Carbonate Precipitation Research Articles

Were early Archean carbonate factories major carbon sinks on the juvenile Earth?

Abstract. Paleoarchean carbonates in the Pilbara Craton (Western Australia) are important archives for life and environment on early Earth. Amongst others, carbonates occur in interstitial spaces of ca. 3.5–3.4 Ga pillow basalts (North Star, Mount Ada, Apex, and Euro Basalt, Dresser Formation) and are associated with bedded deposits (Dresser and Strelley Pool Formation, Euro Basalt). This study aims to understand the formation and geobiological significance of those early Archean carbonates by investigating their temporal–spatial distribution, petrography, mineralogy, and geochemistry (e.g., trace elemental compositions, δ13C, δ18O). Three carbonate factories are recognized: (i) an oceanic crust factory, (ii) an organo-carbonate factory, and (iii) a microbial carbonate factory. The oceanic crust factory is characterized by carbonates formed in void spaces of basalt pillows (referred to as “interstitial carbonates” in this work). These carbonates precipitated inorganically on and within the basaltic oceanic crust from CO2-enriched seawater and seawater-derived alkaline hydrothermal fluids. The organo-carbonate factory is characterized by carbonate precipitates that are spatially associated with organic matter. The close association with organic matter suggests that the carbonates formed via organo-mineralization – that is, linked to organic macromolecules (either biotic or abiotic), which provided nucleation sites for carbonate crystal growth. Organo-carbonate associations occur in a wide variety of hydrothermally influenced settings, ranging from shallow marine environments to terrestrial hydrothermal ponds. The microbial carbonate factory includes carbonate precipitates formed through mineralization of extracellular polymeric substances (EPSs) associated with microbial mats and biofilms. It is commonly linked to shallow subaquatic environments, where (anoxygenic) photoautotrophs might have been involved in carbonate formation. In the case of all three carbonate factories, hydrothermal fluids seem to play a key role in the formation and preservation of mineral precipitates. For instance, alkaline Earth metals and organic materials delivered by fluids may promote carbonate precipitation, whilst soluble silica in the fluids drives early chert formation, delicately preserving authigenic carbonate precipitates and associated features. Regardless of the formation pathway, Paleoarchean carbonates might have been major carbon sinks on the early Earth, as additionally suggested by carbon isotope mass balances indicating a carbon flux of 0.76–6.5 × 1012 mol yr−1. Accordingly, these carbonates may have played an important role in modulating the carbon cycle and, hence, climate variability on the early Earth.

Bioaugmentation of Industrial Wastewater and Formation of Bacterial–CaCO3 Coupled System for Self-Healing Cement

This study investigates the potential of bioaugmentation with Bacillus species to enhance wastewater treatment and develop a bacterial–CaCO3 system for self-healing cement applications. Utilizing microbiologically induced calcium carbonate precipitation (MICP), this study evaluates the dual functionality of Bacillus licheniformis and B. muralis strains. For wastewater treatment, the bioaugmentation process achieved significant pollutant reductions, including a 99.52% decrease in biochemical oxygen demand (BOD5), a 92.13% reduction in chemical oxygen demand (COD), and a substantial removal of heavy metals and nutrients. This process also produced high concentrations of CaCO3 precipitate enriched with viable bacterial cells, demonstrating an eco-friendly approach to improving water quality. For self-healing cement applications, bioaugmented CaCO3 crystals were coated with nutrient and sodium silicate layers to form a bacterial–CaCO3 coupled system. This system demonstrated a 92% recovery in compressive strength after 180 days, highlighting its ability to autonomously repair microcracks in cement-based materials. The layered encapsulation strategy ensured bacterial viability and a controlled activation mechanism, offering a scalable and sustainable solution for infrastructure resilience. This dual-function approach addresses critical environmental and construction challenges by linking efficient wastewater treatment with innovative self-healing material development, contributing to global sustainability and circular economy goals.

Fatigue of SFRC in compression: Size effect & autogenous self-healing

This review synthesizes prior research on size effect and autogenous self-healing in steel fiber-reinforced concrete (SFRC) under compressive fatigue. It explores the fatigue behavior of SFRC, focusing on fiber reinforcement’s role in post-cracking toughness, crack propagation, and fatigue endurance. The review demonstrates that larger SFRC specimens have reduced fatigue lives, attributed to increased elastic energy driving microcrack growth, aligning with classical size effect theory. Additionally, it highlights autogenous self-healing, where fatigue-induced microcracks release occluded water, promoting rehydration and calcium carbonate precipitation, which enhances residual strength. The interaction between size effect, fiber content, and self-healing is examined, offering insights into improving SFRC’s durability under cyclic loading. These findings have practical implications for designing SFRC structures subjected to compressive fatigue, such as wind turbine towers and railway slabs.

