J Computer-Aided Mater Des DOI 10.1007/s10820-006-9030-0 On the formation of cementitious C–S–H nanoparticles H. Manzano · A. Ayuela · J. S. Dolado Received: 3 June 2006 / Acceepted: 1 September 2006 © Springer Science+Business Media B.V. 2007 Abstract This work explores from a theoretical viewpoint the underlying growth mechanisms which govern the formation of the most important hydration product present in cementitious environments, the so called C–S–H (calcium–silicate–hydrate) gel. Aiming at identifying the basic building blocks which make up the cementitious C–S–H gel, we have performed ab-initio calculations at the Hartree Fock (HF) level. Two different growth mechanisms have been identified depending on the amount of Si and Ca ions, which naturally lead to the appearance of tobermorite-like and jennite-like nano-crystals. Keywords Cement-based materials · C–S–H gel · Tobermorite · Jennite · ab-initio calculations · Self-assembly 1 Introduction The chemistry involved in the formation of cement-based materials is certainly complex and appealing. Just by pouring cement powder to water, a myriad of chem- ical reactions take place to form a rigid, complex and porous structure that is called cement paste. This cement paste is a multiphase material [1] which evolves over H. Manzano · J. S. Dolado Labein Centro Tecnológico, C/Geldo Ed. 700, Parque Tecnológico de Bizkaia, 48160 – Derio, Bizkaia, Spain A. Ayuela Departamento de Física de Materiales, Facultad de Químicas, Universidad del País Vasco, Unidad de Física de Materiales, Centro Mixto CSIC-UPV/EHU and Donostia International Physics Center (DIPC), P.O. Box 1072, 20018 San Sebastian/Donostia, Spain J. S. Dolado (B ) Nanostructured and Eco-efficient Materials for Construction Unit, Associated Unit LABEIN-Tecnalia/CSIC, 48160 Derio, Bizkaia, Spain e-mail: jsanchez@labein.es