The distribution and morphology of CH particles in various cementitious systems was investigated to determine which parameters influence their nucleation and growth. Morphological image analysis of SEM-BSE images of polished cross-sections was used to measure CH distribution, particle size and shape within the microstructure. It was observed that CH crystallises according to two main patterns, the first consisting of a few, large, convoluted and unevenly distributed clusters, while the second consists of numerous smaller particles, evenly and closely distributed in the matrix. The growth of CH in Portland cement type systems seems to be the result of a specific interaction between the availability of gypsum and alumina, with CH crystals nucleating in the vicinity of gypsum particles. (c) 2007 Elsevier Ltd. All rights reserved.