The isolation of biosilica from diatoms followed the modified sol-gel method described by (Utama et al., 2025). Diatomaceous earth was purchased from a certified supplier. A total of 20 g of diatomaceous earth was weighed using an analytical balance (KERN ABS 220-4) and homogenized in 500 mL of 1 M NaOH solution at 60–80°C for 3 h. The resulting suspension was titrated with 1 M HCl at and allowed to stand overnight to form a silica hydrogel. The hydrogel was washed with distilled water and oven-dried to produce biosilica in powder form.

Barnacle shells were collected in July 2025 from coastal structures in Teluk Kunyit and from the shorelines of Sariringgung and Mutun, all located within Lampung Province, Indonesia. The samples were then cleaned, sun-dried, and ground using a grinder at the Agricultural Product Processing Laboratory. The preparation procedure was based on the method of (Prihanto et al., 2022) to obtain powder using the dry milling method. A total of 100 g of CaCO₃ barnacle shell powder was placed in a ball mill (MYRA-YFI56) together with 16 steel balls with a diameter of 2 cm, and run at a speed of 200 rpm for 70 h. The milled powder was then sieved and dried at a low temperature to obtain stable nano-sized particles without any changes in the phase. The samples were analyzed for particle size using a particle size analyzer (Beckman Coulter) to confirm the nanoscale range. Element characterization was performed using XRF (PANalytical, model Minipal 4) to identify calcium (Ca) and FTIR to detect CO₃ functional groups (Hariyati et al., 2019).

The concrete job mix formula (JMF) was designed to achieve a target compressive strength of 25 MPa at 28 days. The formulation followed the SNI 7656:2012 (BSN, 2012) mix design method and was conducted at the Materials and Construction Laboratory of the University of Lampung. Additives of CaCO₃ from barnacles (Balanus sp.) and Bacillariophyceae biosilica in a 1:1 ratio were incorporated at substitution levels of 0, 5, 10, 15, 20, and 25% relative to the cement weight. This range was used to evaluate a broad performance spectrum, from low-dosage enhancement to higher substitution levels, and to identify the specific threshold where the bio-additives transition from improving the matrix to potentially reducing compressive strength. The process began with formulation calculations, aggregate weighing, and mixing using a concrete mixer (TIGER GT300 GE). The resulting mixture was subjected to a slump test, and each variation was cast into three cylindrical molds (15 cm diameter × 30 cm height) and three beam molds × 60 cm). After compaction using a vibrator and plastic covering, the concrete was cured before testing on days 14 and 28. The concrete formulation is presented in Table 1.

Compressive strength tests were performed using a CTM (Vertex Scientific) in accordance with SNI 1974:2011 (BSN, 2011) at the Materials and Construction Laboratory. The samples were positioned with a maximum plate spacing of 1 cm, and the indicator needle was reset to zero before loading at a rate of 0.15–0.35 MPa/s until the maximum load (Pmax, in kN) was reached. The test results are presented as mean ± standard deviation to reflect sample variability.

Flexural strength tests were conducted following SNI 4431:2011 (BSN, 2011) using a UTM (Zwick Roell) at the Materials and Construction Laboratory. Once the specimen was properly positioned, the load was gradually applied at a rate of 8–10 kg/cm² per minute to prevent sudden impact and then reduced near the cracking point. The peak load causing failure was recorded at three measurement points with a precision of 0.25 mm, and the average value was calculated.

Data were analyzed descriptively, including silica (Si) and calcium (Ca) detection using X-ray fluorescence (XRF) (with normalized data), and compound identification of CaCO₃ using Fouriertransform infrared spectroscopy (FTIR) and particle size analysis (PSA). The mechanical properties of the concrete were determined based on the maximum values recorded from the UTM and CTM tests and further statistically analyzed using IBM SPSS version 25.

