Assessing Durability Properties and Economic Potential of Shellfish Aquaculture Waste in the Construction Industry: A Circular Economy Perspective

Citation: Peceño, B.; Bakit, J.; Cortes, N.; Alonso-Fariñas, B.; Bonilla, E.; Leiva, C. Assessing Durability Properties and Economic Potential of Shellfish Aquaculture Waste in the Construction Industry: A Circular Economy Perspective. Sustainability 2022,14, 8383. https://doi.org/ 10.3390/su14148383 Academic Editors: Jinhui Li and Abhishek Kumar Awasthi Received: 7 June 2022 Accepted: 2 July 2022 Published: 8 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article Assessing Durability Properties and Economic Potential of Shellfish Aquaculture Waste in the Construction Industry: A Circular Economy Perspective Begoña Peceño 1,* , JoséBakit 2,3,* , Niris Cortes 1, BernabéAlonso-Fariñas 4, Enzo Bonilla 1 and Carlos Leiva 4 1 Escuela de Prevención de Riesgos y Medioambiente, Facultad de Ciencias del Mar, Universidad Católica del Norte, Larrondo 1281, Coquimbo 1780000, Chile; [email protected] (N.C.); [email protected] (E.B.) 2 Departamento de Acuicultura, Facultad de Ciencias del Mar, Universidad Católica del Norte, Larrondo 1281, Coquimbo 1780000, Chile 3EqualSea Lab-CRETUS, Departamento de Economía Aplicada, Universidad de Santiago de Compostela, 15705 Santiago de Compostela, Spain 4Departamento de Ingeniería Química y Ambiental, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos s/n 41092, 41004 Seville, Spain; [email protected] (B.A.-F.); [email protected] (C.L.) *Correspondence: [email protected] (B.P.); [email protected] (J.B.) Abstract: The construction industry’s high demand for natural resources, combined with the waste generated by agriculture, creates an opportunity for the circular economy. This experiment used the CaCO 3 found in scallop shells as an ingredient for the manufacture of fire-resistant materials, replacing gypsum in compositions of 40% and 50% by weight. The mechanical compressive strength was estimated for both freeze-thaw cycles and acid and sulfate attacks. The cost of disposing of scallop shell waste in landfills, savings from substitution, and the payback period relative to the amount of production were determined. The compressive strength of the materials decreased by 80% when subjected to freeze-thaw cycles and sulfate attack. In response to acid attack, they showed a 100% increase in strength during the first three weeks and a decrease thereafter. The savings amounted to $46.36 (22.4%) for 40% replacement and $58.93 (28.4%) for 50%. Respectively, return on investment is achieved at 800- and 630-per-metric ton produced. The difference between the costs of waste disposal (in aquaculture) and the potential savings from using CaCO 3 as a raw material (in construction) creates an opportunity for commercialization between the two industries, serves as a reference for decision-makers, and complies with circular economy principles, reducing both inputs of raw materials and outputs of waste. Keywords: construction materials; durability; circular economy; aquaculture waste; Argopecten purpuratus; seashells wastes; freeze-thaw cycles; recycling 1. Introduction The construction industry has the highest demand for natural resources worldwide: 44% of mineral resources [ 1 ], 40% of energy resources [ 2 ], 12% of water [ 3 ], and also produces 40% of all waste [ 3 ]. Urban expansion and densification can also lead to further soil degradation and deterioration [ 4 ]. According to projections from the United Nations [ 5 ], if there is no change in how the construction sector operates, these impacts will be exacerbated by the scarcity of material resources, land use changes, and growing populations. In order to reduce the impacts of construction, the incorporation of waste as a raw material [ 6 , 7 ] or proposals to improve the mechanical properties of the material or the maintenance of buildings [8,9] are essential. Although there is no official record of waste produced by the aquaculture industry, shells account for approximately 40% to 70% of the total weight of shellfish waste [ 10 ]. Sustainability 2022,14, 8383. https://doi.org/10.3390/su14148383 https://www.mdpi.com/journal/sustainability Sustainability 2022,14, 8383 2 of 16 Seashells have little to no commercial value and are generally dumped into the sea or deposited in landfills [11]. Incorrect management of this waste causes: • Environmental problems: these wastes can produce odors due to the decomposition of organic matter or of salts contained in the shells, emitting gases such as H 2 S, NH 3 (ammonia) and organic compounds such as amine [11]. • Economic problems: pollution and odors have an economic impact on tourism given that aquaculture enterprises are located on seashores with heavy tourism [12]. • Health problems: illegal dumping of these wastes attracts biological vectors such as rats and mosquitoes in nearby populations [13]. • Social problems: currently, there are few economic and effective solutions for managing shell waste, which has led to the proliferation of