Oxidation of pyrite (FeS2) and troilite (FeS) impurities in kaolinitic clays after calcination

Franco Zunino,Karen Scrivener

doi:10.1617/s11527-021-01858-9

Franco Zunino, Karen Scrivener

Open Access

https://doi.org/10.1617/s11527-021-01858-9

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Abstract

Valorisation of locally available clays for producing blended cements is crucial for a widespread adoption of sustainable binders incorporating these materials. In some places, clays can be intermixed with small amounts of iron sulfides, which could eventually expand in the alkaline media of concrete and lead to cracking if clay particles are sufficiently fine. This study explored the stability of iron sulfides, namely troilite and pyrite, during calcination of clays and their influence in reactivity. It was found that both troilite and pyrite decompose and oxidize into hematite under typical calcination conditions for clays. Furthermore, there is no negative influence of the presence of iron sulfide phases on the reactivity of calcined clays. This shows that these clays are suitable for use in blended cements, provided that adequate quality control is conducted to ensure a complete decomposition of the iron sulfide phases.

Highlights

The substitution of clinker by supplementary cementitious materials (SCMs) is the most effective way to reduce the carbon footprint associated with the Portland cement industry [1]
This paper studied the suitability of using kaolinitic clays containing iron sulfides as sources of SCMs
Based on the results presented, the following conclusions can be drawn: 1. Pyrite (FeS2) and troilite (FeS) oxidised in an air atmosphere when subjected to common calcination conditions for calcined clay production (800 °C, 30 min), leading to the formation of hematite (Fe2O3) and the release of SOX gas during the process

Summary

Introduction

The substitution of clinker by supplementary cementitious materials (SCMs) is the most effective way to reduce the carbon footprint associated with the Portland cement industry [1]. The combination of limestone and calcined clays in cement (LC3: limestone calcined clays cement) enables the clinker factor to be reduced down to 50% while reaching comparable strength to OPC at 7 days [3]. Clay minerals are made up of a stack of alternating layers [4]. Kaolinitic clays are the most promising alternative for calcined clay production, as they exhibit higher reactivity than others such as illite and smectites [5, 6]. Clays with kaolinite content of 40% or above are suitable for use in LC3 cements [6, 7]

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