Statistical damage model with strain softening for lime-stabilized rammed earth after elevated temperature

Many historical earthen buildings are damaged due to fire exposure in the past. It is important to understand the strength degradation of rammed earth after elevated temperature for guiding the strategy of building protection or rehabilitation. A total of 24 unconfined compression tests are conducted on lime-stabilized rammed earth specimens after elevated temperature up to 700°C. A quasi-linear reduction in strength and stiffness is found for rammed earth with the increase of temperature. At high temperature, the ductility of rammed earth is enhanced, e.g., strain at peak strength of 2.5% and 1.5% at 700°C and 20°C, respectively. Microstructural analyses demonstrate that with the increase of temperature, the specimen becomes more porous with reduced calcium carbonate precipitation, explaining the strength reduction. A new thermal damage model is proposed to describe the behavior of rammed earth after elevated temperature, in which the closure of pores is captured to show unrecoverable deformation, and the skeleton part is simulated using a thermal damage variable in a statistical manner to present the damage evolution (strain softening). By comparing with the measured stress-strain curves, one can confirm that the proposed method can provide effective prediction for the response of rammed earth after elevated temperature.

Experimental formation of carbonates from perchlorate and sulphate brines: Implications for Jezero crater, Mars.

Extensive carbonate precipitation has occurred on Mars. To gain insight into the carbonation mechanisms and formation processes under ancient Martian aqueous conditions, we examine the precipitation of carbonates resulting from atmospheric carbon fixation, focusing on interactions between various brines and silicate and perchlorate solutions in alkaline environments. The micro-scale morphology and composition of the resulting precipitates are analysed using ESEM micrographs, EDX chemical compositional analysis, X-ray diffraction, and micro-Raman spectroscopy. Our findings indicate a significant atmospheric carbonation process involving chlorate and sulphate brines reacting with alkaline perchlorate solutions, leading to the precipitation of calcium carbonate polymorphs, including vaterite, aragonite, and calcite, as well as other carbonates like siderite (iron carbonate) and zaratite (nickel carbonate). Some precipitates exhibit biomorphic structures (such as globular spherical aggregates, fine branched tubes, and flower-like morphologies) that should not be mistaken for fossils. These experiments demonstrate that various precipitates can form simultaneously in a single reaction vessel while being exposed to different micro-scale pH conditions. We propose that systematic laboratory studies of such precipitate reactions should be conducted in preparation for the analysis of the Mars Sample Return collection on Earth, aiding in the interpretation of carbonate presence in natural brine-rock carbonation processes under Martian conditions while also helping to distinguish potential biosignatures from purely geochemical processes.

3D Printing and Biocementation of Hierarchical Porous Ceramics

AbstractCeramics with controlled porosity are used as bio‐scaffolds, insulators, electrodes and lightweight materials. While their high surface area and low weight are attractive functionalities, such porous ceramics often suffer from poor mechanical properties and need energy‐intensive, high‐temperature sintering for manufacturing. The present work reports a low‐temperature approach for the manufacturing of mechanically efficient porous ceramics. The process relies on the 3D printing of inks loaded with ceramic hollow spheres, which are biocemented by the precipitation of calcium carbonate induced by ureolytic bacteria. Electron microscopy, thermogravimetric analysis and mechanical tests are performed to study the kinetics of the biocementation process and its effect on the calcification and mechanical properties of extruded and printed samples. Hierarchical porous ceramics with a grid‐like architecture and filament sizes in the order of one millimeter are effectively biocemented at ambient temperature after 2 days of calcification. The calcified structures display higher mechanical efficiency than previously reported monoliths of comparable porosity, thus demonstrating the potential of 3D printing and bacteria‐driven biocementation for the low‐temperature fabrication of hierarchical porous ceramics.