illegal waste dumping near populated areas [ 14 ]. The foul odors and contamination from dumping have led to protests and complaints from neighboring communities [15]. Seashells are primarily composed of CaCO 3 [ 12 ]. As such, recycling has been widely studied as a source of CaCO 3 or as CaO [ 16 – 21 ]. Seashell wastes were used in 15th-century Latin America as a raw material for the manufacture of Tabby concrete [ 22 ]. Currently, 9% of global seashell waste is recycled as a substitute for limestone in fertilizers and feed additives in poultry farming [ 23 – 25 ]. There are research studies that have analyzed the use of CaCO 3 and CaO as adsorbents in polluted waters [ 26 – 28 ], as a catalyst in biodiesel production [ 29 , 30 ], and as polymer fillers [ 31 – 33 ]. In the construction sector, seashell wastes have been studied for use as fine and/or coarse aggregates in concretes and mortars [ 34 – 37 ], as a cement substitute [ 38 , 39 ], as a raw material for cement clinker [ 40 , 41 ], and as cement blocks [ 42 ]. Fireproofing materials prevent or retard the passage of extreme heat in order to protect more important elements, such as steels that lose their mechanical properties at high temperatures, and prevent the spread of fire throughout the floors of a building. When heated, such as during a fire, gypsum undergoes endothermic decomposition according to the reaction: CaSO4·2H2O→CaSO4·1/2H2O + 3/2H2O (1) CaSO4·1/2H2O→CaSO4+ 1/2H2O (2) Reaction 1 starts at around 120 ◦ C when semi-hydrate gypsum is produced. In reaction 2 , semi-hydrate gypsum is converted to anhydrous gypsum at around 200 ◦C [43–45] . Both Reactions 1 and 2 are endothermic reactions, in which heat is removed from the environment. CaCO 3 exhibits similar behavior. At high temperatures (600 ◦ C and 800 ◦ C), CaCO 3 undergoes endothermic decomposition (reaction 3): CaCO3→CaO + CO2(3) The use of aquaculture wastes as inputs for materials from other industries is a productive strategy that has been developed in recent decades [ 46 – 50 ]. Initially recognized as recycling strategies and studied through economic or life cycle cost (LCC) analysis, they are nowadays associated with circular economy strategies and studied using life cycle assessment (LCA). LCA- and LCC-type economic analyses are very flexible and can be used for economic evaluation in areas as different as building material reuse [ 51 , 52 ], e.g., as PET for manufacturing blankets [ 53 ], and extending the life of household appliances [ 54 ]. Some findings indicate that LCA and LCC economic analyses indicate that high resource and energy consumption, direct emissions, and the transport of raw materials during cement production are the main processes contributing to most environmental categories and economic costs [52,55]. In order to address both environmental and economic impacts, trends in concrete technology are currently moving towards sourcing sustainable alternative materials for making concrete to minimize over-reliance on natural resources [ 56 ]. In this scenario, the use of waste from aquaculture, specifically scallop shell waste, appears to be a promising solution [ 11 , 38 ], capable of bringing about a circular economy between aquaculture and Sustainability 2022,14, 8383 3 of 16 construction. However, the implementation of a circular economy between these industries must address challenges in both accessing capital and strengthening the trade network between supply and demand, identified as barriers, and in strengthening the organizational culture of every company [ 57 ]. To advance a circular economy, a qualitative estimate has been conducted in 2021 of the economic benefits of using scallop shell powder as a cement substitute in the development of blended cements [ 58 ], not only to improve sustainability, but also to reduce production costs. An approach to the utilization of seashell wastes, specifically scallop (Argopecten purpuratus), as fine and coarse additives in the manufacture of building materials has been carried out [ 55 ]. The authors present these wastes as an alternative that consumes little energy in recycling compared to raw materials coming from mining and find that despite the additional cost of substituting scallop shells, the associated benefits include longer pavement life and better resistance to deformation and humidity. Looking at the performance of the aquaculture and construction industries in Chile allows us to identify linear economic models, where scallop shell waste is discarded without use [ 46 , 59 ] and more than 70% of raw materials for construction materials come from virgin natural resources [ 60 ]. Figure 1shows both the amount of calcium carbonate and gypsum extracted to supply the resource demand for the production of construction materials, and the amount of scallop landings (i.e., muscle and shells). Producers of building materials make up the demand for natural resources and are identified in green on the central map among the industries where a circular economy could emerge. The performance of scallop shells as an