Dredged sediments used as novel supply of raw material to produce Portland cement clinker

Abstract

The maintenance of waterways generates large amounts of dredged sediments that are an environmental issue. This paper focuses on the use of fluvial sediment to replace a portion of the raw materials of Portland cement clinker, which would otherwise come from natural resources. The mineralogy of the synthetic cement was characterised using X-ray diffraction and scanning electron microscopy and its reactivity was followed by isothermal calorimetry. Comparisons were made to a commercial ordinary Portland cement (CEM I 52.5). Compressive strength measurements were conducted on cement pastes at 1, 2, 4, 7, 14, 28 and 56 days to study strength development. The results showed that Portland cement clinker can be successfully synthesised by using up to 39% sediment. The compressive strengths developed by the cement made from sediment were equivalent to those obtained with the reference at early ages and 20% higher at long term.

Introduction

With more than 10 billion metric tons of concrete being produced worldwide per year, the cement industry has a large environmental footprint. The tools and strategies to meet the environmental challenges should involve the use of recycled materials in place of natural resources. This study focuses on the recycling of sediment to replace a portion of raw materials in conventional Portland cement manufacture. Waterway sediments are dredged either to maintain navigability, prevent flooding or, more recently, to improve the environmental quality of the aquatic ecosystem. Presently, in France about 60% of waterway-dredged sediments are deposed in dedicated sites. However, this style of management is unacceptable to most local communities. In addition, several technologies, including the thermal elimination of organic matter, the stabilisation of heavy metals [1], mineral processing [2] and phytoremediation [3] have been explored either to treat sediments or to manage the risk of pollution transfer to surrounding environments. Most of these treatments are under validation but are either too expensive or not available at this stage.

From the viewpoint of resource recovery and recycling, the use of by-products or wastes as raw materials for cement production is of interest and has been widely investigated [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Judging by the chemical composition, sediment could also be used as substitute for cement raw materials. Dredged sediments present the advantages of a renewable resource and are available in huge quantities, easily transportable through waterways, which is in accordance with cement industry needs. Research conducted on the valorisation of sediment in cement-based materials has mostly focused on the classical sand substitution for mortar production [1 and citations therein]. This type of recovery requires treated or unpolluted sediment. Dalton et al. [14] demonstrate the potential of using contaminated marine sediment as feedstock replacement in Portland cement manufacture. A bench scale manufacture was carried out with feedstock mixtures containing up to 12% of dredged material from the New York/New Jersey harbour. Results showed a decrease of alite and increase of belite for contents higher than 6%. In addition, this study treated marine sediment with a high chloride content, which causes two difficulties. First, chloride trapped in the clinker is well known to reduce the concrete strength and may speed up the corrosion of reinforcing steel in concrete [15]. Second, large fractions of chloride are volatilised at approximately 980 °C and travel up the kiln to cooler regions to finally precipitate along the kiln walls. Thus, periodic maintenance is needed, and production time is lost [16].

Here we present the use of fluvial contaminated sediment to replace a portion of raw materials in conventional Portland cement manufacture. The sediment is classified as contaminated because of the presence of organic and inorganic contaminants. At the high temperature required for cement manufacture (1450 °C), organic contaminants are degraded, and heavy metals are stabilised [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. In addition, previous research indicates that the heavy metals present in wastes and alternative fuels used in clinker kilns are incorporated into the clinker crystalline phases [20], [21], and cement producers have extensive experience with their impact on concrete quality and cement production [17], [22].

This use of sediment is an advantageous use of existing technologies to manage dredged fluvial sediment and leads to a reduction of the cement industry demand for raw materials. In addition, cement producers can be paid for using the polluted sediments and developing a sustainable way to reuse significant amounts of sediment. The goal of this study was first to synthesise Portland cement with the highest possible content of polluted sediment and secondly to compare its reactivity to commercial Ordinary Portland Cement (OPC) CEM I 52.5. The crystalline phases of the resultant cement clinker and cement pastes were identified by X-ray powder diffraction. SEM–EDS was used to investigate the clinker. The hydration kinetics was followed by isothermal calorimetry and by measurements of the compressive strength of hardened cement pastes.