The nature of C-S-H in hardened cements

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.