ingredient in the fabrication of building materials has been addressed in the literature, which nonetheless indicates challenges such as the economic evaluation of substitution [38] and comparison with the production cost of conventional materials [11]. Sustainability 2022, 14, x 3 of 17 construction. However, the implementation of a circular economy between these industries must address challenges in both accessing capital and strengthening the trade network between supply and demand, identified as barriers, and in strengthening the organizational culture of every company [57]. To advance a circular economy, a qualitative estimate has been conducted in 2021 of the economic benefits of using scallop shell powder as a cement substitute in the development of blended cements [58], not only to improve sustainability, but also to reduce production costs. An approach to the utilization of seashell wastes, specifically scallop (Argopecten purpuratus), as fine and coarse additives in the manufacture of building materials has been carried out [55]. The authors present these wastes as an alternative that consumes little energy in recycling compared to raw materials coming from mining and find that despite the additional cost of substituting scallop shells, the associated benefits include longer pavement life and better resistance to deformation and humidity. Looking at the performance of the aquaculture and construction industries in Chile allows us to identify linear economic models, where scallop shell waste is discarded without use [46,59] and more than 70% of raw materials for construction materials come from virgin natural resources [60]. Figure 1 shows both the amount of calcium carbonate and gypsum extracted to supply the resource demand for the production of construction materials, and the amount of scallop landings (i.e., muscle and shells). Producers of building materials make up the demand for natural resources and are identified in green on the central map among the industries where a circular economy could emerge. The performance of scallop shells as an ingredient in the fabrication of building materials has been addressed in the literature, which nonetheless indicates challenges such as the economic evaluation of substitution [38] and comparison with the production cost of conventional materials [11]. Figure 1. Geographical distribution in Chile of raw, whole scallops, including shells in metric tons (t) versus resource demands (t) in the construction industry. The recycling of mollusk shells into fireproof materials has been evaluated in this context [46]. However, the durability of building materials using scallop shell waste has not been evaluated for these applications. In order to contribute to the knowledge of how to implement a circular economy strategy between the construction and aquaculture industries, the objectives of this study are: (1) to evaluate the durability properties of the construction material with gypsum substitution by CaCO 3 from scallop (Argopecten purpuratus) aquaculture waste. (2) The economic performance of the experience was measured to evaluate the circular economy potential between the two industries. Figure 1. Geographical distribution in Chile of raw, whole scallops, including shells in metric tons (t) versus resource demands (t) in the construction industry. The recycling of mollusk shells into fireproof materials has been evaluated in this context [ 46 ]. However, the durability of building materials using scallop shell waste has not been evaluated for these applications. In order to contribute to the knowledge of how to implement a circular economy strategy between the construction and aquaculture industries, the objectives of this study are: (1) to evaluate the durability properties of the construction material with gypsum substitution by CaCO 3 from scallop (Argopecten purpuratus) aquaculture waste. (2) The economic performance of the experience was measured to evaluate the circular economy potential between the two industries. 2. Materials and Methods 2.1. Materials Our study used Chilean scallops (Argopecten purpuratus) produced using aquaculture, hereinafter scallops (S), from Invertec, located in Tongoy (Coquimbo Region) and commercial gypsum (G) in accordance with European Standard EN 13279-1 [ 61 ]. The scallop shells were washed in a tank to remove salt from their surface, using 1.5 L of water per kg of Sustainability 2022,14, 8383 4 of 16 scallop shells to be treated. Subsequently, organic matter was eliminated using an electric oven at 300 ◦ C for 3 h. The scallop shells were crushed using a jaw crusher and sieved to 600 µm. Figure 2shows the particle-size distribution of scallop shells and gypsum. Sustainability 2022, 14, x 4 of 17 2. Materials and Methods 2.1. Materials Our study used Chilean scallops (Argopecten purpuratus) produced using aquaculture, hereinafter scallops (S), from Invertec, located in Tongoy (Coquimbo Region) and commercial gypsum (G) in accordance with European Standard EN 13279-1 [61]. The scallop shells were washed in a tank to