Marine Concrete Research Articles

Effects of interlayer-modified layered double hydroxides with organic corrosion inhibiting ions on the properties of cement-based materials and reinforcement corrosion in chloride environment

Developing novel, environmentally friendly, and efficient corrosion inhibitors is of great significance for improving the durability of marine concrete against chloride ion erosion. This paper aims to explore the potential of layered double hydroxides (LDHs) as nanocontainers, incorporating organic corrosion inhibitors between LDHs layers to synergistically enhance their effectiveness. In this study, Mg/Al-pAB-LDH was synthesized through the interlayer modification of LDHs with an organic corrosion inhibitor, p-aminobenzoic acid (pAB), employing a calcination-rehydration method. Chloride ion (Cl−) adsorption behavior was quantitatively and qualitatively analyzed and the effect on mortar properties was investigated. The corrosion resistance of steel bars in the mortar was assessed under chloride salt simulated concrete pore solution (SCPs) and chloride salt wet-dry cycles via electrochemical tests and microscopic characterization of Mg/Al-pAB-LDH. The results demonstrate that Mg/Al-pAB-LDH efficiently captures Cl− and releases corrosion-inhibiting ions pAB, with the adsorption process conforming to pseudo-second-order kinetics and Langmuir adsorption isotherm. Mg/Al-pAB-LDH enhances the pore structure of mortar, effectively improving mechanical properties and resistance to chloride ion penetration, with the optimal effect observed at a 4 % addition rate. Mg/Al-pAB-LDH demonstrates outstanding corrosion resistance to steel bars in both SCPs and mortar. In SCPs, it serves as a corrosion inhibitor by adsorbing Cl− and releasing pAB, whereas in mortar, it functions as a corrosion inhibitor by enhancing the physical barrier effect of mortar, adsorbing Cl−, and releasing pAB. This study demonstrates the promising potential of utilizing Mg/Al-pAB-LDH as a novel corrosion inhibitor to mitigate the corrosion of steel bars in marine concrete.

Mechanical properties and micro-mechanism of seawater cementitious materials reinforced by in-situ polymerization

Understanding the potential mechanism of in-situ polymerization of acrylamide (AM) for modifying seawater cementitious materials is crucial for designing high-strength and durable marine concrete. Herein, the acrylamide (AM) in-situ polymerization was investigated for its effects on the hydration behavior, micro-morphology, and pore structure of cementitious materials mixed with seawater and freshwater through a series of elaborately designed microscopic characterization methods. The results reveal that the hydration process of cementitious materials proceeds simultaneously with in-situ polymerization. However, compared with freshwater mixtures, seawater provides a large number of metal ions and SO42- ions, which can cross-link with the generated polyacrylamide (PAM) during in-situ polymerization to form a three-dimensional network structure. The synergistic effect of the hydration, in-situ polymerization, and cross-linking processes of cementitious materials can improve the pore structure of seawater-mixed paste, enhance erosion resistance, and improve the stability and toughness of microstructure. These findings were further confirmed by comparing infrared spectroscopy results, hydration products, pore size, and micro-morphology analysis as well as flexural performance tests. This is of great significance to guide the design of novel materials in marine infrastructure.

Effect of compound mineral capsules on the self-healing performance of cementitious materials under marine environment