Microbial Use as an Agent to Improve the Durability of Sub-Structure Concrete: A Comprehensive Structured Review

The increasing degradation of sub-structure concrete due to environmental factors and aging has intensified the need for innovative and sustainable solutions to enhance its durability. To increase the longevity of sub-structure concrete, microbial use is becoming more and more important. One key technique in this exploration of the literature is Microbial-Induced Calcium Carbonate Precipitation (MICP). We reviewed over 50 peer-reviewed journal articles and conference papers published between 2020 and 2024, selecting those that specifically address microbial applications in concrete. To achieve this, we conducted an extensive search of scholarly articles from reportable databases, such as Scopus, IEE, and Science Direct, focusing on studies from 2020 to 2024. The flow of studies is based on the PRISMA framework. Our methodology involved analyzing the effectiveness of microbial treatments in extending the service life of concrete through qualitative and quantitative data synthesis. The studies found that n=29 final primary data was analyzed. The finding was divided into three themes, which are (1) Microbial Applications in Soil and Foundation Crack Stabilization, (2) Microbial Applications in Concrete and Construction Materials and (3) Advanced Microbial Technologies and their Impacts. Additionally, this study demonstrates the environmental sustainability of microbial applications compared to traditional chemical-based methods. The review concludes that while using microbes in concrete shows significant potential for enhancing durability and sustainability, further research is required to optimize these biological processes for mainstream construction practices and assess long-term of their impacts. This study lays the groundwork for future investigations into scalable microbial solutions, aiming to revolutionize the construction industry’s approach to prolonging the lifespan of concrete infrastructures.

Carbonate precipitation-derived CO2 outgassing offsets the mineral weathering sink in the orogenic regime of southwestern Taiwan: Insights from triple Sr isotopes

Secondary carbonate precipitation plays a crucial role in determining the CO2 outgassing from carbonate-saturated rivers and influences the short-term terrestrial carbon cycle. The fractionation of stable isotopes has been extensively employed to quantify post-weathering reactions in rivers and assess the quantities of metals removed by secondary carbonates. However, a major challenge is that water isotopic compositions usually reflect both lithological mixing and biogeochemical fractionation, which complicates distinguishing between these signals. In this study, we applied triple Sr isotopes (87Sr/86Sr and δ88/86Sr) to determine the “unfractionated” δ88/86Sr of water. This method allowed us to precisely quantify the precipitation-derived fractionation. Our results show that a median of 48 % of Sr was incorporated into carbonates in southwestern (SW) Taiwan, corresponding to the removal of 69 % of the originally weathered Ca. This process is closely related to a significant increase in HCO3− concentration in the water. In SW Taiwan, high river discharge during wet periods typically enhances carbonate dissolution, leading to increased carbonate precipitation along with CO2 outgassing. Thus, hydrology and river discharge constraints significantly influence CaCO3 precipitation and CO2 outgassing processes and their associated CO2 budget. Using a machine learning model, we estimated that the precipitation-derived CO2 outgassing from rivers is +7.9 × 108 mol per year. This flux is nearly twice the long-term (~10 kyr) emission flux of +4.2 ± 1.4 × 108 mol per year in this region. This indicates that precipitation-driven CO2 emissions from rivers constitute a significant carbon source within the terrestrial carbon cycle of orogenic regions.

Environmental Changes Recorded in Sedimentary Rocks in the Clay-Sulfate Transition Region in Gale Crater, Mars: Results From the Sample Analysis at Mars-Evolved Gas Analysis Instrument Onboard the Mars Science Laboratory Curiosity Rover.

The Curiosity rover explored the region between the orbitally defined phyllosilicate-bearing Glen Torridon trough and the overlying layered sulfate-bearing unit, called the "clay-sulfate transition region." Samples were drilled from the top of the fluviolacustrine Glasgow member of the Carolyn Shoemaker formation (CSf) to the eolian Contigo member of the Mirador formation (MIf) to assess in situ mineralogical changes with stratigraphic position. The Sample Analysis at Mars-Evolved Gas Analysis (SAM-EGA) instrument analyzed drilled samples within this region to constrain their volatile chemistry and mineralogy. Evolved H2O consistent with nontronite was present in samples drilled in the Glasgow and Mercou members of the CSf but was generally absent in stratigraphically higher samples. SO2 peaks consistent with Fe sulfate were detected in all samples, and SO2 evolutions consistent with Mg sulfate were observed in most samples. CO2 and CO evolutions were variable between samples and suggest contributions from adsorbed CO2, carbonates, simple organic salts, and instrument background. The lack of NO and O2 in the data suggest that oxychlorines and nitrates were absent or sparse, and evolved HCl was consistent with the presence of chlorides in all samples. The combined rover data sets suggest that sediments in the upper CSf and MIf may represent similar source material and were deposited in lacustrine and eolian environments, respectively. Rocks were subsequently altered in briny solutions with variable chemical compositions that resulted in the precipitation of sulfates, carbonates, and chlorides. The results suggest that the clay-sulfate transition records progressivelydrier surface depositional environments and saline diagenetic fluid, potentially impacting habitability.

