Influence of activator type on hydration kinetics, hydrate assemblage and microstructural development of alkali activated blast-furnace slags

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Abstract

The hydration of two slags with different Al2O3 contents activated with sodium hydroxide and hydrous sodium metasilicate (commonly named water glass) is studied using a multi-method approach. In all systems, C-S-H incorporating aluminium and a hydrotalcite-like phase with Mg/Al ratio ~ 2 are the main hydration products. The C-S-H gels present in NaOH activated pastes are more crystalline and contain less water; a calcium silicate hydrate (C-S-H) and a sodium rich C-N-S-H with a similar Ca content are observed at longer hydration times. The activation using NaOH results in high early strength, but strength at 7 days and longer is lower than for the sodium metasilicate systems. The drastic difference in C-S-H structure leads to a coarser capillary porosity and to lower compressive strength for the NaOH activated than for the sodium metasilicate activated slags at the same degree of slag reaction.

Introduction

In order to reduce the CO2-emissions related to cement manufacturing, the application of industrial by-products rich in aluminium and silicon oxide in alkaline activated binders is a promising option [1], [2]. Materials generally used are blastfurnace slag, fly ash or metakaolin, which are activated by the addition of alkalis like alkali silicates or hydroxides [3], [4], [5], [6].

Alkali activated slags (AAS) can have high strength development and using the adequate activators can lead to rapid setting, good durability and high resistance to chemical attack [2], [3], [4], [5], [6], [7], [8]. The main hydration products found in AAS are C-S-H with a low Ca/Si ratio related mainly to the composition of the slag and the nature of activators used, hydrotalcite intimately intermixed with the C-S-H in the MgO containing slag and in some cases also an AFm phase, most probably strätlingite [9], [10], [11], [12], [13]. The C-S-H produced by alkali activated systems may incorporate a higher content of Al2O3 based on the initial composition of the slag [12], [14], [15]. Many variables influence the reaction of alkali activated slags such as fineness, chemical composition, water/binder (w/b) ratio, temperature or the pH [16], [17], [18], [19], [20], [21], [22], [23]. A better understanding of the effects of alkaline activators such as NaOH and hydrous sodium metasilicate (Na2SiO3·5H2O) on the hydration mechanisms of alkali activated slag could indicate ways to optimise the use of alkaline activators.

An important factor that influences the porosity and thus the compressive strength is the kinetics of the hydration. In Portland cement systems it has been observed that a fast initial reaction as e.g. caused by elevated temperatures or accelerators, results in the formation of a dense hydration product and less reduction of the initial porosity that affects adversely the mechanical properties at later ages [24], [25].

This paper investigated the major hydration products formed in alkali activated slags and their composition, morphology and spatial distribution. The study was carried out using two slags with different Al2O3 content to investigate the influence of the slag composition on the types of hydrates formed. An image analysis method was adopted to quantify the degree of hydration and the amount of coarse porosity directly from the backscattered electron (BSE) image of polished slag pastes samples. By studying the coarse porosity and degree of hydration using different alkaline activators, the relationship between microstructural properties and compressive strength was established.

Section snippets

Materials and experiments

Two ground granulated blast furnace slags (GGBFS) with different Al2O3 contents were studied: GGBFS LA exhibits a low Al2O3 content of 7.1 wt.%, and GGBFS HA a higher content of 12.0 wt.% Al2O3 (Table 1) with a similar particle size distribution. Two types of activators were used: NaOH and hydrous sodium metasilicate Na2SiO3·5H2O (labeled here as NSH5 and commonly referred to as water glass). The dosage of both activators and the amount of water were selected to obtain the same Na2O content and

Strength development

Depending on the activators used, the development of the compressive strengths differs (Table 3). NaOH as activator gives a high compressive strength already after 1 day. In contrast, the NSH5 activated pastes have no measurable compressive strength after 1 day, but the compressive strength after 7 days is higher than for the NaOH activated slags. After 180 days, the compressive strength of the NSH5 activated slags is twice as high as the strength of the NaOH activated slags. The samples activated

Conclusions

The investigated alkaline activators NaOH and sodium metasilicate pentahydrate (NSH5) have a strong impact on microstructure and compressive strength of slag based binders by influencing the hydration kinetics, porosity and hydration products. Two slags with different Al2O3 contents (7 wt.% and 12 wt.%) are used in this study. In all systems, a C-S-H incorporating aluminium and a hydrotalcite phase are the main hydration products.

Clear differences are observed between the NaOH and the NSH5

Acknowledgements

The authors would like to acknowledge Walter Trindler and Boris Ingold for their help with the experimental work. The authors would like to acknowledge also the anonymous reviewers that improved the quality of the paper with their helpful comments.

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