This experiment uses different proportions of shells as concrete aggregate, prepares cement mortar according to a certain mix ratio and curing conditions, uses different test equipment and methods to measure its compressive strength, flexural strength and water absorption rate, in order to study the effect of shell replacement of crushed stone on the performance of cement mortar.
2.1 Experimental materials and proportions
2.1.1 The performance of the shell
Shells are the hard protective coverings of various mollusks, such as oysters, clams, mussels, and scallops. They are composed mainly of calcium carbonate, also known as limestone, which is a common ingredient of cement[19]. Shells have a complex hierarchical structure, consisting of different layers of organic and inorganic materials, arranged in various patterns and orientations. This gives them high strength and toughness, as well as resistance to fracture and damage [25].
The properties of seashell concrete depend on several factors, such as the type, size, shape, and proportion of seashells, as well as the curing conditions, admixtures, and chemical treatments. Some of the effects of seashells on concrete are:
Setting time: Seashells can increase the setting time of concrete, due to their alkaline nature and water absorption capacity[25][26] .
Workability: Seashells can decrease the workability of concrete, due to their irregular shape and rough surface [25].
Density: Seashells can increase the density of concrete, due to their higher specific gravity than cement or sand [25][26].
Compressive strength: Seashells can decrease the compressive strength of concrete, due to their lower bonding strength with cement paste and higher porosity 234. However, adding admixtures or applying chemical treatments can improve the compressive strength of seashell concrete[26][27] .
Tensile strength: Seashells can increase the tensile strength of concrete, due to their fibrous structure and crack-bridging effect [13][25].
Flexural strength: Seashells can increase the flexural strength of concrete, due to their higher modulus of elasticity and toughness[26].
Modulus of elasticity: Seashells can decrease the modulus of elasticity of concrete, due to their lower stiffness and higher deformation [27].
2.1.2 Experimental materials
Table 1
The main component of cement %
CaO | SiO2 | Al2O3 | Fe2O3 | SO3 | Na2O | K2O | MgO |
64.12 | 22.31 | 6.42 | 4.37 | 1.1 | 0.75 | 0.56 | 0.37 |
Table 2
Physical and mechanical properties of cement
Standard Consistency Water Requirement/% | Specific surface area/ (m2/kg) | Coagulation time/min | Compressive strength/MPa | Flexural strength/MPa |
Initial setting | Finalization | 3 d | 28 d | 3 d | 28 d |
28 | 360 | 175 | 235 | 27.5 | 49.0 | 5.5 | 8.0 |
Cement: P·O R45 cement, The main chemical composition is presented in Table 1, while the physical and mechanical properties are displayed in Table 2, which complies with the requirements of EN 197-1 for chemical composition, strength, setting time, soundness, and fineness.
Water: ordinary tap water.
Sand: natural river sand, in line with the construction sand standard, the bulk density is less than 1.5g/m3, the fineness modulus is 1.9, the moisture content is less than 1%.
Stone: fine stone meets the requirements of EN 12620 for geometrical and physical properties, the bulk density is not less than 2.6g/m3, the particle size is 5mm- 7mm.
Shell: according to the European Standards (EN − 12620), the aggregates used in the production of concrete are inert granular materials such as gravel, crushed stone, sand, slag, recycled concrete, and geosynthetic aggregates. The aggregates may be natural, manufactured, or recycled, hence, the substitution of shells for aggregate meets the standards. After calcination and crushing, the particle size is less than 0.5mm, the bulk density is less than 2.9g/m3, the fineness modulus is 2.9, and the moisture content is less than 1%, refer to Fig. 3 and Fig. 4 for details.
The compressive strength of the concrete samples was tested at 7 days and 28 days after casting, according to EN 12390-3. The average compressive strength at 7 days was 25 MPa, 35 MPa on 28 days, which met the design requirement of 30 MPa for the structural elements.
The slump test was performed on the fresh concrete mix, according to EN 12350-2. The slump value was 75 mm, which indicated a medium workability of the concrete, suitable for the casting and compaction methods used in this project.
2.1.3 Material ratio and specimen design
According to the study's conclusions and curve analysis, it is believed that as the water-binder ratio increases, the impact of coarse aggregate becomes more pronounced, resulting in lower dry shrinkage[28], refer Fig. 5. According to the studies of previous researchers, increasing the water-cement ratio results in higher porosity and lower strength of the concrete[29]. After analyzing Figs. 5 and 6, and considering EU standard EN 206 + A2 and Chinese standard GB 50010 − 2010, the water-cement ratio in the range of 0.45–0.6 is relatively appropriate[30], to ensure sufficient strength and reduce porosity, it is necessary to choose a smaller water-cement ratio[31]. So our team take a water-cement ratio of 0.5 as optimal for ensuring the strength and preventing cracking of the concrete in the experiment[31][32].
In this test, shells and other materials were used as a substitute for natural stone. The mix ratio (kg/m3) of the cement mortar in the benchmark group was as follows: m (cement): m (sand): m (stone): m (water) = 500: 600: 900: 250, the mix ratio (kg/m3) of the cement mortar in the 10% shell replacement rate was as follows: m (cement): m (sand): m (stone) : m(shell): m (water) = 500: 600: 810:90: 250, the mix ratio (kg/m3) of the cement mortar in the 30% shell replacement rate was as follows: m (cement): m (sand): m (stone) : m(shell): m (water) = 500: 600: 530: 270: 250, the mix ratio (kg/m3) of the cement mortar in the 50% shell replacement rate was as follows: m (cement): m (sand): m (stone) : m(shell): m (water) = 500: 600: 450: 450: 250. All the mixes conform to standard EN 206-1, the mixture is shown in Fig. 7. Additionally, the superplasticizer content was 0.2% of the cement mass. The gravel is replaced based on shell gradients of 10%, 30%, and 50%. The mixture was evenly stirred and poured into a mold with dimensions of 400 mm×400 mm×1600 mm, the test prisms is shown in Fig. 8, followed standard EN 12390-5:2009. Thirteen specimens are cast for each different gradients, 39 specimens in total for bending experiments. The specimens for compressive experiments are 100 mm×100 mm×100 mm, followed the standard EN 12390-3:2019, with 13 specimens cast at the each gradient, 39 specimens in all. After segmental vibration compaction, the specimens were hardened at 23°C, demoulded after 7 days, and cured at 20°C with a relative humidity of 95% until the specified age[33].
2.1.4 Experimental procedure
After curing for 28 days, the specimen underwent test using the DS2-1000N compressive strength tester for axial compressive resistance and three-point bending. The "Hydraulic Pressure Testing Machine-GB/T3722" was used to conduct a uniaxial compressive test on the specimen, aligning the axis with the pressure center of the testing machine's pressure plate. The load was applied at a speed of 10 kN/s to 30 kN/s until the specimen failed, and the failure load was recorded to determine the compressive and flexural strengths[33]. The test block was soaked in water for 2 days, taken out to dry completely, recorded the weight before and after. The water absorption rate was calculated to evaluate the frost resistance strength.