The increase of anthropogenic CO2 emissions and their impact on the greenhouse effect result in rising social awareness as well as in business & economic paradigms change. The CO2 footprint becomes then a product selection choice. This concerns in particular Ordinary Portland Cement (OPC), as the cement industry accounts for about 8% of global CO2 emissions [1]. In addition, the expected application of CO2 taxes will lead to a significant price rise of this product.
In this context, several new types of more environmentally friendly hydraulic binders have been studied and suggested as potential OPC substitutes. Many of them comprise Fe and Al oxides, which may be present as a Ca4Al2-xFe2+xO10-like phase (commonly named C4AF in the Cement nomenclature). Upon hydration, this phase leads in particular to the precipitation of Ca-Layered Double Hydroxides (i.e. Ca-LDH or AFm in the Cement nomenclature). Ca:Al-based AFm phases have been extensively considered in the literature. In contrast, only a few studies have been reported on the Ca:Fe-based AFm phases, and none about the possible existence of Ca:(Fe/Al) AFm solid solutions [2,3]. Investigating such model systems is considered to be a prerequisite to properly characterize the hydration products of the above-mentioned new low-CO2 binders.
Ca:(Al/Fe) AFm (Ca2Al1-xFex(OH)6×X×nH2O) phases were synthesized by the co-precipitation method with different Fe/Al molar ratios. Various interlayer stabilizing anions X were considered, including Cl-, NO3-, and CO32-. The experimental conditions (in particular the fixed pH value) were optimized to make possible the formation of both the two end-members of the AFm phases (Al-AFm and Fe-AFm) [4]. The synthesized AFm phases were then characterized by using several methods. XRD measurements with Rietveld refinements analysis were performed to demonstrate the existence of a solid solution of the two AFm end-members. Spectroscopic techniques, including FTIR and Raman, were used to confirm the intercalation of the targeted anion. Finally, thermogravimetric analyses were carried out to quantify the water contained in the interlayers.
In order to mimic the durability of such AFm phases in an actual binder, the stability of the synthesized AFm phases has been also studied over time. The AFm phases were stored under various conditions such as under alkaline water, under saturated moisture atmosphere to promote carbonation, under chloride water to promote anionic exchanges, etc. XRD analyses were performed on the samples after various storage time periods to investigate the AFm phases stability.
This study on synthesized model phases is expected to be helpful for understanding the more complex hydration mechanisms of Fe/Al rich low-CO2 binders, in particular the likely formation of the Fe/Al AFm phases and their stability over time.
                                   
1.         L. Proaño, A.T. Sarmiento, M. Figueredo, M. Cobo, J. Clean. Prod., 263 (2020) 121457.
2.         B. Dilnesa, Ph.D. Thesis, 2012.
3.         J.-P. Rapin, Ph.D. Thesis, 2001.
4.         I. Rousselot, C. Taviot-Guého, F. Leroux, P. Léone, P. Palvadeau, J.-P. Besse, J. Solid State Chem., 167 (2002) 137–144.