S T R U C T U R E O F M A T E R I A L S

S C I E N T I F I C H I G H L I G H T S

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PRINCIPAL PUBLICATION AND AUTHORS

The crystal structure of a new calcium aluminate phase containing formate, M.C. Dalconi (a), G. Artioli (a), N. Masciocchi (b), C. Giacobbe (c), F. Castiglioni (d), G. Ferrari (d), J. Cem. Con. Res. 146, 106490 (2021); https:/ www.doi.org/10.1016/j.cemconres.2021.106490 (a) Department of Geosciences and Circe Center, Unipd (Italy) (b) Department of Science and High Technology and To.Sca.Lab., University of Insubria (Italy) (c) ESRF (d) Mapei S.p.A. (Italy)

REFERENCES

[1] P.-C. Aïtcin & R.J. Flatt, Science and Technology Of Concrete Admixtures, Woodhead Publishing Series in Civil and Structural Engineering (2016). [2] P.C. Hewlett & M. Liska, Lea s Chemistry of Cement and Concrete, Elsevier (2019). [3] H. Pöllmann et al., Proceedings International Conference, Avignon (2014). [4] J. Wright et al., Curr. Opin. Solid St. M. 100818 (2020).

using synchrotron X-ray single-crystal diffraction thanks to the micro-focussing capabilities of the nanoscope station of beamline ID11 [4].

The cement paste was prepared mixing CEM-I 52.5 R with Ca-formate at high dosage (27.5 wt%) in excess of water (water/cement ratio of 3.67) and maintaining the suspension stirring at 70°C for 24 hours. Analysis of X-ray powder diffraction data of the air-dried sample revealed the presence of distinctive hydration products of Portland cement (ettringite, portlandite and C-S-H) plus extra diffraction peaks related to a new crystalline phase not reported in the crystallographic databases. Direct imaging of the new phase was obtained by SEM-EDX, revealing the presence of acicular crystals (Figure 107) mainly composed of CaO and Al2O3. The same acicular crystals analysed by Raman spectroscopy were also revealed to be composed of formate molecules.

Tiny single acicular crystals were directly picked up from the cement paste, mounted on a microloop and irradiated with an X-ray beam of 1-µm diameter. The data quality allowed to obtain approximate unit cell parameters and a preliminary structural model that was used as input for the Rietveld refinement, enabling confirmation of the final structure. The new phase has been indicated as the M-phase with molecular formula Ca6(Al(OH)6)2(HCOO)6, and its crystal structure has been solved in R-3 space group (a = 13.739 Å, c = 10.589 Å, Vol = 1730.99 Å3, density = 2.18 g/cm3).

The main structural features of the M-phase encompass columns composed of octahedral AlO6 alternating with groups of three CaO7 polyhedra and running parallel to

c crystallographic direction (Figure 108). The formate ions are located in the intercolumn space bridging the Ca polyhedra. This structural arrangement of Al and Ca polyhedra sharing edges and forming columns resembles that of ettringite (Ca6(Al(OH)6)2(SO4)3⋅(H2O)26), a calcium aluminate containing sulfate ions and water molecules.

Dissimilarly to ettringite, where the columns are well- separated and the intercolumn space hosts water molecules, in the M-phase, the columns are connected to each other through edge-sharing Ca polyhedra, which thus form a more compact structure, and no water molecules can be hosted. The crystal structure of the M-phase testifies to the strong interaction occurring between small organic molecules such as formate and calcium aluminate components of the cement phases, and it sheds light on the possible fate of formate in hydrated cement pastes. The proposed structural model of the M-phase fills a gap in the knowledge of the possible hydration products of Portland cement and on their interaction with formate ions.

Fig. 108: a) Structural model of M-phase view along [001] (top) and (b) [120] (bottom) directions. Blue: Al octahedra;

green: Ca polyhedra; brown: formate; red: oxygen.