Friedel's Salt Research Articles

Durability and thermal insulation properties of glazed hollow bead concrete with nanoparticles in marine atmospheric environment

Industrial and civil buildings in coastal areas have been in the marine atmosphere for a long time and are especially susceptible to the combined effects of carbonation and salt spray, which accelerate the deterioration of concrete durability and the reduction of thermal insulation performance. This paper focused on the effect of nano-SiO2 and nano-Al2O3 on the durability and thermal insulation of glazed hollow bead concrete (GC) under a salt spray carbonation environment, and analyzed the mechanism of nanoparticles on the improvement of durability and thermal insulation of GC by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) microscopic tests. The results showed that both nano-SiO2 and nano-Al2O3 incorporation could improve the durability and thermal insulation of GC to a certain extent. The optimal amount of both nanoparticles was 2 %, and nano-SiO2 had a stronger beneficial effect than nano-Al2O3. In comparison to GC, the free Cl− content in the surface layer of GCNS20 and GCNA20 after 56 cycles was decreased by 33.33 % and 27.22 %, respectively. Furthermore, the carbonation depths of GCNS20 and GCNA20 were decreased by 42.83 % and 36.38 %, respectively. The thermal conductivity of GCNS20 and GCNA20 decreased by 24.86 % and 17.34 %, respectively, while the thermal diffusion coefficients were reduced by 33.22 % and 23.73 %, respectively. The incorporation of nanoparticles effectively reduced the microcracks and pores in GC, improved its internal structure, and reduced the possibility of thermal bridge formation. In addition, the incorporation of nanoparticles significantly reduced the production of erosion products such as CaCO3 and Friedel's salt in GC.

Ion transportation properties of cement-based material under standard curing and steam curing

In coastal and saline environments, the infiltration of chloride and sulfate ions into cementitious materials can result in erosion and deterioration. This research focuses on the impact of temperature-related curing methods on the ion transport properties and microstructure of these materials. The results show the curing method does not alter the hydration products of cementitious materials. High-temperature curing reduces the content of ettringite, resulting in decreased Friedel's salt and gypsum after chloride ion corrosion. Meanwhile, high-temperature curing reduces the content of Ca(OH)2 and SiO2, decreasing the content of gypsum after sulfate ion erosion. It indicates that high-temperature curing reduces the binding capacity of both chloride ions and sulfate ions. Besides, high-temperature steam curing deteriorates the pore structure compared to standard curing. As a result, the resistance of concrete to ion corrosion is reduced under high-temperature steam curing. Besides, the surface ion concentration and binding capacity of chloride ions and sulfate ions both increase with the duration of erosion. Under the same erosion age, the binding capacity of sulfate ions is significantly higher than that of chloride ions. This research provides new insights into the transport characteristics of eroding ions under high-temperature steam curing.

Study on the microscopic transmission characteristics and coupled erosion depth of tailings recycled concrete under the coupling effect of carbonization and salt-fog

In ocean and salt-lake district, especially in some industrial areas, concrete structures suffer from the coupling erosion of carbonation and salt-fog. As an eco-friendly material, tailings recycled concrete(TRC) has been used to its alleviate the deterioration of living environment. Based on series of physical and chemical equations of hydration, carbonization and salt-fog erosion, this paper introduces parameters such as hydration rate, carbonization rate and Friedel salt degradation coefficient. Then，a comprehensive analysis method that can reflect the erosion performance of TRC quantitatively is established, and the microscopic transmission characteristics and erosion depth calculation method under coupling effect are studied. The results indicate that aggregates have a “blocking effect” on the diffusion of corrosive substances, resulting in higher concentration around the front edge of aggregates, and the erosion of salt-fog plays a leading role under the coupling effect. Overall, the CO2 concentration decays faster than that of Cl−. The longer the erosion age, the higher the CaCO3 content, while the content of CH and CSH is approximately inversely proportional to that of CaCO3. In addition, the method for determining the depth of carbonation and salt-fog erosion under coupling effect is studied, and the calculated values are in good agreement with the experimental values. It shows that the coupling erosion depth is greater than that under single erosion, which leads to the salt-fog erosion depth increased by about 30 %, the longer the erosion age, the greater the erosion depth. The effect of iron ore tailings(IOT) on coupled erosion depth is similar to that of single erosion. The results also show that the coupled erosion performance of TRC can be analyzed by using the model proposed in the paper.