remove salt from their surface, using 1.5 L of water per kg of scallop shells to be treated. Subsequently, organic matter was eliminated using an electric oven at 300 °C for 3 h. The scallop shells were crushed using a jaw crusher and sieved to 600 µm. Figure 2 shows the particle-size distribution of scallop shells and gypsum. Figure 2. Particle-size distribution. The chemical compositions of the scallop shells and gypsum were determined according to American standards, ASTM D3682 [62], using an absorption spectrometer (Model 3100, Perkin-Elmer, Waltham, Massachusetts). The specific gravity of the materials used in this study was determined according to American standards, ASTMD854 [63]. As seen in Table 1, the majority component of the scallop shells was CaCO 3 . Losses due to calcination were high, mainly because CaCO 3 decomposes due to reaction 1, which is represented in the table as loss on ignition (LOI). Commercial gypsum is primarily composed of CaSO 4 . The average scallop shell particle size was almost 72 times larger than the average gypsum particle size. Furthermore, the specific gravities of scallop shells and gypsum were similar. Table 1. Chemical composition (weight), average particle size (µm) and specific gravity (g/cm 3 ). S (%) (1) G (%) (2) SiO 2 N.D. (3) 0.88 Al 2 O 3 N.D. (3) 0.18 Fe 2 O 3 N.D. (3) N.D. (3) MnO N.D. (3) 0.02 MgO N.D. (3) N.D. (3) CaO 54.76 40.56 Na 2 O 0.57 0.07 K 2 O N.D. (3) N.D. (3) TiO 2 N.D. (3) N.D. (3) P 2 O 5 0.13 N.D. SO 3 0.32 45.56 PC 44.42 12.52 Figure 2. Particle-size distribution. The chemical compositions of the scallop shells and gypsum were determined according to American standards, ASTM D3682 [ 62 ], using an absorption spectrometer (Model 3100, Perkin-Elmer, Waltham, Massachusetts). The specific gravity of the materials used in this study was determined according to American standards, ASTMD854 [ 63 ]. As seen in Table 1, the majority component of the scallop shells was CaCO 3 . Losses due to calcination were high, mainly because CaCO 3 decomposes due to reaction 1, which is represented in the table as loss on ignition (LOI). Commercial gypsum is primarily composed of CaSO 4 . The average scallop shell particle size was almost 72 times larger than the average gypsum particle size. Furthermore, the specific gravities of scallop shells and gypsum were similar. Table 1. Chemical composition (weight), average particle size (µm) and specific gravity (g/cm3). S (%) (1) G (%) (2) SiO2N.D. (3) 0.88 Al2O3N.D. (3) 0.18 Fe2O3N.D. (3) N.D. (3) MnO N.D. (3) 0.02 MgO N.D. (3) N.D. (3) CaO 54.76 40.56 Na2O 0.57 0.07 K2ON.D. (3) N.D. (3) TiO2N.D. (3) N.D. (3) P2O50.13 N.D. SO30.32 45.56 PC 44.42 12.52 Median size (µm) 180 25 Specific gravity (g/cm3)2.76 2.77 Note: (1) S: scallop shells, (2) G: gypsum, (3) N.D: not detectable. Product samples were fabricated following a simple, low-cost method under standard laboratory conditions (temperature: 25 ◦ C and 45% humidity). To prepare the samples, the solid components were weighed in compositions of 40% scallop shells and 60% gypsum (40% S), 50% scallop shells and 50% gypsum (50% S), and the control sample, 100% gypsum (0% S). The technical feasibility of using shells in these compositions for this application was shown in [ 46 ]. All solids were mixed into a homogeneous mixture using a mixer, and water was added at a water/solid ratio equal to 0.5. The water ratio was kept constant for the Sustainability 2022,14, 8383 5 of 16 fabrication of all samples [ 44 ]. It was mixed again until a homogeneous paste was obtained. All the flame retardant materials were removed from their molds after 24 h and cured at room temperature (20 ◦ C on average) with constant humidity (45% relative humidity on average) for an additional 28 days. The specimens were created using cylindrical molds with a diameter of 34 mm and a height of 40 mm. 2.2. Methods 2.2.1. Product Durability Tests The durability of the material was determined using three aspects: resistance to acid attack, resistance to sulfate attack, and resistance to freeze-thaw cycles. Compressive strength was analyzed each week for 10 weeks total. Before undergoing durability tests, the manufactured materials (0% S, 40% S, 50% S) were measured for compressive strength and bulk density. The compressive strength (Sc) of the samples was determined using a compression testing machine (Controls, 65-L28F12, 300 kN), in accordance with the European standard, EN 13279-2, [ 64 ]. The method described in the American standard, ASTM E 605 [ 65 ], was used to measure density (d). For compressive strength and bulk density, 3 tests were performed for each composition. Compressive strength and bulk density were the arithmetic mean of 3 specimens tested. For each of the compositions and properties (compressive strength and bulk density), the standard deviation was determined. •Resistance to Acid Attack Resistance to acid attack (Ra) was determined using the procedure detailed in previous studies [ 66 ]. The specimens were first cured for 28 days. Half of the fabricated samples were immersed in