Granular Skeleton Optimisation and the Influence of the Cement Paste Content in Bio-Based Oyster Shell Mortar with 100% Aggregate Replacement

Sustainability in construction and building materials involves various aspects, including raw materials, durability, and life cycle assessment. Concrete is a widely used material that is in high demand worldwide. However, the production of raw materials for concrete, including cement and aggregates, has numerous environmental impacts, such as climate change and resource depletion. To design more sustainable concrete, we can reduce the amount of traditional cement in the mix, use clinker-free cement, or replace extracted aggregate with recycled industrial waste. In particular, the construction industry has shown an interest in the circular economy concept by using wastes as by-products in concrete. In addition, combining the natural properties of bio-based materials with conventional construction materials can enhance environmental quality and performance [1,2]. It is, therefore, logical to recycle and use industrial biological wastes in cementitious material. Crushed seashell waste is a great option because commercial mollusc farming is a vital part of the global aquaculture industry, accounting for 23% of the world’s total production [3]. These farms produce a large amount of waste, with up to 90% of the mollusc’s weight being represented by its shell [4]. Additionally, some seashells exhibit unique and outstanding mechanical properties [5,6,7], which make them ideal for use.

According to Robert et al. [8], in the 2010s, Europe produced 800,000 tons of molluscs per year, with a turnover of EUR 1100 million. This led to 37,000 direct employments and accounted for 50% of global EU aquaculture production by weight and 30% of the value. The main farmed species are oysters, mussels, and clams. In 2010, France produced 194,000 tons of molluscs, comprising 59% oysters, Crassostrea gigas (113,000 tons), 39% mussels, and 2% other molluscs such as cockles, clams, the King scallop, and the flat oyster [8]. (In 2017, the World Register of Marine Species (WoRMS) renamed Crassostrea gigas to Magallana gigas, but this name change is still debated in the community (see, e.g., [9,10]). In this paper, we will keep the former nomenclature, Crassostrea gigas.) Crassostrea gigas oysters are widely cultivated in France, with the region of Nouvelle-Aquitaine being the largest contributor (33.6%). Oyster farming generates a significant amount of waste (dead or diseased oysters, cleaning of parks), along with that from food consumption. The careless disposal of oyster seashell waste may cause several environmental problems such as soil contamination and a strong odour due to the organic matter or the microbial decomposition of salts into gases (NH₃ and H₂S) [11].

Oyster shell (OS) by-products have been valued in different activities such as heavy metal removal (e.g., [12]), soil supplement in agriculture (e.g., [13]), bio-applications (e.g., [14,15]), or construction and building materials. Regarding construction and building materials, significant research has been conducted in the last few years on utilising OS by-products as a supplementary cementitious material, alkaline-activated material, or aggregate replacement, and several review papers have been published recently (e.g., [16,17,18,19,20]). Yang et al. [21,22] examined the mechanical properties and durability of concrete with up to 20% crushed OS replacing fine aggregate. Eo and Yi [23] presented several concrete mixtures in which they substituted crushed OS for fine aggregate (ranging from 0 to 50%) and coarse aggregate (ranging from 0 to 100%), while considering various water–cement ratios. The replacement of conventional aggregate was examined individually for both fine and coarse aggregates in these concrete mixtures. Kuo et al. [24] conducted an analysis on the replacement of sand with OS (ranging from 5% to 20%) to create controlled low-strength materials. Meanwhile, Wang et al. [25] suggested an OS mortar that incorporated fly ash, with different replacement rates of traditional fine aggregate (sand): 5%, 10%, 20%, and 30%. Liao et al. [26] conducted experiments where they used a mixture of natural river sand and crushed waste oyster shells with varying particle sizes as fine aggregates in the preparation of mortar. Their findings showed that, as the content of oyster shells increased, the compressive and flexural strengths of the mortar decreased. Bamigboye et al. [27] mixed river sand, Senilia senilis seashells, and granite in varying proportions in concrete. It was observed that, as the proportion of seashells in the mixture increased up to 20%, there was a significant reduction in the compressive strength. However, the results in terms of split tensile strength were relatively good. After conducting a short literature survey, it can be concluded that using crushed OS particles as full or partial aggregate replacement in cementitious materials poses a challenge. Crushed OS aggregates have elongated and angular shapes, which is different from conventional spherical aggregates. This irregular shape and flatness may cause difficulties in terms of granular skeleton packing, increasing the porosity, and ultimately, leading to a decrease in mechanical performance [26]. This is why previous studies examined small replacement rates, but full aggregate replacement, especially in mortar, remains limited in the literature. Different methods have been employed in the previous articles to partially substitute traditional aggregate with OS by-products. Wang et al. [25] and Kuo et al. [24] replicated standard particle size distributions (ASTM C33 and C136), while Yang et al. [21,22] and Eo and Yi [23] used a 5 mm sieved crushed OS mixture without emphasising any particular OS granular skeleton packing strategy. However, in classical mortar and concrete, having a compact granular skeleton is important to improve the mechanical and durability properties [28,29].

Various particle-packing models have been developed for classical cementitious materials to predict their packing density and increase compactness. The most widely used approach for optimising spherical aggregate packing density is to follow an ideal grading curve [30,31], which was first proposed by Fuller and Thompson in 1907 [32] and later developed by different authors (e.g., [33]). This approach is considered a trustworthy method for achieving good concrete mixes with spherical aggregates and is widely used by European standards. The method assumes a continuous particle size distribution, which means that the voids left by larger particles are filled by smaller particles and so on. As the studies on this topic have progressed, researchers have been able to identify the structural and interaction effects that occur between particles. The main effects that have been observed are as follows [31,34]. The loosening effect: This occurs when a fine particle disturbs the packing of a coarse particle frame, causing it to become loosened. The wall effect: This happens when additional voids are created by a coarse particle among a frame of fine particles. The filling effect: This occurs when fine particles tend to fill smaller voids created among the coarse particles’ frame. The occupying effect: This happens when coarse particles are placed in the bulk volume of fine particles. Based on these considerations, different theoretical packing models have been proposed (e.g., [35,36,37]). Additionally, computational models using the discrete element method have been developed, which can simulate particle packing in both 2D and 3D structures (e.g., [30]). These models are effective at estimating the particle packing density for spherical particles. However, replacing conventional aggregates with non-spherical by-products remains a challenge for these models, particularly when performed at high rates.

Limited studies have been conducted to develop models for predicting the packing density of a blend with non-spherical particles [34]. Non-spherical packing theories are usually based on the assumption that their packing system can be considered similar to a spherical one. Yu et al. [38,39] described an approach for a binary system where non-spherical particles are related to spherical particles through an equivalent packing diameter, allowing the prediction of mix porosity. Goltermann et al. [37] worked out the concept of the Eigen packing degree and proposed an equivalent packing diameter for a multi-component aggregate. However, the definition of such a diameter is not obvious for OS particles because their shape changes with the particle size and the OS particles can be considered as lamellar particles (resting horizontally) or as standing needles in the concrete. Due to this problem, it is necessary to find another way to optimise the granular packing without using the equivalent diameter theory.