Original paper

Thermal behaviour of Al-rich tobermorite

Biagioni, Cristian; Bonaccorsi, Elena; Lezzerini, Marco; Merlino, Stefano


The tobermorite supergroup is composed by a number of calcium-silicate-hydrate (C-S-H) minerals characterized by different hydration states and sub-cell symmetries. Taking into account their basal spacing, closely related to the hydration state, phases having a 14 Å (plombièrite), 11 Å (tobermorite, kenotobermorite, and clinotobermorite), and 9 Å (riversideite) basal spacing have been described. Tobermorite and kenotobermorite belong to the so-called tobermorite group and differ for their thermal behaviour which can be ''normal'' (the phase shrinks to a 9 Å phase at 300°C) or ''anomalous'' (the phase preserves its 11 Å basal spacing at 300°C). Specimens of Al-rich tobermorite from Montalto di Castro and Vallerano, Latium, Central Italy, showing a ''normal'' thermal behaviour, were studied in order to describe the transition from the 11 Å to the 9 Å phase by means of thermogravimetric–differential scanning calorimetry (TG-DSC) analyses as well as in situ and ex situ X-ray diffraction experiments. The TG-DSC analyses showed a continuous mass loss from 100°C up to 700°C, with different mass loss gradients between 100°C up to 300°C and between 300°C up to 700°C, corresponding to the dehydration of tobermorite and dehydroxylation of ''tobermorite 9 Å '', respectively. Above 700°C, ''tobermorite 9 Å '' is replaced by wollastonite. The X-ray powder diffraction data were collected at the GILDA beamline of the ESRF, Grenoble, France, from room temperature up to ca. 840°C. Tobermorite is completely replaced by the 9 Å phase at ca. 300°C, whereas the latter is transformed into wollastonite at ca. 700°C. The transition from the 11 Å to the 9 Å phase seems to be favoured by the transient appearance of a clinotobermorite-like compound.


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