water (reference samples) (Figure 3a) and the other half in 1 N sulfuric acid (Figure 3b). As shown in Equation (4), the compressive strength of the attacked samples was measured each week and compared with the compressive strength of the reference samples. Resistance to acid attack was sampled in three specimens each week for 10 weeks. Ra = CSacid CSwater (4) where, CS acid is compressive strength of samples attacked by acid and CS water is Compressive strength of samples attacked by water. Sustainability 2022, 14, x 6 of 17 (a) (b) Figure 3. Submerged samples during (a) water immersion and (b) acid immersion. • Resistance to Sulfate Attack Resistance to sulfate attack was determined in accordance with the American standard, ASTM C 1012-13 [67]. The samples were immersed in a dilution of Na2SO4 at a concentration of 50 g/L of Na2SO4 (Figure 4). Each week, three specimens were tested for compressive strength over a period of 10 weeks. Figure 4. Samples were submerged in sulfate salts. • Resistance to Freeze-Thaw Cycles Resistance to freeze-thaw was determined in accordance with the European standard, EN 12390-9 [68]. The samples were subjected to freezing phases at −18 ± 3 °C for 8 h during the day and thawing phases at 20 ± 2 °C for 16 h. Three specimens were tested for compressive strength each week over a period of 10 weeks. 2.2.2. Economic Analysis • Benefits/Savings Related to The Use of CaCO3—Construction The economic analysis was carried out to determine both the costs (e.g., scallop shell conditioning, construction material manufacturing) and benefits (e.g., revenue, savings) of using scallop shells to substitute gypsum [46]. For the economic analysis, two different compositions, by weight, of gypsum and calcium carbonate from scallop shells were used: the first composition was 40% scallop shells and 60% gypsum (40% S), and the second composition was 50% scallop shells and 50% gypsum (50% S), which were compared with Figure 3. Submerged samples during (a) water immersion and (b) acid immersion. •Resistance to Sulfate Attack Resistance to sulfate attack was determined in accordance with the American standard, ASTM C 1012-13 [ 67 ]. The samples were immersed in a dilution of Na 2 SO 4 at a concentration of 50 g/L of Na 2 SO 4 (Figure 4). Each week, three specimens were tested for compressive strength over a period of 10 weeks. Sustainability 2022,14, 8383 6 of 16 Sustainability 2022, 14, x 6 of 17 (a) (b) Figure 3. Submerged samples during (a) water immersion and (b) acid immersion. • Resistance to Sulfate Attack Resistance to sulfate attack was determined in accordance with the American standard, ASTM C 1012-13 [67]. The samples were immersed in a dilution of Na2SO4 at a concentration of 50 g/L of Na2SO4 (Figure 4). Each week, three specimens were tested for compressive strength over a period of 10 weeks. Figure 4. Samples were submerged in sulfate salts. • Resistance to Freeze-Thaw Cycles Resistance to freeze-thaw was determined in accordance with the European standard, EN 12390-9 [68]. The samples were subjected to freezing phases at −18 ± 3 °C for 8 h during the day and thawing phases at 20 ± 2 °C for 16 h. Three specimens were tested for compressive strength each week over a period of 10 weeks. 2.2.2. Economic Analysis • Benefits/Savings Related to The Use of CaCO3—Construction The economic analysis was carried out to determine both the costs (e.g., scallop shell conditioning, construction material manufacturing) and benefits (e.g., revenue, savings) of using scallop shells to substitute gypsum [46]. For the economic analysis, two different compositions, by weight, of gypsum and calcium carbonate from scallop shells were used: the first composition was 40% scallop shells and 60% gypsum (40% S), and the second composition was 50% scallop shells and 50% gypsum (50% S), which were compared with Figure 4. Samples were submerged in sulfate salts. •Resistance to Freeze-Thaw Cycles Resistance to freeze-thaw was determined in accordance with the European standard, EN 12390-9 [ 68 ]. The samples were subjected to freezing phases at − 18 ± 3 ◦ C for 8 h during the day and thawing phases at 20 ± 2 ◦ C for 16 h. Three specimens were tested for compressive strength each week over a period of 10 weeks. 2.2.2. Economic Analysis •Benefits/Savings Related to The Use of CaCO3—Construction The economic analysis was carried out to determine both the costs (e.g., scallop shell conditioning, construction material manufacturing) and benefits (e.g., revenue, savings) of using scallop shells to substitute gypsum [ 46 ]. For the economic analysis, two different compositions, by weight, of gypsum and calcium carbonate from scallop shells were used: the first composition was 40% scallop shells and 60% gypsum (40% S), and the second composition was 50% scallop shells and 50% gypsum (50% S), which were compared with the commercial material (gypsum, 0% S). Scallop shell pre-treatment accounted for the costs incurred for the consumption of wash water required for removing impurities and the energy costs required for both the