The nature of C-S-H in hardened cements

Calcium silicate hydrates (C-S-H) are the main binding phases in all Portla nd cement-based systems. This paper considers the morphology, composition, and nanostructure of C-S-H in a range of hardened cements. Inner product (I p) C-S-H present in larger Portland cement grains typically has a fine-scal e and homogeneous morphology with pores somewhat under 10 nm in diameter. I p 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 di stributions on backscattered electron images. In cement-slag blends, the fi brillar morphology of outer product (Op) C-S-H is gradually replaced by a f oil-like morphology as the slag loading is increased. It seems likely that this change in morphology is largely responsible for the improved durabilit y performance possible with slag-containing systems. The Ca/Si ratio of C-S -H in neat Portland cement pastes varies from similar to 1.2 to similar to 2.3 with a mean of similar to 1.75 The Ca/(Si + Al) ratio of C-S-H in water activated cement-slag pastes (0-100% slag) varies from similar to 0.7 to s imilar to 2.4; these limits are consistent with dreierkette-based models fo r the structure of C-S-H. Al substitutes for Si in C-S-H only in the "bridg ing" tetrahedra of dreierkette chains; this is true for a range of systems, including blends of Portland cement with slag, fly ash: and metakaolin. Th ese 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 g ood. (C) 1999 Elsevier Science Ltd. All rights reserved.