Dual Effects of Brown Algae Impurities on Coral Concrete: Sulfate Resistance Decay and Multiscale Predictive Modeling

Effects of Desert Sands on the Fresh and Hardened Properties of Cement-Based Materials

The use of desert sands as an alternative to river sand in concrete may be limited by the presence of internally sourced sulfate and chloride ions, which may affect hydration and long-term performance. This study investigates the influence of naturally contaminated desert sands on the fresh and hardened properties of cement-based materials. Mortar mixtures produced with sands from three sites of the Atacama Desert were compared with mixtures containing artificially contaminated sands designed to reproduce equivalent ionic release during mixing. Hydration kinetics by isothermal calorimetry, strength development, and susceptibility to internal sulfate attack (ISA) were evaluated using isothermal calorimetry, compressive strength testing, and length change measurements. Results show that chloride contents up to 1.0% by mass of dry sand accelerated early hydration and increased 7-day compressive strength by up to 15% relative to the reference mixture. Conversely, sulfate contents of 0.24% reduced 28-day compressive strength by up to 18% and promoted long-term expansion associated with ISA. Mixtures containing combined moderate sulfate and high chloride levels exhibited enhanced early hydration without significant strength loss at later ages. The novelty of this study lies in linking ion release from natural desert sands to equivalent artificial contamination, enabling a systematic evaluation of internal sulfate–chloride interactions in cement-based materials.

Posterosion Performance of Seawater–Sea Sand Concrete: Synergistic Reinforcement by Ultrafine Metakaolin and Nano-TiO2 under Tidal Action

The direct use of untreated seawater and sea sand for concrete production may adversely affect the mechanical properties and durability of concrete, thereby limiting their widespread application in civil construction. To address this issue, this study has investigated the performance evolution of seawater–sea sand concrete (SWSSC) under long-term exposure to a natural marine tidal environment. The effects of adding ultrafine metakaolin (UMK) and nano - TiO 2 (NT) were also explored in their enhancement on the SWSSC performance. Seven mix designs, including control, binary (UMK), and ternary (UMK and NT) mixtures, were exposed to real-world tidal wetting–drying cycles for up to 360 days. Mechanical properties, including cubic and axial compressive strengths, elastic modulus, and flexural strength, were periodically evaluated. The results showed continuous strength development in all mixtures, with the ternary group exhibiting the most balanced and cost-effective improvement, where SWSSC-UMK-NT10 reached a 28-day compressive strength of 65.4 MPa. The synergistic action of the UMK and the NT also markedly enhanced the matrix compactness and cracking resistance. Scanning electron microscopy (SEM) revealed the refined pore structures and denser interfacial transition zones in the modified SWSSC. X-ray diffraction (XRD) confirmed a reduction in the harmful crystalline phases and an increase in the formation of dense hydration products. Furthermore, mercury intrusion porosimetry (MIP) demonstrated the reduced total porosity, along with a shift in the pore size distribution toward the finer gel pores, thereby restricting the fluid transport pathways. In conclusion, the combined addition of UMK and NT is an effective strategy to improve the microstructure and durability of SWSSC in marine environments.

Influence of Aggregate Size Distribution on the Mechanical and Hydraulic Performance of Pervious Concrete

Pervious concrete is an environmentally sustainable construction material with significant potential for mitigating stormwater runoff, enhancing groundwater recharge, and reducing urban heat island effects. This study investigates the influence of aggregate size and blend on the fresh and hardened properties, infiltration rate, and compressive strength of pervious concrete providing guidance for optimal mix design balancing strength and permeability. Six mixtures were produced using single aggregate sizes (9.5 mm, 13.2 mm, and 19 mm) and 50:50 blended combinations (9.5 + 13.2 mm, 9.5 + 19 mm, and 13.2 + 19 mm). Key properties evaluated include fresh and hardened density, void content, infiltration rate, and compressive strength at 7 and 28 days. The effect of 5% colour pigment on the compressive strength of the 9.5 mm aggregate mix was examined. Empirical relationships were developed between density and void content, hardened density and compressive strength, and hardened void content and infiltration rate at 7 days, supported by graphical analysis. Results indicate that infiltration capacity and porosity are strongly influenced using single versus blended aggregate sizes. Density was found to be an effective predictor of compressive strength and void content, while void content reliably predicted infiltration rate. Hardened density ranged from 1845.14 to 2071.99 kg/m³, with void contents between 20.14% and 33.03%. Infiltration rates varied from 0.45 to 1.00 cm/s. The highest compressive strengths were 8.52 and 11.1 MPa for 7 and 28 days respectively using the 9.5 + 13.2 mm aggregate blend. Colour pigment reduced compressive strength by 18% at both curing ages.