milling process and the calcination of the scallop shells. Costs were calculated based on one metric ton of material. The average exchange rate used was 792 Chilean pesos for each US dollar ($) [ 69 ]. The analysis does not incorporate transportation costs or other costs that may be associated with shell availability. To calculate the benefits/savings related to the replacement of gypsum with CaCO 3 , the manufacturing costs of construction materials were determined, including both raw materials (gypsum and calcium carbonate) and inputs (energy and water). Given the absence of a market to commercialize building materials with added calcium carbonate from scallop shells, the benefits (e.g., savings) were defined as the difference between the total cost value of building material production and those achieved with the different compositions produced. •Scallop Shell Generation Potential and Disposal Cost—Aquaculture The landings data for Chile were collected from the annual statistical reports of Sernapesca (National Fisheries and Aquaculture Service). Landings of scallop shells were differentiated by fishery (extraction) and aquaculture (farming). To calculate scallop shell availability, the Condition Index (CI) ratio was used, which allows for differentiating the Sustainability 2022,14, 8383 7 of 16 meat and shell weight of bivalve mollusks [ 70 ]. The CI varies according to inherent factors such as gonad weight and muscle growth and other environmental factors that influence the reproductive stage and feed supply. Due to the small number of producers (large-, medium-, and small-scale), the final costs of scallop shell disposal (waste) were collected in a semi-structured interview. The selected producers are located in the region of Coquimbo, where most of Chile’s production is concentrated (Figure 1). •Modified Payback Period The payback period is a financial indicator to determine the time it takes for investments to pay off, i.e., the time it takes for revenue to cover costs. The shorter the payback period, the lower the return on investment. However, given the difficulty of establishing production times, we worked with the modified payback indicator, which contrasts savings versus production, thus quantifying the amount of construction material required to recover the amount of the initial investment. Laboratory adjustments and the acquisition of equipment were considered as initial investments and amounted to $36,995. The performance of the quantity produced versus the savings obtained was graphed. •Circular Economy—Construction and Aquaculture The economic evaluation was complemented by determining the function that calculates the cost of final disposal of scallop shells (given their waste condition). For the construction industry, savings from the addition of scallop shell as a raw material were evaluated. The savings scenarios of 40% S and 50% S scallop shell substitution were graphed individually against aquaculture waste disposal costs. 3. Results and Discussion 3.1. Durability 3.1.1. Initial Properties As can be seen in Table 2, the density and compressive strength (Sc) of the materials decrease with the addition of scallop shell waste. Density decreased because the waste has a higher particle size distribution than gypsum, increasing the porosity between particles. Regardless of their proportion, and in accordance with the European standard, EN 13279-1 [61] , all materials would have a compressive strength greater than 2.0 MPa and, therefore, all mortars produced met this requirement. According to the European standard, EN 12859 [71], all materials were classified as high density. Table 2. Compressive strength and density. Density (kg/m3)Compressive Strength (MPa) 0% S 1528.16 ±2.10 13.51 ±1.37 40% S 1474.00 ±12.54 13.10 ±1.53 50% S 1468.15 ±16.01 10.88 ±0.54 Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. 3.1.2. Acid Attack During the first 3 weeks, in the materials with scallop shells (40% S and 50% S) subjected to acid immersion, the compressive strengths were reduced to 6.1 MPa. However, in the case of the materials subjected to water immersion, the compressive strengths were reduced to 2.2 MPa. After week 3, at 40% S and 50% S, the compressive strengths of the materials subjected to acid immersion dropped drastically to 2.5 MPa. As shown in Figure 5, as the amount of waste increases in the 40% S (40% scallop and 60% gypsum) and 50% S (50% scallop and 50% gypsum) samples, resistance to acid attack (Ra) increases and remains above 1 until week 5. This is because, in an acidic medium, part of the calcium in the scallop shells dissolves in water as Ca 2+ and reacts with SO 42- , producing CaSO 4 (gypsum), as shown in Figure 6. The gypsum that forms around the material has a compressive strength of 5 MPa [ 44 ]. However, after week 3, resistance to acid attack (Ra) begins to decrease, and by week 5, Ra is less than 1. As shown in Figure 6c, this is because the fabricated gypsum Sustainability 2022,14, 8383 8 of 16 produces a volumetric expansion of the material and leads to spalling of the surface layers and, consequently, a decrease in compressive strength [72]. Sustainability 2022, 14, x 8 of 17 Table 2. Compressive strength and density. Density (kg/m 3 ) Compressive Strength (MPa) 0% S 1528.16 ± 2.10 13.51 ± 1.37 40% S 1474.00 ± 12.54 13.10 ± 1.53 50% S 1468.15 ± 16.01 10.88 ± 0.54 Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. 