The Hydration of Slag in Cement, Part 1: Reaction Models for Alkali-Activated Slag
W. Chen
&
H.J.H. Brouwers
Department of Construction Management & Engineering, Faculty of Engineering Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Abstract
Keywords: Reaction models; Hydration product; alkali activation; slag, C-S-H
1. Introduction
Researches on the alkali-activated slag (AAS) grew remarkably in the recent decades because of its numerous advantages such as fast development of strength, high resistance to chemical attack, and low heat release, as compared to normal Portland cement (Glukhovsky (1980)). Furthermore, as the slag is an industrial by-product, its usage in the AAS has many environmental merits such as low energy cost, usage of secondary raw materials and low pollutant gas emission, which make it preferable to conventional binders.
The commonly used alkali activators in AAS are sulfate, Portland cement, sodium silicate and sodium hydroxide, whose anions or anion groups react with the calcium to form insoluble or less soluble calcium products. Typical AAS consists of GGBFS with 3.5%–5.5% (in mass) of Na2O added, usually as sodium hydroxide (NaOH) or water-glass. Increasing the dosage of activator will consequentially accelerate the strength development rate and increase the 28d strength of the paste (Narang & Chopra (1983)).
Till now the reaction mechanism of AAS is not fully understood and the chemistry of slag reaction in the AAS remains unclear. The lack of knowledge on the hydration of slag obstructs the further application of the AAS, on which there are already many researches since 1950s. Much work has been done to analyze the hydration products of slag and to characterize the hardened AAS paste (Narang & Chopra (1983), Schilling et al. (1994), Shi & Day (1995), Wang & Scrivener (1995), Schneider et al. (2001), Brough & Atkinson (2002), Puertas & Jimenez (2003)). Most researches found that the main hydration product from the activated slag was a low-basic calcium silicate hydrate (C-S-H). The other hydration products observed in the pastes differed slightly due to the differences in the slag composition, curing time, temperature, and the activators used. Although there are already numerous researches on the hydration of AAS, to the authors’ knowledge, no study can be found on the reaction occurring in the AAS paste, i.e. what products are exactly formed from the slag, their quantities and the composition of the hydration products. Modeling the hydration of the alkali activated slags will bring conceivable opportunities for evaluating the potential of reactants and predicting performances of the hardened AAS paste without the need of time-consuming and costly experiments. The reaction model, together with the physical properties of the hydration products, can be used in predicting the physical properties of the hardened AAS paste, e.g. the chemical shrinkage evolved during the AAS hydration, the strength development, and the waste binding capability of AAS, which are important while exploring new applications of AAS binder.
In this study, reaction models are proposed to quantify the hydration products and to determine the composition of the main hydration product C-S-H from the activated slag. The models established in this study are validated with a serial of experiments on AAS selected from literature. The models are further applied to compute the water retention by the hydration products of AAS and sequentially to derive the hydration equation of AAS. As illustration to one of the model applications, chemical shrinkage of the AAS paste at different hydration stages is predicted. In Part II of this research, reaction models for the slag blended cement are addressed using models developed in this study (Brouwers & Chen (2004)).