Improving non-uniform gravelly sand using microbially induced carbonate precipitation: An outdoor cubic-meter scale trial by engineering contractors

Soil improvement using microbially induced carbonate precipitation (MICP) remains largely confined to the laboratory, with only a very small number of large-scale experiments having been completed under field conditions and none by engineering contractors. This study presents a cubic-meter scale improvement of heterogeneous natural sand collected from a local quarry, with a wide variation in grain size, via MICP. The MICP trial was conducted by engineering contractors in a cubic test cell under variable temperatures ranging from 5 °C to 19 °C. The upscaling of cultivation of Sporosarcina pasteurii (600 L for each treatment cycle) under non-sterile conditions, as performed by engineering contractors, achieved an optical density (OD600) of 0.89 and a specific urease activity of 2.5 mM urea/min/OD600. Post-MICP-treated sands were subjected to a series of coring, block sampling and laboratory tests. The block sampling process indicated that the majority of sand was effectively cemented, with a small region near a side wall forming a less well-cemented zone, likely induced by less effective fluid delivery in this region. The unconfined compressive strengths of three cores (diameter: 10 cm, length: 22 cm) were 3.6, 4.4, and 7.6 MPa. Consolidated-drained triaxial tests on sub-sampled cores also demonstrated rock-like material behaviour, with a peak friction angle of 43.6° and peak cohesion of 0.64 MPa, and ultimate state frictional angles of 42.0° and ultimate cohesion of 0.12 MPa. The increased shear parameters of the bio-cemented samples (relative to the untreated samples) have many implications for mitigation of geotechnical hazards such as soil liquefaction. The study marks a step forward industrial implementation of MICP for soil improvement by engineering contractors without prior knowledge of MICP.

Microbially Induced Carbonate Precipitation for Soil Improvement: Insights from a Meter-Scale Radial Grouting Trial

Microbially Induced Carbonate Precipitation for Soil Improvement: Insights from a Meter-Scale Radial Grouting Trial

A Comprehensive Review of Biomaterials Synthesized Using the MICP Process for Sustainable Construction

Building materials and infrastructure contribute to approximately 13% of global CO₂ emissions annually, according to the International Energy Agency (IEA, 2022). This underscores the urgent need to transition to more sustainable construction materials. Emerging biomaterials, developed through innovative processes such as the Microbially Induced Calcium Carbonate Precipitation (MICP) process, are being explored as potential alternatives to conventional materials. These biomaterials, including bio-concrete, bio- cement, and bio-bricks, are produced using waste materials and biological processes, such as bacteria and plant-based resources that act as carbon sinks, offering an eco-friendly solution to construction challenges. Many researchers and companies are actively experimenting with these materials to solve pressing environmental problems, with promising results. However, challenges remain in optimizing these materials for large-scale production and ensuring their performance under real-world conditions. Despite these obstacles, ongoing research is continually pushing the boundaries of biomaterials' potential in construction, with numerous studies focused on improving their properties and addressing current limitations. This paper provides a comprehensive review of the advantages and disadvantages of biomaterials in comparison to traditional construction materials. It explores how these bio- based materials—synthesized through the MICP process—can offer significant benefits, such as self-healing properties, low-cost production, and reduced environmental impact. The review also discusses the challenges that still need to be overcome and the ongoing research aimed at making biomaterials a viable alternative to conventional materials. As part of the field of engineering, this paper highlights the critical role of biotechnology in advancing sustainable construction practices and the continued evolution of biomaterials in engineering applications.

