Clay influence on compaction and strength of unfired sediment bricks

Based on particle size, dredged sediments and natural soils generally consist of four main components: clay, silt, sand and gravel. In unfired earth materials, clay and silt provide binding properties, whereas sand and gravel act as fillers. A sufficient proportion of clay is necessary to achieve optimal material stiffness and strength [1]. Amounts of 10 to 22% and 5 to 15% are recommended for compressed earth blocks and rammed earth, respectively [2–4]. Depending on its nature and proportion, clay may offer several advantages or disadvantages concerning the physicochemical and geotechnical characteristics of raw sediment or soil, as well as the strength and durability of material. For instance, an adequate amount of clay and water contributes to the soil’s plasticity and material’s strength. Conversely, low or excessive clay content results in adverse effects, such as cracks, swelling, accelerated deterioration of earth materials due to excessive rain, reduced material tensile and compressive strengths, etc. The influence of clay on the engineering properties of unfired earth materials has been widely studied, but further research is needed due to the variability in clays and sediment characteristics. Combining low-clay soils with pure clays or clayey soils can enhance the plasticity (cohesion, moulding) and densification (compaction, kneading, pressing, etc.) of materials. In this context, sediment dredged from the River Seine in a town near Rouen was reused in unfired earth bricks production. Given its low clay content (less than 3%), the sediment was mixed with other locally available resource, such as excavated clayey soil, to improve its properties. This article presents key findings highlighting the effect of excavated clayey soil (ECS) on the compaction parameters, including optimum moisture content (OMC) and maximum dry density (MDD), as well as the strength of sediment bricks. In the mixtures, the dredged sediment was replaced with ECS at 10wt.% and 20wt.%, while the control sample made of 100wt.% sediment was used for comparison. The compaction test was carried out using the miniature Proctor on both sediment and ECS separately at various water contents, maintaining the same compaction energy of 600kN.m/m3. The flexural and compressive strength tests were conducted on air-dried sediment bricks (cured for 3 weeks), with dimensions of 4×4×16cm3 and 4×4×4cm3, respectively, using a tabletop precision universal tester (Shimadzu, Autograph AGS-X series), at a loading rate of 0.5mm/min. Since clays have a high surface area, which creates intermolecular forces that attract water molecules, the excavated clayey soil needed less water to reach the MDD than the sediment. For ECS, the MDD of 1863kg/m3 was reached at an OMC of 13%. In contrast, for the sediment, the MDD was 1470kg/m3 and was achieved at an OMC of 22%. As expected, the addition of the sediment to the soil rich in clay enhanced the mechanical performance of the bricks. The sediment bricks containing 10wt.% and 20wt.% exhibited flexural strengths (Rf) of 0.95 and 0.99MPa, respectively, while the reference sediment bricks showed the Rf of 0.5MPa. Following the addition of ECS, the compressive strength of the reference sediment bricks (1.9MPa) substantially increased, reaching 3.29MPa and 3.43MPa for bricks with 10wt.% and 20wt.% excavated clayey soil, respectively. Based on these results, the combination of excavated clayey soil with the sediment from the River Seine was beneficial, as it improved the strength of unfired sediment bricks, while promoting sustainability in construction and the circular economy.