Hidratación del cemento de aluminato de calcio en condiciones de muy elevada alcalinidad

Investigating the retarding effect of CAC in alkali-activated cements

“…Regardless of the activator dosage, the dormant period is prolonged up to ~40 h. The 10C-a sample shows several overlapping heat evolution peaks between 35-50 h, resolving into a small narrow peak at ~55 h. The appearance of these secondary exothermic peaks may indicate initial gel formation, i.e., progression of the polycondensation reaction. The subsequent induction-like period spans from 55 to 70 h, after which a visibly suppressed second exothermic peak is apparent with a maximum at ~85 h. The induction period in the 10C-b sample also exceeds 35 h, followed by two more distinct exothermic peaks: a sharp narrow peak at ~50 h and a broader peak at ~80 h. As seen in the case of 10C-a, the slope of the last peak diminishes after 150 h. The exact designation of individual heat evolution peaks to describe the progress of several concurrent and competing reactions sequences in this multicomponent system is difficult, however, it is clear from the data that both the 10C-a and 10C-b systems with the addition of CAC experience a serious retarding effect on the formation kinetics associated with polycondensation, similarly to what is observed for alkali-activated CAC (Pastor et al, 2009).…”

“…Further peaks are referenced to calcium monocarboaluminate (Mc, C 4 A CH 11 PDF#00-041-0219), an AFm-like phase likely to be formed from the carbonation of CA hydration products, and hydrogarnet (C 3 AH 6 PDF# 00-024-0217). Meta-stable hexagonal aluminate hydrates (CAH 10 and C 2 AH 8 ), compounds that conventionally form in the early hydration of CAC, are not observed, suggesting either that in alkaline environments the formation of cubic aluminate hydrate is favoured from the onset of reaction, or that the conversion of the metastable aluminates to hydrogarnet has very fast kinetics (Pastor et al, 2009;Fernández-Jiménez et al, 2011;Reig et al, 2016;Vafaei and Allahverdi, 2016). The formation of AH 3 along with cubic aluminate hydrates is not excluded as a possibility, however this phase is not easily detected in its disordered forms.…”

“…The behaviour of CAC in moderately and highly alkaline environments has been investigated in some studies (Pastor et al, 2009;Fernández-Jiménez et al, 2011), as has its effect on the alkali-activation of precursors including GGBFS (Arbi et al, 2013), metakaolin (Fernández-Jiménez et al, 2008), natural pozzolans (Carrasco et al, 2007;Vafaei and Allahverdi, 2016), red clay brick wastes (Reig et al, 2016), and fly ash (Cao et al, 2018), for the formation of hybrid binders. Most of these studies did not observe the typical hydration products of CAC (CAH 10 , C 2 AH 8 , and AH 3 ); highly alkaline conditions and high temperatures favour instead the formation of cubic aluminate hydrate from the outset, and if sources of silica are present, katoite type phases are preferentially formed over strätlingite, which is not favoured in highly alkaline environments.…”

Improving early compressive strength of precast concrete by increasing active silicon content

“…The addition of alkaline ions promotes the increase of pH in the gelling system, and the hydration process of C 3 A and C 4 AF accelerates, resulting in the crystal structure of the hydration product C 3 AH 6 transform from hexagonal aluminate to stable and dense cubic aluminate (C 3 AH 6 ). 41 As the hydration reaction of cement particles continues, the content of C-S-H gel, N-S-H gel, and Ca(OH) 2 in the hydration product continues to increase. Studies have shown that only a small amount of Ca(OH) 2 can maintain the alkalinity of the hydration environment and maintain the stability of the C-S-H gel and N-S-H gel.…”

“…Some Na + irons in the gelling system replace Ca 2+ and react with OH − to form NaOH. The addition of alkaline ions promotes the increase of pH in the gelling system, and the hydration process of C 3 A and C 4 AF accelerates, resulting in the crystal structure of the hydration product C 3 AH 6 transform from hexagonal aluminate to stable and dense cubic aluminate (C 3 AH 6 ) 41 …”