Phosphate-Induced Surface Roughening Suppresses Bacterial Transport in Coral Sand.

Phosphate input is widely used to improve nutrient availability in coral sand soils on artificial islands, yet its effects on microbial transport remain unclear. This study investigated how phosphate accumulation affects Escherichia coli (E. coli) transport in coral sand. Phosphate treatment promoted Ca-P formation and transformed the coral sand surface into a P-enriched, roughened interface while largely preserving the bulk pore structure. In column experiments, bacterial breakthrough decreased from 77.8 ± 0.8% in untreated coral sand (CS) to 17.4 ± 0.1% in highly phosphate-treated coral sand (HCS), while the deposition rate coefficient increased from 0.722 to 5.67 h-1. Enhanced retention persisted under varying ionic strength, grain size, and flow velocity. Batch adsorption and density functional theory calculations indicated that enhanced direct chemical affinity alone was unlikely to fully explain the observed immobilization. SEM, AFM, micro-CT, and roughness-corrected XDLVO analyses supported that phosphate-induced roughening created microscale trapping sites, lowered bacteria-collector energy barriers, and enhanced physical retention. Laterite colloids reduced bacterial mobility. These findings suggest that phosphate-induced surface reconstruction can limit downward bacterial penetration and potentially reduce microbial contamination risks to island groundwater. This mechanism supports predicting leaching risks under phosphate-loading scenarios and designing nutrient-input strategies that balance fertility improvement with groundwater protection.

MICP and fiber-reinforced soil improvement: mechanisms, engineering performance, and machine learning prediction—a review

Enhanced erosion resistance of seawater-based bio-cemented sand via soybean hull-derived EICP

Coastal erosion severely threatens the stability of sandy shorelines, necessitating low-cost and sustainable bio-mediated treatment methods for shoreline protection. This study investigates an enzyme-induced carbonate precipitation (EICP) approach utilizing crude urease extracted from soybean hulls and concentrated seawater as an ionic source to enhance the erosion resistance of sand. The joint effects of urease activity, solution volume, treatment cycles, seawater concentration, and curing time were systematically evaluated through penetration strength, wind erosion, and hydraulic erosion tests, complemented by carbonate content measurements. Testing results indicate that the urease activity of the soybean hull extract scaled with the dosage, reaching a maximum of 2.27 mmol L -1 ·min -1 . The seawater-based EICP treatment successfully formed a hardened surface crust, increasing the surface penetration strength to 0.841 MPa, while the critical wind velocity and critical flow velocity increased by up to 7.2 and 6.4 times, respectively, compared with untreated sand. Notably, multi-cycle spraying proved superior to a single application with the same total solution volume. While urease activity was the dominant factor governing carbonate production under relatively low-activity conditions, the number of treatment cycles exerted the strongest influence on hydraulic erosion resistance. Furthermore, power-law relationships were established between erosion-resistance indices and both carbonate content and penetration strength. Carbonate content showed consistently stronger correlations with erosion resistance than penetration strength, indicating that it provides a practical and reliable indicator for evaluating the erosion resistance of seawater-based EICP-treated sand. These findings demonstrate the feasibility of coupling agricultural by-product-derived urease with seawater-based EICP for cost-effective and sustainable stabilization of coastal sandy soils.