1. Introduction
Concrete is one of the most consumed materials in the world, which has an effect on the natural material deposits (sand, aggregates, limestone, etc.) that are used directly in the manufacture of concrete. These deposits are becoming increasingly scarce while needs continue to grow. On the other hand, due to modern lifestyles, the progress of industry and technology has led to a significant increase in the amount and type of waste, such as demolition concrete, waste foundry sand and glass. These wastes could be recycled and used as alternative sources of aggregates or cement to produce concrete and meet the challenge of reducing the depletion of raw materials [
1,
2,
3,
4,
5,
6].
Dredging operations carried out on the waterways of the Paris region generate approximately 150,000 m
annually. These sediments require valorisation or storage in waste storage facilities. For inert sediments (86% of the total volume), the main valorisation field is the filling of quarries and ballast. Noninert but nonhazardous sediments are treated before reuse or storage in nonhazardous waste storage facilities (ISDND) [
7]. Approximately 190,000 m
(25%) of the sediments dredged over the period between 2014 and 2018 were directly stored in Inert Waste Storage Facilities (ISDI) at a cost between 5 and 11 EUR/ton [
8] (based on 2012 data; current data may be much higher). For sediment managers, the ISDI sector therefore cost EUR 1 M for these five years (2014–2018). In addition, national and international regulations concerning sediments are increasingly demanding and, thus, in the future, the cost of their current management could be greatly increased.
In recent years, many international studies have been carried out to encourage the valorisation of dredged sediments as alternative materials for civil engineering. These studies indicate various beneficial uses of dredged sediments as raw materials in construction, namely road construction [
9,
10,
11], cement production [
12,
13,
14,
15,
16,
17,
18,
19] and the replacement of conventional aggregates for the manufacture of mortar or concrete [
20,
21,
22,
23,
24]. The sandy fraction (0.08–20 mm) of the Paris region deposit has also shown a potential for reuse as an aggregate in concrete. However, the incorporation of fine particles as a substitute for 30% vol. sand significantly increases the concrete’s setting time and total shrinkage as well as decreases the compressive strength by 50% [
24]. This concrete was formulated by maintaining a slump that was similar to that of the control concrete. For this purpose, the amount of water added to the mixture was increased with the incorporation of fine sediments. These results are in accordance with those of Millrath et al. [
25], who investigated two different methods of mixing: the first one consists of incorporating fine sediments as sand with the same water/cement ratio and the second one is based on keeping the same slump with the incorporation of fine sediments. For the first case, the results of this study showed that as the fine sediment content increases from 0 to 20 wt %, the flow gets reduced by 50% and the compressive strength is slightly affected. However, for the second case, the compressive strength dropped by 50% for a 20% substitution of sand by fine sediments. Therefore, the reuse of fine sediments as sand in concrete offsets any economic or environmental benefits. Nevertheless, their fraction in the deposit is substantial, and it is necessary to find a way to reuse them in concrete (
Figure 1; sediments containing more than 40% of fines (<50
m) represent 70% of the Paris deposit).
Due to their mineralogical and chemical constitution (siliceous, clay, limestone, etc.), sediments could be used to replace the raw materials for Portland cement clinker. Several studies have shown the feasibility and efficiency of this method [
12,
13,
14,
15,
16]. However, the effective use of fine sediments often requires an adequate thermal treatment process aimed at eliminating the organic fraction and certain pollutants. Van Bunderen et al. [
12] studied the hydration of a cement paste formulated with dredged sediments calcined at 865 °C. The results show that treated sediments and fly ash had similar early hydration behaviour. Regarding the mechanical properties, Dang et al. [
13] showed that a cement based on calcined sediments (650 and 850 °C for 5 h) develops a compressive strength better than limestone filler but lower than that of the control mortar. These results are in accordance with those presented by Ez-zaki and Diouri [
14]. On the other hand, the results reported by Benzerzour et al. [
15] showed that the mortar incorporating up to 15 wt % of sediments treated at 850 °C for 1 h develops better mechanical properties than the control mortar. Hadj Sadok et al. [
17] also showed that the incorporation of 15% calcined sediments (at 750 °C for 5 h) as a cement substitute improves the compressive strength by 3% at a curing temperature of 40 °C. In the same sense, Safhi et al. [
16] showed that the use of up to 20 wt % of treated sediments, at 800 °C for 1 h, in concrete offers a compressive strength comparable to that of the control concrete.