Marine concrete has been subjected to the dual threats from cracking and the corrosion of ion. To achieve the self-healing of cracks in concrete and the binding of Cl− and SO42− in marine environment, a novel compound mineral capsule with MgO expansive agent, quicklime and metakaolin as healing agents was developed in this study. To evaluate the ion binding capacity of capsules, after the broken capsules were immersed in synthetic seawater for different times, the concentrations of Cl− and SO42− in seawater were measured, and the binding products on the capsules were analyzed. Then the capsules were incorporated into mortar in varying amounts, and the self-healing capacity at different healing ages was quantitatively characterized by the tests of crack observation, sorptivity and water permeability. Through microstructure analysis of healing products, the self-healing process and mechanism of the mortar containing capsules under marine environment were revealed. Results showed that the compound mineral capsules effectively bound Cl− and SO42− by forming Friedel's salt and ettringite. The addition of capsules significantly improved the crack closure and conspicuously reduced the transport performance of cracked mortar. With the dosage of capsules increased, the self-healing process was accelerated. The underlying reasons were that the capsules promoted the formation of Friedel's salt, ettringite, hydrotalcite and brucite, in addition to portlandite, calcite and C-S-H. Under the filling effect of these products, cracks achieved self-healing while Cl− and SO42− were simultaneously bound. Consequently, the impermeability of the cracked cementitious materials in marine environments was effectively increased.

Experimental study on impact performance of seawater sea-sand concrete with recycled aggregates

On islands distant from the mainland, obtaining raw materials for concrete production is often more challenging. To achieve sustainable development in island reef engineering, using discarded marine concrete and coral waste generated during island construction as recycled aggregates are of considerable significance. The preparation of Recycled Coral Aggregate Concrete (RCAC) for island reef engineering thus holds substantial importance. In this study, RCAC and Natural Aggregate Concrete (NAC), both designed with a compressive strength of C60, were prepared. Initially, the fundamental physical properties of the recycled coarse aggregate, such as apparent density, water absorption, and crushing index, were determined. Subsequently, a comparative analysis of the quasi-static mechanical properties of RCAC with varying proportions of recycled coral coarse aggregate (RCCA) was conducted. Furthermore, the impact compression mechanical properties of different RCAC specimens under various strain rates were examined using the Ф100mm Split Hopkinson Pressure Bar (SHPB) apparatus. The microstructure and long-term drying shrinkage performance of RCAC were also analyzed using Scanning Electron Microscopy (SEM) and a drying shrinkage apparatus. The finding indicated that the 28-day compressive strength of RCAC specimens with 100% coarse aggregate replacement reached a maximum of 62.4 MPa. The quasi-static compressive strength of RCAC specimens with 50% and 100% RCCA replacement was only 11.5% and 14.2% lower than that of NAC, respectively. Under impact loading, the dynamic compressive strength of RCAC specimens increased with the strain rate, with peak stress exhibiting an approximately linear relationship with the strain rate. The energy dissipation of RCAC specimens generally occurred in three stages, with the reflected and absorbed energies of the specimens increasing linearly with strain rate. At the same strain rate, the transmitted energy of RCAC specimens was higher than that of NAC specimens. Microstructural analysis revealed that the morphology of recycled coral aggregate is characterized by its porous and rough surface. The interfacial transition zone between the recycled coral aggregate and the cement mortar was relatively dense. Incorporating recycled coarse aggregate significantly affected the drying shrinkage properties of the concrete, with higher contents of RCCA leading to greater drying shrinkage rates.

Predictions for Carbonation Depth of Marine Concrete Under Different Temperature and Humidity Environments Based on Levenberg-Marquart Algorithm

Predictions for Carbonation Depth of Marine Concrete Under Different Temperature and Humidity Environments Based on Levenberg-Marquart Algorithm

Effects of Hydrostatic Pressure and Cation Type on the Chloride Ion Transport Rate in Marine Concrete: An Experimental Study.

The effect of hydrostatic pressure and cation type on chloride ion transport in marine underwater concrete cannot be ignored. The study of the chloride ion transport behavior of concrete under the effect of hydrostatic pressure and cation type coupling can provide a basis for durability design and the protection of marine concrete. In this work, the chloride ion transport behavior of marine concrete in four common chloride salt solutions under different hydrostatic pressures is studied by a hydrostatic pressure test device developed by the authors. The results show that hydrostatic pressure and its action time significantly influence the chloride ion transport behavior in marine concrete; the higher the hydrostatic pressure of concrete, the faster the chloride ion transport rate. The longer the time, the more chloride ions accumulated in the same position, and the farther the chloride ion transport distance. Cation type has a certain influence on the transport process of chloride ions. Under the same test conditions, the chloride ion transport rate in a divalent cation solution is about 5% higher than that in a monovalent cation solution. The results also show that the chloride ion binding capacity under hydrostatic pressure is only 10~20% of that under natural diffusion. Using the test results, a predictive model of a chloride ion apparent transport coefficient based on the hydrostatic pressure and hydrostatic pressure action time corrected by a cation type influence coefficient is established.