Characterization of biosilica derived from Bacillariophyceae diatoms using X-Ray Fluorescence (XRF) analysis revealed a content of 98.798% (Table 2), indicating high purity and excellent potential as a supplementary material for concrete. The characterization of barnacle-derived showed a calcium content of 91.72% (Table 2). The Fourier transform infrared (FTIR) spectrum of barnacle (Figure 1) exhibited characteristic vibrational bands corresponding to C–H stretching at 2519.7 , O–H stretching between 3750 and 3000terpadu , and C–O stretching within the 1300–e:09/08/2025 9:01:37 1000 cm⁻¹ range. In addition, carbonate ion vibrations were detected between 716.6-1796.6 (Vironika & Rahmawati,

2022). A distinct absorption peak at (Riyanto et al., 2023) indicates the presence of calcium oxide (CaO), confirming that was successfully identified in the synthesized barnacle nanoparticles.

Particle size analysis of barnaclederived revealed that all particles were within the nanometer range, distributed between 10 and 201.5 nm, with an average of 120.1 nm (Figure 2). This confirms that the synthesized material meets the nanoparticle size criteria proposed by Eker et al. (2024), who defined nanoparticles as having dimensions of up to 100 nm in at least one spatial direction (x, y, or z). The small particle size and large specific surface area significantly influence interparticle interactions and enhance the pozzolanic reactivity of . According to Mydin et al. (2023), incorporating nanoparticles into concrete can improve its workability, mechanical strength, durability, and microstructural properties. Barnaclederived nanoparticles act as nucleation sites for the formation of calcium silicate hydrate (C–S–H) gel, which forms through reactions between calcium hydroxide and silicate compounds present in the cement matrix. This mechanism contributes to enhanced bonding within the matrix and leads to improved compressive strength of concrete.

The compressive strength of concrete with CaCO nanoparticles and biosilica is presented in Table 3.

The compressive strength test results showed that the strenght of the concrete increased with the curing age. This finding is consistent with the statment of Marchewka et al. (2025), who explained that a longer curing period allows the cement hydration1 / 1 process to proceed more completely, leading to the formation of more calcium silicate hydrate (C–S–H) compounds that significantly contribute to concrete strength development. Optimal curing conditions, such as temperature, humidity, and time control, ensure that hydration reactions occur eficiently, minimizing the risk of early cracking and promoting a notable increase in strength at later ages. Therefore, the experimental data obtained are in agreement with the theoretical principle that the compressive strength of concrete is directly proportional to the curing duration.Table 3 indicates that the addition of nanoparticles and biosilica influenced the mechanical performance of concrete at diferent curing ages (p<0.05). The control sample (0%substitution) exhibited the highest compressive strength, reaching 24.28 MPa at 14 d and 25.33 MPa at 28 d. This indicates that a conventional concrete composition without substitution still provides optimal bonding and hydration reactions under standard curing conditions. However, the incorporation of nanoparticles and biosilica led to varying efects, depending on the substitution level. At 5% substitution, the compressive strength at 28 d increased to 21.26 MPa, indicating a moderate improvement compared to the 14-day strength. This suggests that small additions of nanoparticles can contribute to secondary hydration and enhance microstructural densification during curing. Nano-CaCO ₃ provides nucleation sites for calcium silicate hydrate (C–S–H) formation, while biosilica reacts with calcium hydroxide to form additional C–S–H, both of which strengthen the matrix. These findings are consistent with those of Suryawanshi and Shaikh (2021), who reported a 6.9% improvement in compressive strength when Meretrix casta shell powder was used at a 5% substitution level. Similarly, Rahmadani et al. (2025) demonstrated that 7% substitution of Anadara granosa shell powder achieved a compressive strength of 28.16 MPa at 28 d, indicating that shell-based materials tend to exhibit optimal compressive performance at low to moderate substitution levels.

