Available online at www.rilem.net Materials and Structures 38 (May 2005) 435-442 1359-5997 © 2004 RILEM. All rights reserved. doi:10.1617/14343 ABSTRACT A study to understand and quantify the growth mode of C-S-H during the hydration of tricalcium silicate grains is presented here based upon number of complementary approaches: study of C-S-H nucleation during the hydration of tricalcium silicate compared with experimental studies of homogeneous nucleation; experimental kinetics studies of C-S-H formation during tricalcium silicate hydration in dilute suspensions; direct observations of C-S-H growth by Atomic Force Microscopy; Simulation of curves obtained from experiments in dilute suspensions from a model based on AFM observations. With AFM the formation of CSH is observed by the agglomeration of identical elements 60x30x5 nm 3 in size on alite surface. This agglomeration takes place perpendicularly and parallel to the surface. Numerical simulation of the experimental curves of the degree of hydration versus time enabled us to quantify the variation of the growth rates parallel and perpendicular to the surface with lime concentration. It is shown that the growth rate of C-S-H only depends on the lime concentration in solution. 1359-5997 © 2004 RILEM. All rights reserved. RÉSUMÉ Une étude pour comprendre et mesurer le mode de croissance de C-S-H pendant l'hydration des grains de silicate tricalcique est présentée ici, basée sur un certain nombre d’approches complémentaires : l’étude de la nucléation de C-S-H pendant l'hydration du silicate tricalcique comparée aux études expérimentales de nucléation homogène ; études expérimentales de cinétique de formation de C-S-H pendant l’hydration du silicate tricalcique dans les suspensions diluées ; observations directes de croissance de C-S-H par microscopie de Force Atomique ; simulation des courbes obtenues à partir des expériences dans les suspensions diluées à partir d'un modèle basé sur des observations AFM. L'AFM permet d'observer la formation de CSH par l'agglomération d’éléments de tailles identiques de 60x30x5 nm 3 sur la surface d'alite. Cette agglomération a lieu perpendiculairement et parallèlement à la surface. La simulation numérique des courbes expérimentales du degré d’hydration en fonction du temps nous a permis de mesurer la variation des taux de croissance parallèle et perpendiculaire à la surface avec la concentration en chaux. On montre que le taux de croissance le taux de C-S-H dépend seulement de la concentration en chaux en solution. 1. INTRODUCTION It is generally agreed that setting and hardening of Ordinary Portland Cement (OPC) are due to the formation of calcium silicate hydrate (CaO-SiO 2 -H 2 O noted C-S-H) [1, 2] during its hydration. An understanding of the factors controlling its formation is very important, particularly during the early period corresponding to the transition from workability to setting, associated with rapid changes in kinetics. It is now generally recognised that the formation of C-S-H occurs by a dissolution-precipitation process [3-6], but the factors controlling the so-called dormant period and acceleration period are the subject of controversy [7-9]. The setting of cement takes place in two mechanical steps [10-12] which are coagulation of cement particles and rigidification. The first step results from attractive force between unhydrated particles and leads to the formation of agglomerates, the second is due to the strengthening of the agglomerates by the precipitation of C-S-H. Since the development of mechanical properties of the paste during setting mainly depends on the growth mode of C-S-H and its ability to develop contacts with and stick together the hydrating cement grains, experimental studies relating to the nucleation and growth of C-S-H during this early period are required. As Portland cement paste is an extremely complex system, it is preferable to use tricalcium silicate, which is the main component, as a model system. A study to understand and quantify the growth mode of C-S-H during the hydration of tricalcium silicate grains is presented S. Garrault 1 , E. Finot 2 , E. Lesniewska 2 and A. Nonat 1 (1) LRRS, UMR CNRS 5613, Dijon, France (2) LPUB, UMR CNRS 5027, Dijon, France Received: 18 December 2003; accepted: 3 June 2004 Study of C-S-H growth on C 3 S surface during its early hydration Editorial Note Dr. André Nonat is a RILEM Senior Member.