Bond properties of GFRP bar embedded in marine concrete subjected to sustained loads

In this work, a total of 20 beam type concrete specimens were prepared to investigate bond behavior of glass fiber reinforced polymer (GFRP) bar embedded in marine concrete. The influence of sustained loads (load level: 0.25, 0.45 and 0.65 ultimate load (Pu)) and stirrup on bond properties was investigated. After sustained load period, flexural bond test was conducted to determine bond stress and slip. Test results indicate that bond strength of GFRP bars shows decreased trend with increasing sustained loading. Compared with specimen not subjected to sustained load, bond strength significantly decreased under 0.25, 0.45 and 0.65 Pu. Also, sustained load deteriorates the bond stiffness and bond toughness. What’ s more, differences in failure modes and bond strength of steel bar and GFRP bars in flexural bonding tests were analyzed and compared. All GFRP bar specimens subjected to sustained load exhibit bars pull-out failure. In contrast to steel bar, the influence of stirrups on GFRP bar-marine concrete bond strength is relatively limited (specimens with stirrups exhibited only 1.8 % increase over specimens without stirrups), due to GFRP bars lower rib heights and weaker mechanical interlocking with concrete. Characterization of different stages of bond-slip curves of GFRP bar beam specimen subjected to sustained loads was carried out and bond-slip curves of GFRP shows two peaks. According bond strength and slip test data, bond stress-slip mathematical relationship model between GFRP bar and marine concrete under sustained load has been fitted.

Advances in Modeling Surface Chloride Concentrations in Concrete Serving in the Marine Environment: A Mini Review

Chloride corrosion is a key factor affecting the life of marine concrete, and surface chloride concentration is the main parameter for analyzing its durability. In this paper, we first introduce six erosion mechanism models for surface chloride ion concentration, reveal the convection effect in the diffusion behavior of chloride ions, and then introduce the corrosion mechanisms that occur in different marine exposure environments. On this basis, the analysis is carried out using empirical formulations and machine learning methods, which provides a clearer understanding of the research characteristics and differences between empirical formulas and emerging machine learning techniques. This paper summarizes the time-varying model and multifactor coupling model on the basis of empirical analysis. It is found that the exponential function and the reciprocal function are more consistent with the distribution law of chloride ion concentration, the multifactor model containing the time-varying law is the most effective, and the Chen model is the most reliable. Machine learning, as an emerging method, has been widely used in concrete durability research. It can make up for the shortcomings of the empirical formula method and solve the multifactor coupling problem of surface chloride ion concentration with strong prediction ability. In addition, the difficulty of data acquisition is also a major problem that restricts the development of machine learning and incorporating concrete maintenance conditions into machine learning is a future development direction. Through this study, researchers can systematically understand the characteristics and differences of different research methods and their respective models and choose appropriate techniques to explore the durability of concrete structures. Moreover, intelligent computing will certainly occupy an increasingly important position in marine concrete research.

