Internationale Ausgabe: DOI: 10.1002/anie.201611858 Mineral Synthesis Very Important Paper Deutsche Ausgabe: DOI: 10.1002/ange.201611858 Ultra-Fast Supercritical Hydrothermal Synthesis of Tobermorite under Thermodynamically Metastable Conditions Marta Diez-Garcia, Juan J. Gaitero,* Jorge S. Dolado, and Cyril Aymonier* Abstract: Tobermorite is a fibrillar mineral of the family of calcium silicates. In spite of not being abundant in nature, its structure and properties are reasonably well known because of its interest in the construction industry. Currently, tobermorite is synthesized by hydrothermal methods at mild temperatures. The problem is that such processes are very slow (> 5 h) and temperature cannot be increased to speed them up because tobermorite is metastable over 130 8C. Furthermore the product obtained is generally foil-like and not very crystalline. Herein we propose an alternative synthesis method based on the use of a continuous flow reactor and supercritical water. In spite of the high temperature, the transformation of tobermorite to more stable phases can be prevented by accurately controlling the reaction time. As a result, highly crystalline fibrillar tobermorite can be obtained in just a few seconds under thermodynamically metastable conditions. Calcium silicate hydrates form a wide family of minerals of varying compositions in terms of the Ca/Si ratio. Of all the phases of the family, tobermorite has probably one of the largest industrial interests owing to its unique properties. In particular, tobermorite)s structure is used as a model to describe the main component of the cement paste, the so- called C-S-H gel. [1–3] Furthermore, well-crystallized tobermor- ite has been found in long-lasting Roman underwater buildings, [4] and it is the main component in some precast construction elements. [5] Some studies have also explored the possibility of using it as seeding addition for cement composites [6] or as a stabilization agent for the capture of contaminants. [7–11] Tobermorite is considered as a rare mineral that in nature only crystallizes under hyper-alkaline hydrothermal condi- tions. [12] In fact, the difficulty of obtaining large and pure crystals is one of the main reasons why its structure has been a matter of debate for many years. [13] Nevertheless, it is generally accepted that calcium oxide layers are parallel to (0,0,1) and flanked on both sides by wollastonite-like chains of silicate tetrahedral. [12, 14] Tobermorite crystallizes with three different degrees of hydration and depending on the amount of water, the basal distance between two consecutive layers of Calcium Oxide changes from 14  to 11  or 9  giving place to the so-called 14  tobermorite, 11  tobermorite or 9  tobermorite. The most common structure under ambient conditions is tobermorite 11  (from here on we will use the term tobermorite in reference to the 11  variety), which can exist in two different polymorphs: normal and anomalous. The main difference between them is that while the normal form transforms into 9  tobermorite upon heating at 300 8C, the anomalous one remains stable in spite of the loss of water. [15–17] Some researchers claim that only in anomalous tobermorite are the silicon chains attached to different calcium layers linked together by Si-O-Si bonds giving a rigid structure. [16] However, others argue that such links exist in both cases and the difference is solely due to the presence of zeolitic calcium ions in the normal form. [13] The presence of such zeolitic calcium ions would also be accompanied by a small change in stoichiometry that, according to Merlino et al., varies from Ca 4.5 Si 6 O 16 OH·5 H 2 O in normal tobermorite to Ca 4 Si 6 O 15 (OH) 2 ·5 H 2 O in the anomalous form. [13] Owing to the scarcity of the mineral, some synthesis methods have been developed. Although the most common one involves a hydrothermal treatment pure crystalline tobermorite is difficult to obtain being often accompanied of amorphous or badly ordered material (Figure 1). Many researchers have studied how different parameters, such as reaction time, [14, 18, 19] Ca/Si ratio, [18, 20] temperature, [14, 21] pH, [18] precursors and partial substitution of aluminum for sili- con [12, 14, 22] affect the resulting product. Some research has also investigated the effect of some additives on the tobermorite synthesis such as EDTA, [18] Borosilicate, [20] sucrose [23] or still Ca-formate. [24] Most of them used CaO and SiO 2 as precur- sors, but other products like calcsilicate alkoxide gel [12] or Ca(NO 3 ) 2 and Na 2 SiO 3 [25] have also been employed. In some cases, a milling step of the precursors was included before the hydrothermal treatment. [21, 26] Under hydrothermal condi- tions, reaction times vary from 5 h to 56 days, with synthesis temperature rarely exceeding 200 8C. [12, 14, 25, 27–32] In fact, this is a critical point because, although tobermorite is considered to be stable over a range of compositions from Ca/Si = 0.8 to 1 at room temperature, [12] its instability under hydrothermal conditions has been widely reported. For instance, Hong and Glasser [33] concluded that tobermorite is stable at 120 8C but starts to transform into xonotlite at temperatures above 130 8C when xonotlite starts to be formed. On the same line, Black et al. [26] said that over the temperature range 170– [*] M. Diez-Garcia, C. Aymonier CNRS, Univ. Bordeaux, ICMCB, UPR 9048 F-33600 Pessac (France) E-mail: cyril.aymonier@icmcb.cnrs.fr M. Diez-Garcia, J. J. Gaitero, J. S. Dolado Sustainable Construction Division Tecnalia Parque tecnológico de Bizkaia C/Geldo, Edif. 700, 48160 Derio (Spain) E-mail: juanjose.gaitero@tecnalia.com M. Diez-Garcia UPV-EHU, Dep. Mining-Metallurgy Engeneering and Mat. Science Alameda Urquijo s/n, 48013 Bilbao (Spain) Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.201611858. A ngewandte Chemi e Zuschriften 1 Angew. Chem. 2017, 129,1–6  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü