The calcium silicate hydrates

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Abstract

This article is concerned with the calcium silicate hydrates, including crystalline minerals and the extremely variable and poorly ordered phase (C-S-H) that is the main binding phase in most concrete. Up-to-date composition and crystal-structure information is tabulated for the most important crystalline calcium (alumino) silicate hydrates and related phases. A number of models for the nanostructure of C-S-H are summarized and compared and it is shown that there is much more of a consensus than might seem apparent at first sight. The value of the recently solved structures of 1.4 nm tobermorite and jennite, together with those of jaffeite and metajennite, for visualizing the nanostructural elements present in the models is demonstrated. The importance of Hal Taylor's contribution to the solution of the structure of jennite is highlighted. The applicability of Richardson and Groves' model is demonstrated using experimental composition-structure observations on the nature of C-S-H in a Portland cement-fly ash blend.

Introduction

Many naturally occurring crystalline calcium silicate hydrates or calcium aluminosilicate hydrates are relevant to Cement Science, either because they are formed hydrothermally — as in deep oil-well or geothermal cements — or in autoclaved or hot-pressed materials, or because their structures have proved to be very useful in modeling the structure of the extremely variable and poorly ordered phase that is essentially the ‘glue’ of the concrete part of the Built Environment. The nature of this phase was discussed [1], [2], [3] at the meeting in London in 1918 that later came to be considered to be the first in the series of international conferences on the chemistry of cement of which this is the twelfth.1 In the opening sentence to the first paper read at that meeting in 1918, Cecil H. Desch noted with regard to calcareous cements that ‘…our knowledge of the scientific nature of the materials and processes involved is even yet imperfect, in spite of many excellent investigations covering various parts of the subject’. The same statement could be used today, but ninety additional years of investigations — utilizing increasingly sophisticated experimental techniques — have of course resulted in many significant advances in our knowledge; the Proceedings of the international conferences in this series have to date been a valuable record of those advances, as is outlined by Francis Young elsewhere in this issue [4]. The purpose of this article is threefold: firstly, to tabulate up-to-date composition and crystal-structure information for the most important calcium (alumino) silicate hydrates and related phases, which should be a useful reference source for others interested in these phases; secondly, to summarize and compare the various models that exist for the nanostructure of the calcium silicate hydrate (C-S-H) phase in hardened C3S and Portland cements — or in blends of Portland cement with supplementary cementing materials —many of which are in fact much more similar to one another than might seem apparent at first sight; and thirdly, to highlight the recent solutions to the structures of jaffeite, jennite, metajennite (as yet unrefined) and 1.4 nm tobermorite, and to demonstrate how the structures of these phases are useful for visualizing the models for the nanostructure of C-S-H. The material in this article is extended in a companion paper [5] in which the cohesion forces that act between individual C-S-H layers or crystallites are considered, and two possible strategies are discussed for tuning the mechanical properties of cementitious materials based on modifying the bonding scheme in C-S-H.

Section snippets

Crystalline calcium silicate hydrates

Crystal-structure data are given in Table 1 for the most important calcium (alumino) silicate hydrates and related phases. The references for the data that are associated with full crystal-structure solutions are shown underlined; a selection of other relevant references are included (not underlined), with those published since the 11th International Congress on the Chemistry of Cement (ICCC) are shown in bold type. The arrangement of the phases in the table is derived from Table II of Taylor

Models for the nanostructure of C-S-H in hardened C3S and Portland cements, or in blends of Portland cement with supplementary cementing materials

Unfortunately, in contrast to the phases in Table 1, the C-S-H that is the principal binding phase in most concrete is nearly amorphous, and so X-ray diffraction techniques are of limited value. Any model that is developed for its nanostructure must of course be consistent with experimental observations from other techniques; most importantly, it must be compatible with the wide ranges in chemical composition and silicate (or aluminosilicate) anion structure that have been observed. The C-S-H

Tobermorite (1.4 nm), jaffeite, metajennite and jennite

It is evident from Table 2 and the discussion in Section 3 that most models for the nanostructure of C-S-H in hardened Portland cement pastes involve elements of tobermorite-like structure. In a number of cases, these are intermixed with others of jennite-like structure (or jaffeite-like; see Section 4.2): the so-called tobermorite-jennite, or T/J, viewpoint [177], [179]. As a consequence, perhaps two of the most significant papers for Cement Science that have been published since the 11th

Visualization of models for the nanostructure of C-S-H.

A consequence of the publication of the crystal structures of jaffeite [112], jennite [70], and 1.4 nm tobermorite [66] is that it is now much easier to build, display and manipulate the type of nanostructural units that are proposed in the various models for C-S-H. The result is that the similarities between the models are more readily apparent. An extensive set of illustrative figures is given in Richardson [189], which were derived from these structures, together with Taylor's trial

Example of the applicability of the models to C-S-H in hardened cements

A number of examples of the applicability of the models to C-S-H in hardened cements are given in Richardson [189], specifically regarding C-S-H present in hardened pastes of C3S, β-C2S, neat Portland cement and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume. Additional examples have also recently been published for C-S-H in a 80% white Portland cement (WPC)-20% metakaolin blend hydrated at 25 °C [191], for neat Portland cement hydrated at 55 °C [192], a 70%

Size of particles of C-S-H in hardened C3S and neat Portland cement pastes

It was noted in the Introduction that a companion paper in this issue is concerned with the cohesion forces that act between individual layers or crystallites of C-S-H [5]; the purpose of this section is to lead into that paper by considering the size of individual particles of C-S-H. The example that is used to illustrate a small particle — which employs J-based (i.e. jaffeite- or jennite-based) type of structure for the C-S-H — also demonstrates possible reasons for the persistence of

Summary

This article is concerned with the calcium silicate hydrates, including crystalline minerals and the extremely variable and poorly ordered phase (C-S-H) that is essentially the ‘glue’ of the concrete part of the Built Environment. Up-to-date composition and crystal-structure information is tabulated for the most important crystalline calcium (alumino) silicate hydrates and related phases, which should be a useful reference source for others interested in these phases. A number of models for the

Acknowledgements

Thanks are due to the Engineering and Physical Sciences Research Council for funding under Grant No. GR/S45874/01, to Dr. Andrew Brown for taking the HRTEM image and to Castle Cement, UK Nirex Ltd., and Lafarge Cements for the additional technical and financial support.

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