Dredged river sediments may contain various types of organic matter that can affect the properties of a cement matrix, with the most representative part of such organic matter consisting of humic substances (HS). This work seeks to investigate the effects of humic substances on the rheology, the hydration and the strength development of a cement paste with different curing times, ranging from 1 to 90 days. Results show that adding organic matter to the cement paste offers greater workability by decreasing the yield stress. This addition also serves to retard the hydration of cement particles. Setting times increase to beyond one week when raising the amount of humic substances in the mixture, while the heat released ultimately reaches values comparable to those of the reference cement paste. Therefore, the addition of organic matter negatively affects strength development of the paste, especially at an early age; no resistance at 1-day has been noticed for HS content greater than 1%. Nevertheless, that there was no significant reduction in the compressive strength up to 20% for HS content less than 1% at 90-day. In parallel, a study on pastes containing calcium lignosulfonate (water reducer) is carried out and the same effects are observed.
Dredged river sediments may potentially be reused as aggregate in concrete production. The variability of dredged sediments has been quantified by measuring their concentrations of heavy metals, several distinct pollutants, specific granular fractions and organic content at various locations in the Seine River watershed over three years (2015)(2016)(2017). It has been shown that the sediment deposit does not significantly change from one year to the next, while the organic content, which delays cement hydration, concentrates in the finest fraction removable by sieving. Moreover, 30% of sediment volume is mainly sand and readily reusable in the form of concrete aggregate. Various sediments have thus been sieved to coarse aggregates, sands and fines in order to formulate concretes. Substituting 30% of aggregate volume by the resulting coarse aggregates or sands, once depleted of silt and organic matter: marginally affects hydration, extends setting time to just under 3 hours, decreases compressive strength by 10%, and increases shrinkage strain by 15%. In contrast, the finest part of the sediment significantly alters these properties, owing to its soluble organic matter and silt contents.
Large amounts of sediments are dredged each year to ensure navigation. These materials, classified as waste, seem to be promising alternatives to conventional construction materials. Dredging operations, carried out by the Territorial Directorate of the Seine Basin (DTBS), generate an annual volume of sediments of about 150,000 m3, of which nearly 50% are fine sediments (<80 μm). For these fine sediments, it is necessary to look for possible ways of valorisation, knowing that the coarse sediments, sands and gravels are already easily reused in concrete. The valorisation of fine sediments, such as concrete with 30% sand, has already been evaluated. However, it was found to significantly affect concrete performance; it extends setting time from 3 to 18 h, decreases compressive strength by an average of 50% and increases shrinkage deformation up to 200%. This paper seeks to evaluate the effects of ten different fine sediments, used as substitutes for 10% of cement by volume, on physico-chemical and mechanical properties. The experimental results show that fine sediments marginally affect concrete properties. The main peak of the released heat flux is delayed to less than 4 h, the compressive strength is decreased by 8% on average and the increase in shrinkage deformation does not exceed 17%, except for in two fine sediments. This incorporation method also has an environmental advantage over substituting 30% of concrete with sand, as it reduces CO2 emissions by almost 10% (instead of 0.2%).
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