Natural calcium silicate hydrates build up a wide family of minerals extraordinarily interesting for the variety of their structural arrangements, the peculiarity of the transformation (in particular dehydration) processes in which they are involved, the relationships with compounds which form during the hydration of the Portland cement. Particular attention will be devoted to four groups of minerals.1. Minerals of the tobermorite group. The relationships among the various natural compounds in the group, clinotobermorite, tobermorite 11Å, riversideite (tobermorite 9Å), plombierite (tobermorite 14Å) and the OD character of each single compound, have been clarified by recent structural studies. Particularly interesting are the dehydration processes which occur in a fairly topotactical way, with preservation of the basic building module which characterizes the whole group of minerals. 2. Okenite and nekoite. Okenite is a unique example of silicate structure presenting simultaneously tetrahedral chains (double wollastonite chains of peculiar shape) and tetrahedral layers. The thermal behaviour of okenite indicates successive steps in the dehydration process and points to the formation of intermediate phases before final 'topotactical' transformation to wollastonite. 3. Minerals of the gyrolite group. The various phases build up a modular family in which the building modules are calcium octahedral layers (O) and two types of tetrahedral sheets (S1 and S2). By different combinations of these modules the distinct members of the family are obtained: reyerite and truscottite (O-S1-O-S2-S2-O…); fedorite (and synthetic K-phase) (O-S2-S2-O…); gyrolite (O-S1-O-S2-X-S2-O-S1….); Z-phase (O-S2-X-S2-O…). Both Z-phase and gyrolite presents similar thermal behaviour, with topotactical transformation to fedorite and truscottite, respectively, on dehydration at 400-500°C and to pseudowollastonite at higher temperatures (800°C).4. Jennite and metajennite. Very recent structural studies have clarified the structural arrangements of both phases and the elationships between them. As the various members of the tobermorite group, they display OD features. As in the case of the other mineral groups presented in this contribution, jennite presents a stepwise dehydration, with a first step (70-140°C) representing the formation of meta-jennite, and final formation (680-800°C) of the anhydrous phases Ca2SiO4 o beta-CaSiO3.
Calcium silicate hydrate (CSH) minerals: structures and transformations
BONACCORSI, ELENA;
2004-01-01
Abstract
Natural calcium silicate hydrates build up a wide family of minerals extraordinarily interesting for the variety of their structural arrangements, the peculiarity of the transformation (in particular dehydration) processes in which they are involved, the relationships with compounds which form during the hydration of the Portland cement. Particular attention will be devoted to four groups of minerals.1. Minerals of the tobermorite group. The relationships among the various natural compounds in the group, clinotobermorite, tobermorite 11Å, riversideite (tobermorite 9Å), plombierite (tobermorite 14Å) and the OD character of each single compound, have been clarified by recent structural studies. Particularly interesting are the dehydration processes which occur in a fairly topotactical way, with preservation of the basic building module which characterizes the whole group of minerals. 2. Okenite and nekoite. Okenite is a unique example of silicate structure presenting simultaneously tetrahedral chains (double wollastonite chains of peculiar shape) and tetrahedral layers. The thermal behaviour of okenite indicates successive steps in the dehydration process and points to the formation of intermediate phases before final 'topotactical' transformation to wollastonite. 3. Minerals of the gyrolite group. The various phases build up a modular family in which the building modules are calcium octahedral layers (O) and two types of tetrahedral sheets (S1 and S2). By different combinations of these modules the distinct members of the family are obtained: reyerite and truscottite (O-S1-O-S2-S2-O…); fedorite (and synthetic K-phase) (O-S2-S2-O…); gyrolite (O-S1-O-S2-X-S2-O-S1….); Z-phase (O-S2-X-S2-O…). Both Z-phase and gyrolite presents similar thermal behaviour, with topotactical transformation to fedorite and truscottite, respectively, on dehydration at 400-500°C and to pseudowollastonite at higher temperatures (800°C).4. Jennite and metajennite. Very recent structural studies have clarified the structural arrangements of both phases and the elationships between them. As the various members of the tobermorite group, they display OD features. As in the case of the other mineral groups presented in this contribution, jennite presents a stepwise dehydration, with a first step (70-140°C) representing the formation of meta-jennite, and final formation (680-800°C) of the anhydrous phases Ca2SiO4 o beta-CaSiO3.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