Beyond the 10% substitution level, a decline in compressive strength was observed, with values decreasing progressively to 17.11 MPa at 15%, 16.18 MPa at 20%, and 12.49 MPa at 25% substitution, on day 28. This reduction may be attributed to the excessive replacement of cement, which limits the availability of calcium hydroxide necessary for pozzolanic reactions and disrupts the matrix continuity. Dhairiyasamy et al. (2025) explained that nanomaterials start to clump together rather than spread evenly at higher concentrations. These clumps or agglomerates create internal weak spots and tiny voids in concrete. From a durability perspective, these extra pores are problematic because they can increase water absorption, which eventually weakens the structure. In a major earthquake, these microscopic gaps act as ‘stress points’ where the concrete can start to crush under the heavy vertical pressure of a building (El-Feky et al., 2024), making the 10% limit a critical threshold for maintaining the structural safety.

Similar findings were reported by Al-Lezami and Devi (2022), who noted that high nanoparticle loading can cause particle agglomeration, reducing the packing eficiency and overall strength. Overall, the compressive strength data exhibited a parabolic trend, improving at low substitution levels (up to 5%) before declining as the percentage increased. This trend reflects the balance between the beneficial microstructural refinement at optimal nanoparticle concentrations and the detrimental efects of excessive substitution. Microstructurally, and biosilica enhanced the particle packing density and filled the capillary pores, potentially improving the interfacial bonding between the aggregates and cement matrix. Although the compressive strength decreased beyond 10% substitution, this microstructural uniformity may enhance the ductility and energy absorption capacity of the material under cyclic loading, which are crucial properties for earthquake-resistant concrete.

The flexural strength of concrete with CaCO nanoparticles and biosilica is presented in Table 4.

The flexural strength results of concrete with varying levels of nanoparticles and biosilica substitution are presented in Table 4. Overall, the data indicate that the flexural strength of all mixtures increased with curing age, consistent with the findings of Jin et al. (2025), who reported that prolonged curing enhances cement hydration and leads to a denser microstructural bond within the concrete matrix. This densification improves the material’s ability to resist tensile strength and crack propagation, resulting in higher flexural strength values at later ages.

At 14 days, the control sample (0% substitution) exhibited the highest flexural strength (3.27 MPa), whereas increasing substitution levels resulted in a gradual decrease in strength. However, by day 28, a significant strength gain was observed across all formulations, indicating that the hydration and pozzolanic reactions of the supplementary materials continued to progress over time. The 5% substitution level achieved a flexural strength of 4.10 MPa at 28 days, approaching that of the control (4.30 MPa), suggesting an optimal balance between the filler and binder phases at this concentration. Similarly, Suryawanshi and Shaikh (2021) reported a modest flexural strength increase of approximately 3.9% with the addition of Meretrix casta shell powder at a 5% substitution level. In contrast, at higher substitution levels (≥20%), the flexural strength declined significantly (p<0.05), likely due to the dilution efect and reduced calcium hydroxide availability, which limits secondary C–S–H formation. This phenomenon weakens the interfacial bonding and increases the porosity, as reported by Al-Lezami and Devi (2022). From a microstructural perspective, moderate nanoparticle incorporation improves the packing density and refines the pore structure, enhancing load transfer and crack-bridging capacity under bending stress.

These results suggest that the moderate incorporation of CaCO₃ nanoparticles and biosilica can contribute positively to the flexural performance of concrete, particularly when suficient curing time is provided to ensure full hydration and pozzolanic reactivity. However, excessive substitution may hinder matrix cohesion, resulting in reduced flexural capacity. The observed trends align well with the established hydration mechanisms and the theory that longer curing durations enhance the development of interfacial bonding and load transfer eficiency within the cementitious composite. Furthermore, the improvement in flexural strength observed at low substitution levels indicates enhanced ductility and energy absorption capacity, which are key characteristics of earthquakeresistant concrete. The refined microstructure formed by the addition of nano-CaCO ₃ and biosilica not only strengthened the matrix but also increased its ability to dissipate energy and delay crack propagation under cyclic loading, thereby improving the overall seismic resilience of the structure.