

Abstract

This study explored the behavior of laboratory-synthesized tricalcium aluminate (C 3 A) in alkaline media. In all the trials, the C 3 A was hydrated with water containing a certain percentage of alkali activators (4% Na 2 CO 3 , 4% Na 2 SO 4, or 8% Na 2 SO 4 by weight of C 3 A). Two-and 28-day mechanical strength values were determined and the products were characterized with XRD, DTA, and NMR. The compressive strength of the C 3 A pastes rose in the presence of the alkaline activators used in this study. The alkalis induced the formation of calcium monocarboaluminate (C 4 AČ H 11 ) as the main hydration product in all cases. In addition, when Na 2 SO 4 was added, a sodium calcium sulfoaluminate, phase U (C 4 ASˇNH 16 ), formed. The presence of this mineral, whose formation is associated with high pH values, furnished indirect information on the alkalinity in the medium.

Very High Volume Fly Ash Cements. Early Age Hydration Study Using Na2SO4 as an Activator

“…even the appearance of moderate amounts of ettringite in FAN‐4 pastes could be surprising as these authors failed to observe ettringite in a pure system comprised solely of Na 2 SO 4 and C 3 A. Although Na 2 SO 4 is not alkaline in itself (a solution of Na 2 SO 4 should have a pH of around 7), when mixed with hydrating C 3 A, the formation of phase U, only considered stable at very high pH′s, has recently been observed . Although Phase U or AFm was not observed in FAN‐4 paste XRD, it should be noted that in our pastes, the estimated C 3 A content is only ~1%, making it impossible to observe any alternative C 3 A hydration products that are poorly crystalline.…”

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

“…An analysis of the tetrahedral aluminum based on the 27 Al MAS NMR spectra for the alkaline-belite hybrid cement and its pastes ( Figure 3B) revealed that the intensity of the signal at +80 ppm, associated with calcium aluminates in the clinker (Baptiste d'Espinose de Lacaillerie et al, 2008;Sánchez-Herrero et al, 2012;Skibsted and Andersen, 2013), was barely perceptible in the 28 d materials and disappeared almost entirely after 180 d.…”

“…The 27 Al NMR spectra for the 28 and 180 days pastes (see Figure 3B) exhibited signals at around +12 ppm, +9.4 ppm, and +4 ppm, indicative of octahedral aluminum and respectively associated with ettringite (C 6 AŜ 3 H 32 ), calcium monosulfoaluminate (C 4 AŜH 12 ) or calcium monocarboaluminate (C 4 AĈH 11 ) and the third aluminate hydrate (TAH) (Shimada and Francis Young, 2004;Baptiste d'Espinose de Lacaillerie et al, 2008;Sánchez-Herrero et al, 2012;Skibsted and Andersen, 2013) none of these hydration products were detected with XRD. In contrast, with its high sensitivity to 27 Al nuclei, MAS NMR identifies compounds difficult to detect with other techniques due to the small amounts present or their low crystallinity.…”

“…Inasmuch as the presence of alkalis in the medium affects, aluminosilicate and Portland clinker reactivity, this study undertook an in-depth analysis of calcium silicate and calcium aluminate behavior after hydration by a number of alkaline solutions of different concentrations. The findings revealed that the compressive strength of C 3 A, C 3 S and β-C 2 S pastes rose in the presence of alkaline additives (Sánchez-Herrero et al, 2012. Most significantly, given the low initial reactivity of β-C 2 S under normal hydration conditions, the presence of such alkaline FIGURE 1 | (A) 29 Si MAS NMR spectrum for fly ash and second derivative of the spectrum; (B) second derivatives of the original 29 Si MAS NMR spectrum for fly ash and of the envelope generated by deconvolution.…”

Influence of Calcium Sulfate Type on Evolution of Reaction Products and Strength in NaOH- and CaO-Activated Ground Granulated Blast-Furnace Slag

“…It is worth noting that Na 2 SO 4 formed in both samples. In earlier studies [12,25,26], U-phase often formed when Na 2 SO 4 was present in cementitious systems, and it competed with ettringite under lowered pH environments. Thus, in the NaOH-activation, due to the substantial formation of U-phase, the ettringite formation seemed to be suppressed.…”