Controls on microbially-induced carbonate precipitation in geologic porous media

Microbially-induced carbonate precipitation (MICP) provides a natural biomineralization approach to secure the geologic storage of gases (e.g., carbon dioxide, hydrogen and methane). Cracks in embrittled wellbore cement, for example, provide a pathway for atmospheric gas leakage, while permeability heterogeneities in the storage reservoir leads to fingering effects that diminish the storage capacity. The design of MICP processes, however, remains a challenge due to limited understanding of the coupled nonlinear reaction kinetics and multiphase transport involved. Specifically, previous attempts at MICP through porous media have been encumbered by carbonate precipitation localized to the first ∼ cm of the bulk injection surface. In this study, we investigate the reactive transport controls on MICP necessary to enable deep MICP penetration into the formation. We use a micromodel with pore geometry and geochemistry representative of real geologic media to image direct pore- and pore-ensemble-level mineral, fluid, and microbial distributions. An approach to adsorb microbes uniformly across the micromodel, rather than local accumulation near the inlet, is developed that enables deep MICP penetration into the porous medium. A sensitivity analysis was performed to investigate the impact of injection conditions (e.g., rates, concentrations) required to maximize CaCO3 precipitation away from the injection site. With multiple cycles of MICP, a ∼ 78 % reduction in permeability was achieved with ∼8 % carbonate pore volume occupation. Overall, this study establishes the possibility of MICP as an effective and controllable method to enhance the security of gas storage in geologic media.

Exploration of urease-aided calcium carbonate mineralization by enzyme analyses of Neobacillus mesonae strain NS-6.

Urease containing nickel cofactor is crucial for urea-hydrolytic induced calcium carbonate (CaCO3) precipitation (UICP). However, limited information exists regarding the influence of amino acid residues interacting with nickel ions in its structure on induced CaCO3 mineralization. Herein, RT-qPCR was used to demonstrate that the addition of NiCl2 dramatically upregulated the expression of urease structural gene ureC that was correlated with nickel binding in Neobacillus mesonae strain NS-6. Homology modeling and molecular docking were employed to construct the three-dimensional structure of urease and seek the key residues involved in nickel binding process, and virtual mutation technology was adopted to inform three key residues coordinated with nickel ions and urea, His249, His275, and Asp363. Four metrics, including root mean square deviation values for mutations of those key residues in urease-urea complexes severally and wild-type, were calculated by molecular dynamics simulations when they were mutated into alanine, respectively. Subsequently, the mutations of H249A, H275A, and D363A were characterized using western blotting to reveal a decrease in the relative expression and activity of urease, along with a corresponding reduction in CaCO3 precipitation. Ultimately, the mutations also exhibited that they had lower substrate affinity and catalytic efficiency for urea through enzymatic properties analysis. The findings suggested that those residues played a pivotal role in UICP of strain NS-6, which would expand the theoretical basis for modulating urease activity.IMPORTANCEUrease-producing bacterium is of great importance in diverse application fields, such as environmental remediation, due to its key driving characteristics in catalyzing urea hydrolysis via urea-hydrolytic induced CaCO3 precipitation (UICP). As essential cofactors of urease, nickel ions play a crucial role in regulating urease catalysis and maintaining structural stability. Numerous investigations have emphasized the impact of nickel ions on urease activity in recent years, to our best knowledge, only a few literatures have studied the molecular-level regulation of nickel-ligand residues. This study focused on the highly urease-producing bacterial Neobacillus mesonae NS-6 to explore the effects of specific nickel-ligand residues on the urease-aided CaCO3 mineralization process using molecular simulation predictions and targeted mutation experiments. The aim was to provide a molecular-level understanding of the interactive effects between urea and critical residues associated with the urease active center, as well as propose an effective modification strategy to enhance the application of UICP in future environmental areas.

Mesoarchean Microbial Cd, Ba, and Ni Cycling: Evidence for Photosynthesis in Pongola Group Stromatolites through Novel Stable Isotopes and High-Resolution Trace Element Maps.