3.1.2. Acid Attack During the first 3 weeks, in the materials with scallop shells (40% S and 50% S) subjected to acid immersion, the compressive strengths were reduced to 6.1 MPa. However, in the case of the materials subjected to water immersion, the compressive strengths were reduced to 2.2 MPa. After week 3, at 40% S and 50% S, the compressive strengths of the materials subjected to acid immersion dropped drastically to 2.5 MPa. As shown in Figure 5, as the amount of waste increases in the 40% S (40% scallop and 60% gypsum) and 50% S (50% scallop and 50% gypsum) samples, resistance to acid attack (Ra) increases and remains above 1 until week 5. This is because, in an acidic medium, part of the calcium in the scallop shells dissolves in water as Ca 2+ and reacts with SO 42- , producing CaSO 4 (gypsum), as shown in Figure 6. The gypsum that forms around the material has a compressive strength of 5 MPa [44]. However, after week 3, resistance to acid attack (Ra) begins to decrease, and by week 5, Ra is less than 1. As shown in Figure 6c, this is because the fabricated gypsum produces a volumetric expansion of the material and leads to spalling of the surface layers and, consequently, a decrease in compressive strength [72]. Figure 5. Variation of compressive strength against acid attack. Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. Figure 5. Variation of compressive strength against acid attack. Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. Sustainability 2022, 14, x 9 of 17 (a) (b) (c) Figure 6. (a) 0% S (gypsum), (b) 40% S (40% Scallop and 60% gypsum), 50% S after two weeks of acid immersion and (c) 40% S after five weeks of acid immersion. 3.1.3. Sulfate Attack Sulfate attack is one of the most important aggressive risks to construction materials, causing losses in mechanical properties [73]. As shown in Figure 7, the compressive strength of 40% S and 50% S suffered a drop of almost 80% compared to 0% S (gypsum), which dropped 70%. After the first week, the mechanical strengths decreased further, and the 40% S and 50% S samples broke completely in the sixth week. During the first 3 weeks, the compressive strength was between 2.3 and 2.5 MPa. From the fourth week, the compressive strength was reduced below 2 MPa (minimum compressive strength required [61]). The deterioration seems to be related to the sedimentation of the products of physical crystallization, causing a higher pressure in the open pores of the material [74]. As shown in Figure 2, the scallop shells had a larger particle size, causing greater porosity [75]. Figure 7. Variation of compressive strength against sulfate attack. Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. 3.1.4. Resistance to Freeze-Thaw All the mixtures exhibited a decrease in compressive strength after freezing and thawing (Figure 8). The 40% S and 50% S samples suffered a 70% decrease as of cycle 28 (week 4). After the first week of freeze-thaw cycles, the mechanical strengths of the scallop 0 2 4 6 8 10 12 14 16 012345678910 Sc (MPa) Week 0%S 40%S 50%S Figure 6. ( a ) 0% S (gypsum), ( b ) 40% S (40% Scallop and 60% gypsum), 50% S after two weeks of acid immersion and (c) 40% S after five weeks of acid immersion. 3.1.3. Sulfate Attack Sulfate attack is one of the most important aggressive risks to construction materials, causing losses in mechanical properties [ 73 ]. As shown in Figure 7, the compressive strength of 40% S and 50% S suffered a drop of almost 80% compared to 0% S (gypsum), which dropped 70%. After the first week, the mechanical strengths decreased further, and the 40% S and 50% S samples broke completely in the sixth week. During the first 3 weeks, the compressive strength was between 2.3 and 2.5 MPa. From the fourth week, the compressive strength was reduced below 2 MPa (minimum compressive strength required [ 61 ]). The deterioration seems to be related to the sedimentation of the products of physical crystallization, causing a higher pressure in the open pores of the material [ 74 ]. As shown in Figure 2, the scallop shells had a larger particle size, causing greater porosity [ 75 ]. Sustainability 2022,14, 8383 9 of 16 Sustainability 2022, 14, x 9 of 17 (a) (b) (c) Figure 6. (a) 0% S (gypsum), (b) 40% S (40% Scallop and 60% gypsum), 50% S after two weeks of acid immersion and (c) 40% S after five weeks of acid immersion. 