Evaluation of corrosion potential in near-shore piles and analysis through RSM modelling

Corrosion is a significant problem in marine environments that ultimately leads to structural failure and economic losses. The assessment of the conditions of concrete piles is an essential part of corrosion prevention measures in marine environments. This research investigated the corrosion potential of concrete marine piles using a half-cell potentiometer. Following this, the observed potential values are subjected to RSM modeling. Approximately ten piles were chosen in Chennai Fishing Harbor, and the corrosion potential values along the depth of the piles in three different zones, namely, the atmospheric, splash and submerged zones, were measured at three-month intervals. Based on the visual inspection, the piles were damaged, and minor cracks developed on the surfaces of the concrete due to the dynamic impact of wave action, specifically in the splash zone. From the results obtained in the field for the chosen piles in the fishing harbor, the corrosion potential values are in the range of −361.45 mV to −527.57 mV in the atmospheric zone, − 520.67 mV to −784.85 mV in the splash zone and– 401.23 mV to—631.28 mV in the submerged zone. RSM was noted to be the most accurate predictor with the highest R2 and least errors. The RSM model captured the variability of the data according to the R2 threshold (R2 > 0.9855). The splash zones have higher corrosion potentials and are more prone to corrosion than the other zones.

Steel corrosion process in ultra-high performance concrete monitored by fiber bragg grating sensor

The steel corrosion in Ultra-High Performance Concrete (UHPC) would cause degradation of structural performance or even structural failure. It is an urgent need to monitor and predict the steel corrosion in UHPC, however it is still a tough task. In this paper, Fiber Bragg Grating (FBG) sensors are applied for monitoring the steel corrosion process in UHPC, by combining with steel bar, which is equidistant winding with the sensors, and connected with power supply for accelerating corrosion. The UHPC specimens embedded with steel bars are surrounded by sodium chloride (NaCl) solution, and the steels are artificially corroded with different current densities for investigation. The steel corrosion in Original C60 and Marine Concrete 60 (Marine C60) are applied for comparison. The volume expansion coefficient for monitoring the steel corrosion process, is carried out by FBG sensors, and further comprehensively monitored by three grating sensors. The results suggest that the steel corrosion process in UHPC is slowly with current density lower than 100 μA/cm2, and largely increases with current density higher than 500 μA/cm2, the volume expansion coefficient of corroded steel in UHPC is in range of 2.02–2.62 in this test, which is much different from Original C60 and Marine C60.

Asymmetric deterioration of reinforced concrete marine piles subjected to nonuniformly localized corrosion: Experimental study

Reinforced concrete (RC) marine piles usually suffer from local stiffness deterioration caused by chloride attack in splash and tidal zones. Moreover, pre-existing concrete cracks and defects at the steel–concrete interface usually lead to nonuniformly localized corrosion, which has seldomly been discussed. In this study, the effects of nonuniformly localized corrosion on the lateral behaviours of piles were experimentally studied. The localized electrochemical chloride erosion (LECE) technique was employed, and the corrosion level was represented by the number of corroded steel bars and the length of the corroded sections. Results show that the nonuniform localized corrosion caused asymmetric force–displacement hysteresis curves and changed the failure mode of pile structure under cyclic loading. The highly contrast mechanical responses suggest the importance of the loading directions in assessing the mechanical behaviours of corroded piles. Skeleton curves, stiffness degradation and energy dissipation capacity were further analysed to investigate the bearing capacity of corroded piles. These findings can benefit the comprehensive understandings and assessment of the asymmetric stiffness deterioration of corroded marine RC piles.

Degradation of GFRP Bars with Epoxy and Vinyl Ester Matrices in a Marine Concrete Environment: An Experimental Study and Theoretical Modeling

Degradation of GFRP Bars with Epoxy and Vinyl Ester Matrices in a Marine Concrete Environment: An Experimental Study and Theoretical Modeling

Experimental study on the mechanical properties and uniaxial compressive constitutive relationship of sea sand coral concrete