Nontraditional stable isotopes of bioactive metals emerged as novel proxies for reconstructing the biogeochemical cycling of metals, which serve as cofactors in major metabolic pathways. The fractionation of metal isotopes between ambient fluid and microorganisms is ultimately recorded in authigenic minerals, such as carbonates, which makes them potentially more reliable than standard biomarkers in organic matter. Stromatolitic carbonates are geochemical archives that allow for the study of the long-term interplay of the biosphere, atmosphere, and hydrosphere through deep time, with the unique potential to investigate early life environments and the evolution of the metallome. The present study uses stromatolites from the ∼2.95-billion-year-old Pongola Supergroup (S. Africa) as a field laboratory for combined in situ trace metal mapping and layer-specific, novel stable metal isotope compositions to infer early Earth microbial metal cycling via phototrophic and chemo-litho-autotrophic metabolisms. Quantitative in situ trace element maps reveal intrinsic biosedimentary enrichments of nickel (Ni), cadmium (Cd), phosphorus (P), iron (Fe), and manganese (Mn) in stromatolitic laminae. In contrast, barium (Ba) shows a more homogeneous distribution. Authigenic carbonates from pristine stromatolite laminae show distinct δ138Ba and δ112Cd fractionation above detrital background and bulk silicate Earth values, but opposing correlation with trace metal concentrations. Authigenic δ60Ni values overlap with Mesoarchean diamictite compositions. Nickel isotopic compositions in authigenic stromatolitic carbonates, potentially a new proxy for methanogenic metal uptake, do not show any proof of the presence of this metabolism in the samples of this study. Meanwhile, Cd isotopic compositions in authigenic carbonates follow typical Rayleigh-type isotope fractionation; that is, the isotopic composition of Cd evolves to heavy values close to modern surface compositions. Correlations of δ112Cd with the micronutrients copper (Cu), molybdenum (Mo), and P, at positively fractionated carbon (C) isotopes (δ13C ∼+2‰), argue for active photosynthesis in the Pongola microbial habitat. We show that Ba isotopes can be used to infer carbonate precipitation rates similar to modern microbial carbonates. Thus, the combination of Cd and Ni isotopes has the unique potential as novel isotope biomarkers for the biochemical sedimentary record of early Earth where traditional lipid biomarkers are not applicable due to the incomplete preservation of organic matter. Key Words: Early life-Stromatolites-Novel stable isotopes-Cadmium isotopes-Nickel isotopes-Barium isotopes-Trace element mapping.

Effect of carbonation on the properties of self-healing concrete with expanded perlite as a microbial carrier

Expanded perlite (EP) is a microbial carrier used in microbial self-healing concrete (MSHC). However, the type of EP microbial self-healing agent (EPMSA) affects MSHC performance, especially with coupled carbonation and microbial-induced calcite precipitation (MICP). In this study, two EPMSAs were prepared (one coated and one uncoated by magnesium potassium phosphate cement (MKPC)) and two MSHC specimens were developed. The effects of carbonation on the carbonation depth, compressive strength, chloride penetration, and crack self-healing properties of MSHC were investigated. The results showed that the MSHC prepared with uncoated EPMSA had merely 1–2.2 mm carbonation depth and 63.8 MPa compressive strength after 28 days of carbonation. The uncoated EPMSA in MSHC induced more calcium carbonate than the coated EPMSA, thus improving the pore structure of MSHC, which is why MSHC exhibits high carbonation resistance and compressive strength. Although carbonation reduced the crack self-healing performance, the maximum healed crack width of MSHC incorporated with uncoated EPMSA could still reach 0.81 mm. Due to the positive effect of EP, MHSC has excellent resistance to chloride penetration. The dual action of EP and MICP makes uncoated EPMSA-incorporated MHSC exhibit excellent durability attributes. Using uncoated EPMSA is a more prudent approach when the strength and durability of MSHC are of critical concern.

Modeling the uncoupled damage-healing behavior of self-healing cementitious material with phase-field method

Microbially induced calcium carbonate precipitation (MICCP), which is a biochemical process that utilizes microbial metabolic activities to precipitate calcium carbonate (CaCO3), can be used for damage-healing by crack filling. In this study, we develop a computational uncoupled damage-healing framework to predict the fracture-healing responses of a multifunctional vascular cementitious composite. The framework integrates a numerical model for self-healing cementitious materials into a damage model developed using the phase-field method. The numerical self-healing model is utilized to compute the MICCP healing based on the chemical and enzyme kinetics of microbial activities, whereas the phase-field method is employed to capture the fracture response of the material. This framework comprises three stages: Stage 1 — damage, Stage 2 — healing, and Stage 3 — re-damage of the healed structure. Using the load–displacement curve of the healed cementitious material, the proposed uncoupled damage-healing models can predict the recovery in material stiffness based on the MICCP healing ratio. The proposed framework can be applied to a wide range of fracture-healing problems in self-healing cementitious materials utilizing MICCP as the healing methodology.

