1. Introduction
Today, the environmental issues related to the management of natural resources are currently considered a significant priority in nature (COP). Indeed, the need to find alternative solutions, particularly by implementing circular economy concepts and adopting new economic models, is more than necessary. In this perspective, the substitution of natural materials for alternative materials presents itself as an interesting solution vis-à-vis this socio-economic and environmental challenge, which is fully in line with a sustainable development and circular economy approach. According to this vision, numerous investigations have highlighted that dredged sediments may be reused as major or minor components in the construction industry sector. For example, in the research carried out by [
1], the sediments were incorporated into the brick-manufacturing process instead of quartz sand. In addition, it has been shown that a 50% replacement of natural brick-making clay by sediments allows for reaching the compressive strength required for the American Society for Testing and Materials (ASTM) standards [
2,
3]. The feasibility of using dredged sediments as a partial replacement for cement in mortars was assessed by several authors [
4,
5]. Benslafa et al. [
4] studied the variation of compressive strength at varying sediment incorporation rates (5, 10, 15 and 20% by mass of cement). The results highlighted that sediments can most suitably be substituted for 5% of the cement used. Lightweight aggregates manufactured from dredged sediments have been studied in many research works [
6,
7,
8,
9,
10], and the results have shown their suitability for large-scale production due to their availability, homogeneity, mineralogical and chemical composition.
In the SEDIMATERIAUX regional framework launched in France in 2009, several innovative ways for recycling non-submersible sediments have been studied by the port of Dunkirk: landscape remodeling, use in road building as well as manufacturing of mortar blocks [
11]. Today, 150,000 cubic meters have been reused in the port’s territory in the form of landscape remodeling. This landscape remodeling is designed to promote the development of biodiversity in an area of low species richness. In May 2012, the first port road was built by using dredged sediments and natural aggregates. By the end of 2013, concrete blocks, including dredged sediments, were made and used in the port’s territory to strengthen defenses against the sea. Currently, the port is studying the feasibility of using sediments to produce artificial aggregates that it will use to strengthen the coastline, which is subject to erosion. In this way, the authority port hopes to identify several treatment ways for recycling non-submersible dredged materials. More recently, the industrial research project entitled «SEDIPLAST» was launched (2015–2018) in France within the SEDIMATERIAUX framework to assess the feasibility of reusing waterways and harbor sediments in thermosetting and/or thermoplastic matrices in order to manufacture polymer mortars which could be used for floor covering applications. Technical investigations have shown that sediments can be incorporated as major component of composite products and by replacement of natural aggregates with a substitution rate of 50% [
12,
13]. Composite products were evaluated by mechanical, thermal and chemical tests according to UPEC specifications to validate their technical use as construction products in floor-covering applications [
14].
According to the European regulation, it is commonly accepted that the use requirements of construction materials must include proof they will not have adverse effects on human health and the environment. It is expected that expected pollutant emissions in soil and water need to be quantified during the service life of construction products. Laboratory test procedures to determine the amount of substances released from construction products were established by the CEN TC 351 “construction products: assessment of release of dangerous substances”. One of the tests—CEN/TS 16637-2:2014—was developed to investigate leaching from monolithic construction products. The dynamic surface-leaching test (DSLT) intends to describe diffusion-controlled leaching processes. However, the test results cannot be used directly to derive expected environmental concentrations.
Concepts for transferring results obtained under laboratory exposure conditions to service-life conditions still need to be developed or refined [
15,
16]. Otherwise, the test indicates whether target substances can be leached from investigated construction products. It is also possible to compare the leachability of the target substances from different construction products according to regulatory levels from the Netherlands (Soil Quality Decree, 2008) or Germany. These tests have often been used to characterize cementitious materials [
17,
18]. There is very little work on the characterization of waste-based material monoliths. Often, the works carried out on the valorization of waste in the construction materials are based on leaching tests of the crushed fraction (0–4 mm) [
19].
The present study is the first to assess the leaching of soluble inorganic substances from polymer mortars based on waterways sediments according to the specifications of NF EN 15863 standard. Indeed, mineral fillers (limestone and/or sands), usually used for the formulation of polymer mortars, have been partially replaced by waste that is the dredged sediment. We investigated the dynamic leaching behavior of several polymer mortar samples, including various epoxy resin rates; (12, 14, 16, 18, 20, and 25%) and two dredged sediment-incorporation rates (30 and 50% in mass) [
12,
19]. The objective was to identify the mechanisms for the release of chemical substances from monolith samples into water. The results of this work will subsequently make it possible to establish an environmental impact study and a life-cycle analysis of this type of material-incorporating waste.
4. Conclusions
The present study assessed the release of inorganic substances from polymer mortar samples, including dredged sediment as a replacement for a sand fraction—these construction products are typically used for floor coverings in Europe. A diffusion test with the sequential renewal of water was performed in lab conditions for different resin epoxy concentrations and sediment incorporation rates. This test was conducted according to specifications described in the NF EN 15863 standard. It can be concluded that the release of soluble substances is very limited in these hydrodynamic conditions. This is particularly true for high epoxy-resin concentration leachates where no trace element, except for small quantities of barium, cobalt, copper, and vanadium, are quantified. Their leaching is mainly controlled by diffusion in the first renewal steps, and then the depletion is observed for all formulations. This is because of the low hydraulic conductivity (low porosity and water absorption) and the low polarity of the polymer binder of these specimens. The percentage of sediment included in the polymer mortars plays important on the leaching of sulfates and major elements (Ca, Fe, K and Mg) by improving their diffusion/dissolution/surface leaching at the highest incorporation rate (50%).
No adverse effect is to be expected in terms of environmental health from the leachates of these polymer mortars because all the measured parameters were below the Soil Quality Decree limits established in the Netherlands. These data about the environmental performances of road construction materials, including dredged sediments, are the first to be published and may serve as a basis for identifying the amounts of hazardous substances that such construction products may release in water. They are of great interest to potential users of secondary raw materials and are required for the CE-marking procedure of construction products. Finally, it seems that pollution from runoff water is more likely to be related to the chemical products applied to the polymer mortars than to the materials themselves. However, an extrapolation of these results to the field conditions must be done with great caution due to the very different hydrodynamic conditions (L/A ratio, leachate renewal, etc.) and evolution of the construction materials under climatic changes (degradation, oxidation of polymer matrix) may be observed during floor covering use. Moreover, further experiments should be performed on a larger panel of dredged sediments to gain a better knowledge of their potential to release substances into the water.