The use of seawater, sea sand and coral to formulate concrete for marine engineering construction is an effective measure for rationally allocating resources and alleviating resource shortages. To promote the engineering application of marine concrete, it is necessary to study the mechanical properties and microstructure of marine concrete. In this study, sea sand coral concrete (SSCC) with strength grades of C20–C40 was prepared, and the compressive and flexural tests were conducted to observe the stress process and damage morphology of the specimens and to establish a constitutive model for SSCC under uniaxial compression. In addition, the microstructures of the SSCC hydration products and interfacial transition zone (ITZ) were observed via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the damage surface of the SSCC occurred directly through the coral aggregate, and the damage process was sudden and brief. Under uniaxial compression, the trend of the rising section of the stress-strain curve of SSCC with different strength grades are basically the same, but the shape of the descending section vary greatly, and the higher the strength grade is, the steeper the slope of the descending section. Quantitative relationships between compressive strength, elastic modulus, peak strain and peak stress and cube compressive strength were established. Based on the experimental results, a two-stage uniaxial compression constitutive model for SSCC was proposed, and the established constitutive model can effectively predict the stress-strain behavior of SSCC. In addition, the XRD patterns show that the early hydration rate of SSCC is faster than that of ordinary concrete (OC), which leads to a faster early strength development of SSCC. The SEM images show that the coral aggregate binds better to the cement matrix than the crushed gravel aggregate does and that the microstructure of the SSCC is denser than that of OC.

Study on the durability deterioration law of marine concrete with nano-particles under the coupled effects of freeze-thaw cycles, flexural fatigue load and Cl− erosion

The marine concrete in cold region such as harbor, wharf and sea-bridge is inevitably subjected to the coupling effects of flexural fatigue load, freeze-thaw cycles and chloride ion (Cl−) erosion. This paper studies the durability of unreinforced concrete under the coupling effects of three factors. And in order to highlight the effects of freeze-thaw cycles, the two-factor coupling durability test of flexural fatigue load and Cl− erosion and the three-factor coupling durability test of freeze-thaw cycles, flexural fatigue load and Cl− erosion are designed. And the microstructures of various concretes are analyzed by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results show that under the coupling effects of three factors, on the one hand the icing of pore water can improve the deformation resistance of concrete; on the other hand, the expansion pressure can accelerate the deterioration of pores and promotes the development of fatigue cracks. The coupling effects of three factors significantly reduces the flexural fatigue life of concrete, and the more the number of freeze-thaw cycles, the more significant the adverse effects on the flexural fatigue life of concrete. The addition of nano-particles (The cement in concrete is replaced by the same content of nano-particles) can improve the adverse effects of freeze-thaw cycles on the flexural fatigue life of concrete, and enhance the flexural fatigue life of concrete. When the amount of nano-particles is optimal, the fatigue life of NC10 and NS20 is increased by 47.6% and 52.3% respectively. The Cl− erosion resistance of concrete increases firstly and then decreases with the increasing amount of nano-particles. The free Cl− content of the compression zone in NC10 and NS20 can be reduced by 22.4% and 24.2% respectively, and the free Cl− content of the tensile zone in NC10 and NS20 can be reduced by 16.6% and 20% respectively. Under the coupling effects of three factors, with the continues erosion of Cl−, the Ca/Si value and the atomic percentage of Ca in interface transition zone (ITZ) of concrete gradually are decreased, which makes the decreased extent of C–S–H gel and Ca(OH)2 in concrete. The addition of nano-particles can reduce the decrease extent of Ca atom percentage in ITZ, reduce the decomposition of C–S–H gel and Ca(OH)2, and improve the fatigue life and Cl− erosion resistance of concrete.

Mitigating CO2 emanations within the marine concrete industry with different structures of silica nanoparticles

