Rehydration of katoite as a layered double hydroxide: an in situ study

Antonia Alana Lima Pacheco,Thiago Ricardo Santos Nobre,Sérgio Cirelli Angulo,Antonio Carlos Vieira Coelho,Marcel Hark Maciel,Celso Valentim Santilli

doi:10.21809/rilemtechlett.2021.130

Antonia Alana Lima Pacheco, Thiago Ricardo Santos Nobre + Show 4 more

Open Access

https://doi.org/10.21809/rilemtechlett.2021.130

Copy DOI

Abstract

Calcium aluminate layered double hydroxides (Ca-Al LDHs) constitute a considerable part of cementitious waste fines. Although cementitious waste fines have proven to be recyclable by thermal treatment at moderate temperatures (400–700 °C), understanding how each phase rehydrates and contributes to the binding properties of rehydrated cementitious materials is still necessary. In this study, the de(re)hydration of katoite is investigated through in situ techniques, and its applicability as an alternative cement or supplementary cementitious material (SCM) is discussed. The research employed X-ray diffraction, isothermal calorimetry, in situ wide-angle X-ray scattering (WAXS), and rotational/oscillatory rheometry. Katoite synthesized by a mechanochemical process was dehydrated at 400 °C, producing mainly mayenite. During rehydration, calorimetry presents high heat production in the first minutes. WAXS shows prompt recovery of katoite and increasing formation of monocarboaluminate (Ca-Al LDH) after 30 s of rehydration. The findings confirm the direct association between rapid heat release and phase reformation. Rehydrated pastes present a high yield stress and an increasing storage modulus, indicating rapid binding properties. The consolidation is also correlated with cumulative heat and monocarboaluminate formation. The results indicate the potential of calcined katoite for use as rapid set alternative cement or SCM.

Highlights

Calcium aluminate layered double hydroxides (Ca-Al LDHs), which include sulfo/carbo calcium aluminates in Portland cements (AFm phases), constitute a considerable part of the phases present in cementitious waste materials [1,2,3]
Cementitious waste fines are recyclable as cement or supplementary cementitious materials (SCMs) when thermally treated [4]
Studies on the recycling of calcium aluminate cement (CAC) as an alternative binder are scarce in the literature

Summary

Introduction

Calcium aluminate layered double hydroxides (Ca-Al LDHs), which include sulfo/carbo calcium aluminates in Portland cements (AFm phases), constitute a considerable part of the phases present in cementitious waste materials [1,2,3]. To explore the binding capacity of the material, rotational and oscillatory rheometry are performed Through these techniques, the study established correlations between phase consumption and formation with the heat release and consolidation of the rehydrated paste. The dehydrated katoite powder was rehydrated in deionized water with a minor quantity of carbonate ions (pH 5.8) at a water/solid ratio (in mass) of 1.2 This quantity of water was required to ensure plastic consistency of the paste, as the calcined powder demanded more water because of its high surface area. A pre-shear of 5 s-1 for 30 s was performed to eliminate the previous stress of the material Both rotational and oscillatory tests started 4 min after the first contact with water.

Results and discussion

Conclusion

C.V. Santilli