Impact of gluconate and hexitol additives on the precipitation mechanism and kinetics of C-S-H

TL;DR

This study explores how gluconate and hexitol additives affect the formation and growth of calcium silicate hydrate (C-S-H), revealing a retarding mechanism that influences nucleation kinetics and cluster aggregation.

Contribution

It provides new insights into the role of organic additives in modifying C-S-H precipitation mechanisms and kinetics through experimental and theoretical analysis.

Findings

Additives increase the kinetic pre-factor of nucleation.

Organics cause steric hindrance, retarding cluster aggregation.

Multi-step nucleation pathway persists with organics.

Abstract

The present paper investigates the influence of gluconate and hexitol additives on the precipitation mechanism and kinetics of C-S-H. To this end, wet chemistry C-S-H precipitation experiments were performed under controlled conditions of solution supersaturation, under varying silicate concentration, while the transmittance of the solution was followed. This allowed determining induction times for the formation of C-S-H precursors in the presence and absence of gluconate and three hexitol molecules. Characterization of the precipitates was performed via small angle X-ray scattering and cryo-transmission electron microscopy experiments, which allowed the identification of a multi-step nucleation pathway also in the presence of the organics. Analysis of the induction time data in the framework of the classical nucleation theory revealed a significant increase of the kinetic pre-factor, which is associated to physical rates of cluster collision and aggregation during the nucleation process. Values of the kinetic pre-factor increase in the same order as the complexation constants of calcium and silicate with the each of the organics. This points to a retarding mechanism of crystallization related to steric hindrance of the aggregation of the early formed clusters via adsorption of the organics at their surfaces.