Effect of Exposure Environment and Calcium Source on the Biologically Induced Self-Healing Phenomenon in a Cement-Based Material

Microbially induced calcium carbonate precipitation (MICP) presents a sustainable, environmentally friendly solution for repairing cracks in cement-based materials, such as mortar and concrete. This self-healing approach mechanism enables the matrix to autonomously close its own cracks over time. In this study, specimens (50 mm in diameter and 25 mm in height) were exposed to submersion and a wet–dry cycle environment. The solution considered a nutrient-rich suspension with calcium lactate, urea, calcium nitrate, and Bacillus subtilis or Sporosarcina pasteurii in a biomineralization approach. The self-healing efficiency was assessed through optical microscopy combined with image processing, focusing on the analysis of the superficial crack closure area. S. and B. subtilis exhibited notable capabilities in effectively healing cracks, respectively, 8 mm2 and 5 mm2 at 35 days. Healing was particularly effective in samples placed in a submerged environment, especially with a 69 mM concentration of calcium lactate in bacterial suspensions containing B. subtilis, where 87.5% of a 4 mm2 crack was closed within 21 days. In contrast, free calcium ions in the solution, resulting from anhydrous cement hydration, proved ineffective for S. pasteurii biomineralization in urea-rich environments. However, the addition of an external calcium source (calcium nitrate) significantly enhanced crack closure, emphasizing the critical role of calcium availability in optimizing MICP for bio-agents in cement-based materials. These findings highlight the potential of MICP to advance sustainable self-healing concrete technologies.

A new framework for making carbon compensation standards considering regional differences at different scales in China

Carbon compensation represents an effective strategy to promote carbon neutrality, address inter-regional inequalities, and support sustainable development. However, existing research on carbon compensation standards often neglects disparities in natural resource endowments and socioeconomic conditions among regions. This study proposes a new framework for making carbon compensation standards that account for regional differences. We explore key factors influencing the Carbon Deficit Index (CDI) and develop carbon compensation standards considering the regional differences of these factors. Carbon compensation amounts are identified and assigned at the three administrative scale: province, city, and county scale in China. The study reveals that between 2000 and 2015, the number of carbon deficit provinces increased from 9 to 21, with a spatial expansion outward from North China as the epicenter. The key factors impacting the CDI include urbanization rate, population, forest area, average annual precipitation, silt content, and soil organic carbon content. By considering regional differences in these key factors, the proposed carbon compensation standards can effectively reduce inter-regional inequalities. These standards can be applied consistently across the three administrative scales. Our findings suggest that carbon compensation standards that take into account regional differences in natural resource endowments，socioeconomic conditions, and payment capacity can enhance the fairness of carbon compensation actions at different scales, providing a valuable reference for the future development of cross-regional carbon compensation policies.

Reactivity and potential mobility of metals in human-impacted harbor sediments (port of Rotterdam, the Netherlands)

Abstract Purpose Vast amounts of harbor sediments are being dredged every year globally. These sediments are often enriched in potentially toxic elements (PTEs), the abundance and potential mobility of which are important for sustainable harbor management practices. In this study, we investigate metal (especially PTE) distribution, abundance, and reactivity in sediments along a salinity gradient in the waterways of Europe’s largest harbor, the Port of Rotterdam. Materials and methods Bulk surface sediments were analyzed for general physicochemical properties (e.g. grain size, total metal concentration). From selected locations covering the local salinity gradient, sediments were subjected to three independent chemical extractions to determine chemically reactive metal pools. Additionally, metal retention in two contrasting sediments (high versus low salinity) was further explored using pH-dependent leaching experiments in combination with a geochemical model. Results and discussion The majority of the investigated sediments consisted predominantly of silt and were rich in organic matter. Concentrations of Cd, Pb, Zn, Cu decreased with increasing salinity. Concentrations of Al, Fe, V correlated negatively with grain size, because these geogenic metals are enriched in fine-grained silicates. Results from the chemical metal extractions showed clear differences in the reactivity and mobility potential of metals, that could be grouped into four clusters. The combined chemical and modeling results indicate that sorption onto metal (oxyhydr)oxides and organic matter as well as precipitation of metal sulfides and carbonates control metal retention. High reactivity and mobility potential were observed for Pb, Cd, Zn, particularly at low pH. Limited spatial variability in metal chemistry along the salinity gradient indicates that the highly variable depositional conditions have little impact on metal behaviors. Conclusions Chemical extractions and pH-dependent leaching experiments revealed distinguishing metal reactivities from four clusters. Our results provide insight into metal distribution in the dynamic estuarine environment of the Port of Rotterdam and highlight the importance of understanding chemical speciation in addition to abundance for harbor sediment management.