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The nature of C-S-H in hardened cements

https://doi.org/10.1016/S0008-8846(99)00168-4Get rights and content

Abstract

Calcium silicate hydrates (C-S-H) are the main binding phases in all Portland cement-based systems. This paper considers the morphology, composition, and nanostructure of C-S-H in a range of hardened cements. Inner product (Ip) C-S-H present in larger Portland cement grains typically has a fine-scale and homogeneous morphology with pores somewhat under 10 nm in diameter. Ip from larger slag grains also displays this morphology, but is chemically distinct in having high content of Mg and Al. The hydrated remains of small particles—whether of Portland cement, slag or fly ash—contain a less dense product with substantial porosity surrounded by a zone of relatively dense C-S-H; this has implications for the analysis of porosity and pore-size distributions on backscattered electron images. In cement-slag blends, the fibrillar morphology of outer product (Op) C-S-H is gradually replaced by a foil-like morphology as the slag loading is increased. It seems likely that this change in morphology is largely responsible for the improved durability performance possible with slag-containing systems. The Ca/Si ratio of C-S-H in neat Portland cement pastes varies from ∼1.2 to ∼2.3 with a mean of ∼1.75. The Ca/(Si + Al) ratio of C-S-H in water activated cement-slag pastes (0–100% slag) varies from ∼0.7 to ∼2.4; these limits are consistent with dreierkette-based models for the structure of C-S-H. Al substitutes for Si in C-S-H only in the “bridging” tetrahedra of dreierkette chains; this is true for a range of systems, including blends of Portland cement with slag, fly ash, and metakaolin. These data support Richardson and Groves' general model for substituted C-S-H phases. The bonding of C-S-H to other products of hydration is generally good.

Introduction

Calcium silicate hydrates (C-S-H) are the main binding phases in all Portland cement-based systems; their exact nature is central to the science of cement and concrete. This article presents data principally for the C-S-H phases present in neat Portland cements, in blends of Portland cement with ground granulated blast-furnace slag, and in alkali-hydroxide activated slags; data are also reported which illustrate the similarity of the C-S-H phases present in blends of Portland cement with fly ash and metakaolin to those with slag.

Section snippets

Materials

Details of the materials used in this work are given elsewhere: ordinary Portland cement-granulated blast-furnace slag blends 1, 2; white cement-slag blends [3]; white cement-fly ash and white cement-metakaolin blends [4]; slags activated by KOH solution [5]; synthetic slag-glass [6]. White cement was needed for the nuclear magnetic resonance (NMR) spectroscopy work because of its low Fe content; the presence of paramagnetic ions causes peak broadening in NMR.

Specimen preparation and experimental details

The slag- and pozzolan-Portland

Reference microstructure

Forty years ago Taplin [11] thought it “convenient to designate those products which lie within the original boundaries of the clinker particles ‘inner’ products, and those which lie ‘outside’ ‘outer’ products.” The scheme has been adopted widely (a few examples are 2, 7, 12, 13, 14, 15, 16, 17, 18, 19) and, although there is not necessarily an exact correspondence between the positions of the outer boundaries of inner product (Ip) and the original grains [20], it is straightforward and is

Conclusions

  • 1.

    Taplin's [11] long-standing “inner-outer” classification of the products of cement hydration is supported strongly by the high resolution technique of transmission electron microscopy of ion-thinned sections. The more recent “phenograin-groundmass” classification due to Diamond and Bonen [31] would seem to have little application outside the description of backscattered electron images.

  • 2.

    Ip C-S-H present in larger Portland cement grains typically has a fine-scale and homogeneous morphology with

Acknowledgements

Thanks are due to the Engineering and Physical Sciences Research Council for funding under Grant Nos. GR/H64972, GR/K52089 and GR/K65478, to Dr Geoff. Groves and Prof. Joe Cabrera for encouragement and support, to Prof. Neville Boden (SOMS Centre, University of Leeds) and Prof. Chris Dobson FRS (Inorganic Chemistry Lab., University of Oxford) for provision of the NMR facilities, to Prof. J. Francis Young and Ms. Xiaofeng Zhu for the TMS work (Centre for Cement Composite Materials, University of

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