The extensive utilization of marine concrete results in a significant consumption of cement, thereby leading to an increase in CO2 emanations. Enhancing the potency and longevity of marine concrete serves as an efficacious approach to mitigate CO2 emanations. The objective of this investigation was to introduce colloidal silica nanoparticles with distinct three-dimensional structures (MCM-41, SBA-15, and DFNS) for the purpose of enhancing the mechanical functionality and longevity of remarkable sulfate persistence Portland cement. Meantime, the incorporation of silica nanoparticles with distinct three-dimensional architectures (DFNS, MCM-41, and SBA-15) into marine cement mortar was carried out, and an investigation was conducted on the aggregation of silica nanoparticles, as well as the humidifying process and micro-process of the mortar. According to the findings, the addition of silica NPs improved the resilience, mechanical traits, and longevity of the marine cement cases. When the inclusion of silica nanoparticles with varying three-dimensional structures (DFNS, MCM-41, and SBA-15) was implemented, a decrease in both the chloride diffusion coefficient and porosity was observed. The compressive intensity of cement mortar was enhanced in the presence of DFNS, MCM-41, and SBA-15 when compared to control samples, following a rehabilitating period of 5, 15, and 30 days. These advancements in intensity and longevity suggest that DFNS exhibits considerable potential in mitigating CO2 emanations within the marine concrete industry.

Enhancing Volumetric Stability of Metakaolin-Based Geopolymer Composites with Organic Modifiers WER and SCA

Shrinkage during hardening and curing is one of the largest challenges for the widespread application of metakaolin-based geopolymers (MKGs). To solve this problem, a silane coupling agent (SCA) and waterborne epoxy resin (WER) were used to synthesize MKG composites. The individual and synergistic effects of the SCA and WER on chemical, autogenous, and drying shrinkage were assessed, the modification mechanisms were investigated by microstructural characterization, and shrinkage resistance was evaluated by the chloride ion permeability of MKG composite coatings. The results showed that the SCA and WER significantly decreased the chemical shrinkage, autogenous shrinkage, and drying shrinkage of the MKG, with the highest reductions of 46.4%, 131.2%, and 25.2% obtained by the combination of 20 wt% WER and 1 wt% SCA. The incorporation of the organic modifiers densified the microstructure. Compared with the MKG, the total volume of mesopores and macropores in MKG-WER, MKG-SCA, and MKG-WER-SCA decreased by 11.5%, 8.7%, and 3.8%, respectively. In particular, the silanol hydrolyzed from the SCA can react with the opened epoxy ring of the WER and the aluminosilicate oligomers simultaneously to form a compact network and resist shrinkage during the hardening and continuous reaction of the geopolymer. Furthermore, the apparently lowered chloride ion diffusion coefficient of concrete (i.e., reduction of 51.4% to 59.5%) by the WER- and SCA-modified MKG coatings verified their improved shrinkage resistance. The findings in this study provide promising methods to essentially solve the shrinkage problem of MKGs at the microscale and shed light on the modification mechanism by WERs and SCAs, and they also suggest the applicability of MKG composites in protective coatings for marine concrete.

Effect of coarse calcareous aggregates on corrosion of steel reinforcement in marine concretes

Finely ground limestone, perhaps as part cement replacement, has been shown to reduce concrete permeability and enhance strength. Whether this holds also for coarse limestone aggregates is of both practical and environmental interest, particularly the effect on corrosion of steel reinforcement. This is confirmed herein with exposure tests over 14 years for a range of concretes exposed to a high chloride environment. Importantly, these concretes also showed delayed initiation of reinforcement corrosion and lower long-term corrosion losses compared with similar concretes made with coarse igneous or siliceous aggregates. The observations are attributed to enhanced bonding between cement-hydration products and the limestone aggregate surfaces. These findings are important both for reducing environmental impacts and greenhouse gasses and for extending the durability of reinforced concrete structures.

Internal superhydrophobic marine concrete: Interface modification based on slag microstructure regulation

