S C

IE N

T IF

IC H

IG H

LI G

H T

S S

T R

U C

T U

R E

O F

M A

T E

R IA

L S

1 2 7 I H I G H L I G H T S 2 0 2 1

PRINCIPAL PUBLICATION AND AUTHORS

Dislocation-toughened ceramics, L. Porz (a), A. Klomp (a), X. Fang (a), N. Li (b), C. Yildirim (c), C. Detlefs (c), E. Bruder (a), M. Höfling (a), W. Rheinheimer (a,d), E.A. Patterson (a,e), P. Gao (b), K. Durst (a), A. Nakamura (f,g), K. Albe (a), H. Simons (h), J. Rödel (a), Mater. Horiz. 8, 1528-1537 (2021); https:/doi.org/10.1039/D0MH02033H (a) Technical University of Darmstadt, Department of Materials and Earth Science, Darmstadt (Germany) (b) Peking University, School of Physics, Beijing (China) (c) ESRF (d) Forschungszentrum Jülich GmbH, Institute for Energy and Climate Research, Jülich (Germany) (e) U.S. Naval Research Laboratory, Materials Science and Technology Division, Washington DC (USA) (f) Osaka University, Department of Mechanical Science and Bioengineering, Osaka (Japan) (g) PRESTO, Japan Science and Technology Agency, Tokyo (Japan) (h) Department of Physics, Technical University of Denmark, Kongens Lyngby (Denmark) (h) Norwegian University of Science and Technology, Department of Materials Science and Engineering, Trondheim (Norway)

REFERENCES

[1] J. Amodeo et al., Crystals 8, 240 (2018). [2] J.J. Gilman & W.G. Johnston, Solid State Phys. 13, 147-222 (1962). [3] D. Brunner et al., J. Am. Ceram. Soc. 84, 1161-1163 (2001). [4] A.C. Jakobsen et al., J. Appl. Crystallogr. 52, 122-132 (2019). [5] R.O. Ritchie, Nat. Mater. 10, 817- 822 (2011). [6] C. Wang et al., Science 368, 521- 526 (2020). [7] J. Li et al., Sci. Adv. 5, 5519 (2019).

demonstrated in Figure 105b for dislocations introduced by special polishing. Furthermore, nanopillar compression results already indicate much-improved ductility [7].

It is therefore expected that unprecedented mechanical behaviour of ceramics more like that of metals will be observed in the near future.

Shedding synchrotron light on hydration products of Portland cement with organic admixtures

When Portland cement hydrates in the presence of organic admixtures, new phases can form. This is the case of the M-phase, a calcium aluminate phase containing formate ions. Using synchrotron X-ray microcrystal diffraction, its crystal structure was successfully determined.

The use of organic admixture to modify rheological and mechanical properties of cement is well established in the concrete industry and dates back to 1950s [1]. Increasing the rate of development of early strength in the cement paste is a main goal in many applications. The admixtures accomplishing this goal are indicated as hardening accelerator, a class of materials encompassing

both organic and inorganic compounds. The class of non- corrosive accelerators includes several chemicals and, among these, formates are highly attractive, being readily available and not classified as harmful substances [2].

The net effect of adding formate to cement is dosage- dependent, and the mechanism of function is to increase the rate of hydration of the tricalcium silicate (C3S) and tricalcium aluminate (C3A) main constituent phases of Portland cement. The effect that formate ions exert at the atomic scale is not yet comprehensively understood, and one of the open questions is about the fate of the organic molecules in the hardened paste when hydration is complete. Studies suggest that formate ions can react with calcium aluminate components of cement to form hydration products with a layered structure (AFm phases in cement notation) or with ettringite-like structure [3]. These crystalline phases were synthesised starting from pure reagents, whereas their direct identification in hydrating cement pastes is hampered by their low concentration.

This work reports a new calcium aluminate phase containing formate ions that was identified in a Portland cement paste hydrated in the presence of high concentration of Ca-formate. The new phase has been indicated as the M-phase, and its crystal structure was successfully solved

Fig. 107: SEM-SE image of M-phase, selected acicular crystals picked up from hydrated cement paste.