Few studies have been devoted to the valorisation of untreated or just dried sediments. Zhao et al. [
19] reported the use of a marine sediment, dried at 40 °C and then ground, as a partial substitute for cement in the manufacture of concretes. Three sediments’ contents were used as a substitute for CEM I 52.5 cement (10%, 20% and 30%) to produce concrete. For a substitution of 10 wt % of cement, the slump decreased from 12.5 cm to 9.5 cm and the compressive strength decreased by 6%. Ouédraogo et al. [
26] showed that untreated sediments can be used with 300 kg/m
(which represents 50% of the binder’s total mass) to formulate self-compacting concrete. The results showed that there is no segregation or bleeding in the fresh state, and the compressive strength at 28 days indicates that these sediments can be used for nonstructural concretes.
The sediments’ calcination presents several advantages, such as the elimination of a part of the organic matter, the activation of the pozzolanic properties in the long term and the stabilization of the heavy metals. However, this thermal treatment of sediments is costly, both environmentally and economically. For this reason, the valorisation approach of this study is to use the sediments in their raw state. As the sediments of the Seine basin are nondangerous, physical, chemical or thermal treatment is not necessary for the use of the sediments in cement materials. The aim of this paper is to evaluate the effect of using untreated fine sediments as 10% substitutes of cement on concrete properties. This replacement rate is enough to consume the entire deposit in the concrete industry in the Paris region. Concrete samples based on ten different fine sediments are tested to investigate the effect of incorporating fine sediments on the slump, the hydration, the compressive strength and the shrinkage of concrete. The effects of sediment incorporation as filler and as sand on concrete properties, carbon footprint and cost are compared.
5. Conclusions
The study aimed at investigating the effect of the incorporation of fine sediments from the Seine Basin deposit on the properties of concrete. Chemical analyses showed that these sediments are nonhazardous waste. Therefore, for environmental and economic benefits, these sediments were incorporated into the concrete without thermal treatment. The objective of this paper was first to evaluate the effect of substituting 10% of cement with fine sediments on the workability, the hydration, the compressive strength and the total shrinkage of C30/37 concrete. Then, the properties of concrete incorporating fine sediments as filler (10%) were compared to those of concrete incorporating fine sediments as sand (30%).
Keeping the same water/binder ratio, the substitution of 10% cement by fine sediments does not affect the workability of concrete. However, a slight delay in hydration, less than 4 h, has been observed for sediment-based concrete, which is attributed to the organic matter content of the untreated fine sediments. The compressive strength showed a slight decrease with the incorporation of fine sediments. This decrease remains limited to 8% on average. As for total shrinkage, with the exception of two sediments, the behaviour of the fine sediment-based concrete is similar to that of the control concrete. The presence of mineral components such as clay in these two sediments could be at the origin of these observations. Indeed, as sediments are heterogeneous materials, the observed effects on concrete properties can be attributed to the presence of a given component, such as humic substances or clay, or to a combined effect of several components.
Regarding the comparison between the use of sediments as an addition to cement or as sand in concrete, the results show that the latter has only one advantage, as it allows for the reuse of a greater volume. On the other hand, the incorporation of sediments as an addition to cement offers acceptable technical performance as well as a reduction in the environmental and economic costs of concrete because it aims to substitute cement, which is the most expensive ingredient from an environmental and economic point of view. Moreover, the use of 10% of fine sediments as cement is enough to consume the entire sediment deposit, which is still relatively small compared to the large quantity of concrete produced in the region.