Structures in coastal environments are susceptible to damage from the combined effects of loading, scouring, corrosion, rust-frost-crystallization expansion forces, etc. Superhydrophobic materials can effectively alleviate these problems due to their excellent water rejection properties. However, in practical applications, superhydrophobic performance failure occurs after the surface is subjected to various stresses. To further explore this point, this article prepared an integrated superhydrophobic protective material using solid waste material slag to achieve the resource utilization of solid waste materials. Superhydrophobic particles covered with paraffin wax (SPP) reduce the direct damage of external forces on hydrophobic particles, extend their service life, and achieve self-healing of superhydrophobicity. Its comprehensive performance indices, including all-round wettability, water repellences, carbonisation resistance and electrochemical corrosion performance, were investigated. The results show that the contact angles of the superhydrophobic mortar inside and outside the contact angle are more than 150°, indicating three-dimensional superhydrophilicity. The 30% SPP dosage of superhydrophobic mortar in 48 h water absorption compared to ordinary mortar reduced by 78%, 120 h water permeability up to 0.72%; and carbonized 28 days after no traces of carbonisation; in the electrochemical corrosion experiments to prevent corrosive liquids from entering the reinforcing bars in the concrete has good anti-corrosion properties. In the electrochemical corrosion experiment, the corrosive agent effectively prevents the corrosive liquid from entering contact with the steel reinforcement in the concrete, and the resulting material has good anti-corrosion performance. These key characteristics can provide unique advantages for a wide range of applications in coastal environments, such as construction, bridging, decoration, transportation and power transmission.

Anatomically preserved early Cretaceous lycophyte shoots; enriching the paleontological record of Lycopodiales and Selaginellales

Anatomically preserved lycophytes of the Lycopodiales and Selaginellales have been discovered among a diverse assemblage of plants and fungi in carbonate marine concretions at the Apple Bay locality along the shore of Holbert Inlet near the northern end of Vancouver Island, British Columbia, Canada. Lycopodialean stems are plectostelic and actinostelic, branch dichotomously, and are similar to both Lycopodicaulis oellgaardii and Lycoxylon spp. The Selaginella specimens represent the first anatomically preserved Selaginellales with excellent internal cellular preservation in the fossil record, and are described as Selaginella quatsinoense Rothwell et Stockey sp. nov. Stems have three and five exarch, monarch stelar segments, each of which is surrounded by an aerenchymatous endodermis with trabeculae. The leaf base is indented on the adaxial surface, suggesting the position of a ligule. These fossils document that species with diagnostic internal anatomy of modern Lycopodiales and Selaginellales evolved no later than the Valanginian of the early Cretaceous.

Improving sulfate and chloride resistance in eco-friendly marine concrete: Alkali-activated slag system with mineral admixtures

This study investigates the effects of various mineral admixtures, such as fly ash, metakaolin, and limestone powder, on the mechanical, chloride, and sulfate resistance properties of alkali-activated slag (AAS). The resistance of alkali-activated concrete (AAC) to chloride and sulfate is evaluated using the rapid chloride migration (RCM), wetting-drying cycles, SEM and XRD, TG/DTG. The results reveal that introducing fly ash improves workability but degrades the strength and durability of AAS. The 15% slag substitution by metakaolin is recommended to improve durability. Incorporating 15% limestone powder increases the compressive strength and reduces porosity due to filling and nucleation effects. The replacement ratio of 30% for each mineral admixture induces a considerable Ca2+ dilution, resulting in reducing the strength and durability of AAC. Under short-term wetting-drying cycles, the continued hydration of raw materials under MgSO4 erosion and the subsequent production of gypsum, hydrotalcite, and brucite from the matrix initially increase the AAC strength. However, the continued substitution of calcium in the C-(A)-S-H gel by magnesium and precipitation of gypsum under excessive cycles adversely deteriorates the compressive strength. The chloride migration degree in AAC is closely related to the porosity. Although the porous gel N-A-S-H facilitates chlorine migration in accelerated tests, it prevents further chlorine penetration by adsorbing chloride ions in large quantities, exhibiting the dominant effect on chloride immobilization. The formation of Friedel's salt, stemming from the chemical bonding of chloride, is present only in the high-calcium specimen G100 and the highly reactive aluminum specimen G85M15. Improvements in the gel chemistry and porosity are key factors in bolstering the durability of AAC. The findings of this study contribute to advancing the application of eco-friendly AAC in marine environments.