3.1.3. Sulfate Attack Sulfate attack is one of the most important aggressive risks to construction materials, causing losses in mechanical properties [73]. As shown in Figure 7, the compressive strength of 40% S and 50% S suffered a drop of almost 80% compared to 0% S (gypsum), which dropped 70%. After the first week, the mechanical strengths decreased further, and the 40% S and 50% S samples broke completely in the sixth week. During the first 3 weeks, the compressive strength was between 2.3 and 2.5 MPa. From the fourth week, the compressive strength was reduced below 2 MPa (minimum compressive strength required [61]). The deterioration seems to be related to the sedimentation of the products of physical crystallization, causing a higher pressure in the open pores of the material [74]. As shown in Figure 2, the scallop shells had a larger particle size, causing greater porosity [75]. Figure 7. Variation of compressive strength against sulfate attack. Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. 3.1.4. Resistance to Freeze-Thaw All the mixtures exhibited a decrease in compressive strength after freezing and thawing (Figure 8). The 40% S and 50% S samples suffered a 70% decrease as of cycle 28 (week 4). After the first week of freeze-thaw cycles, the mechanical strengths of the scallop 0 2 4 6 8 10 12 14 16 012345678910 Sc (MPa) Week 0%S 40%S 50%S Figure 7. Variation of compressive strength against sulfate attack. Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. 3.1.4. Resistance to Freeze-Thaw All the mixtures exhibited a decrease in compressive strength after freezing and thawing (Figure 8). The 40% S and 50% S samples suffered a 70% decrease as of cycle 28 (week 4). After the first week of freeze-thaw cycles, the mechanical strengths of the scallop shell materials (40% S and 50% S) were reduced to 10.5 MPa for 40% S and 8.5 MPa for 50% S. In the fourth week, the compressive strengths of the materials with scallop shells (40% S and 50% S) were reduced below 2 MPa. However, in the case of gypsum (0% S), the compressive strength decreased more slowly. After the tenth week, the compressive strength of the gypsum remained above 2 MPa. This was because the scallop shells had a larger particle size, as shown in Figure 2, causing greater porosity. Therefore, the water permeated more when frozen, and as a consequence, generated microcracks [76]. Sustainability 2022, 14, x 10 of 17 shell materials (40% S and 50% S) were reduced to 10.5 MPa for 40% S and 8.5 MPa for 50% S. In the fourth week, the compressive strengths of the materials with scallop shells (40% S and 50% S) were reduced below 2 MPa. However, in the case of gypsum (0% S), the compressive strength decreased more slowly. After the tenth week, the compressive strength of the gypsum remained above 2 MPa. This was because the scallop shells had a larger particle size, as shown in Figure 2, causing greater porosity. Therefore, the water permeated more when frozen, and as a consequence, generated microcracks [76]. Figure 8. Variation of the compressive strength against the cycles of freezing and thawing. Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. 3.2. Economic Analysis 3.2.1. Benefits/Savings Related to the Use of CaCO 3 —Construction The conditioning treatment for scallop shells to be used as ingredients to manufacture building materials results in production costs [11]. Pretreatment includes costs such as wash water for the removal of impurities and the energy expended in grinding and calcination. Table 3 shows the details of the costs for 40% S and 50% S compositions of scallop shell substitution for gypsum. It also shows that the pre-treatment cost is mainly determined by propane gas consumption and that the total cost increases as the composition of the pre-treated shell increases. Table 3. Pre-treatment costs (US$) of scallop shells (S) by composition. Pre-Treatment Cost (US$) 0% S 40% S 50% S Wash water 0 2.12 2.70 Propane consumption 0 25.23 32.07 Grinding energy 0 0.60 0.76 Pre-treatment cost 0 27.95 35.53 Note: 0% S: 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. The addition of calcium carbonate from scallop shells generates savings in the manufacture of building materials compared to the cost of materials with only gypsum. The breakdown of material production costs (Table 4) shows that, although pretreatment of the scallop shells entails costs, they are lower than the use of gypsum alone. For a composition of 40% S, the pretreatment cost was $27.95, a cost that was covered by the savings, which in total amounted to $46.36 (22.4% of the cost without adding shells). For compositions of 50% S, the cost of pretreatment increases to $35.53, and savings increase Figure 8. Variation of the compressive strength against the cycles of freezing and thawing. Note: 0% S : 100% gypsum, 40% S: 40% scallop and 60% gypsum and 50% S: 50% scallop and 50% gypsum. 3.2. Economic Analysis 3.2.1. Benefits/Savings Related to the Use of CaCO3—Construction The conditioning treatment for scallop shells to be used as ingredients to manufacture building materials results in production costs [ 11 ]. Pretreatment includes costs such as wash water for the removal of impurities and the energy expended in grinding and calcination. Table 3shows the details of the costs for 40% S and 50% S compositions of scallop shell substitution for gypsum. 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