Chloride binding behavior and mechanism of phosphoaluminate cement clinker and its hydration products

As one of potential materials for extending the lifetime of reinforced concrete serving in chloride-rich environments, phosphoaluminate cement (PAC) offers excellent chloride binding capacity and superior resistance to chloride penetration. The chloride binding behavior as well as binding routes and mechanisms of the clinker and its hydration products had not been clarified so far. This study revealed that the clinker would preferentially form Friedel's salts through a dissolution-precipitation route in NaCl solutions, thereby binding chloride. This dissolution-precipitation process led to an accumulation of Al3+ in solutions, which negatively affected the subsequent hydration reactivity of the clinker. In the hydration products, the calcium hydroaluminates, mainly as C2(A,P)H8 with minor C(A,P)H10, could bind chloride rapidly via an ion exchange mechanism. Through the ion-exchange route, the chemical binding of calcium hydroaluminates to chloride could be completed in less than 1 hour. In contrast, the unhydrated clinker exhibited a slow chloride binding behavior until 72 hours. All these would provide theoretical guidance for using PAC cementitious materials in reinforced concrete to enhance resistance to chloride attack.

The effects of hardening accelerator on microstructure and mechanical performance of fly ash-cement-based materials using response surface methodology

To enhance the engineering application range of fly ash-cement-based materials (FCM), the combined effects of hardening accelerator on FCM were investigated by using response surface methodology (RSM). Based on a three-factor three-level Box-Behnken design (BBD), the linear, quadratic and interaction relationships between the dosages of calcium chloride (CaCl2), aluminum sulfate (Al2(SO4)3) and triethanolamine (TEA) on the primary properties (setting time, mechanical properties and shrinkage rate) of FCM were discussed. The prediction model was established to determine the optimal ratio of the composite system. The microscopic performance and the mineral fraction of the paste were explored by using X-ray diffraction (XRD), thermogravimetric differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Results indicated that the paste performance was affected by the interaction of multiple factors, and the interaction between CaCl2 and Al2(SO4)3 was the most significant. The optimum ratio of paste was 2.81 % CaCl2, 0.75 % Al2(SO4)3 and 0.12 % TEA, achieving a 28 d compressive strength of 40.48 MPa, closely matching the predicted value. The dominant hydration products of the composite cementitious system were C–S–H gel, AFt and Friedel's salt, with changes observed in the morphology of CH. With the prolong of curing age, the hydration products and unhydrated fly ash particles were bonded to each other, so that the slurry maintains a certain strength performance.

Investigation on solidified/stabilized behavior of marine soil slurry by lime-activated incinerated sewage sludge ash-ground granulated blast furnace slag under multifactor conditions

This study aims to evaluate the possibility of reusing treated marine clayey soils by stabilization/solidification (S/S) technology as geomaterial in reclamation projects from the aspects of engineering strength, chemical modification and environmental risk assessment. The lime-activated incinerated sewage sludge ash (ISSA) together with ground granulated blast furnace slag (GGBS) was employed as the binder. The multi-controlling factors including water content, curing time, salinity, and chemical compositions of mixing solution were taken account to identify the S/S treated Hong Kong marine deposit (HKMD) slurry based on the strength tests, pH measurement, thermo-gravimetric (TG) analysis, X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS) and toxicity characteristic leaching procedure (TCLP) tests, etc. The results show that the S/S treatment using lime-activated ISSA-GGBS can effectively enhance the strength of marine soil at initial water content of 110% and 200%. The water content and curing time have a significant impact on the S/S treated HKMD. The pH of treated soils is higher than 11.1, which proves an alkaline environment for the reactions in the treated soil. A special case is the treated HKMD at 200% water content hydrated by MgCl2 solution, which has a low pH of 10.23 and maintains a slurry state. Based on the TCLP results, the leaching concentration of heavy metals from S/S treated HKMD is environmentally safe and meets Hong Kong standard for reusing treated soil with a low level of <0.2 mg/L. The content of main products such as calcium/magnesium silicate hydrate, ettringite or Friedel's salt depends on the chemical additions (e.g. distilled water, seawater, NaCl and Na2SO4). The products in the specimens mixed with MgCl2 solutions are mainly composed of Mg(OH)2, M-S-H and MgCO3, which is distinct with the neoformations in the other cases. Therefore, this study proves that the S/S treated soil slurry could be reused as geomaterials in reclamation projects, and the S/S process is greatly affected by water content, curing time and solution compositions, etc.

Study on seawater corrosion resistance of calcium sulfoaluminate cement-based novel grouting materials

Grouting materials play a critical role in anchoring structures of cross-sea tunnels by safeguarding the anchors against seawater corrosion and enhancing durability. In this paper, a novel grouting material comprising ferrite-rich sulfoaluminate cement (FCSA), ordinary Portland cement (OPC), anhydrite and limestone were prepared. Accelerated corrosion immersion tests were conducted to assess the corrosion resistance of the grouting material using artificial seawater solutions with varying salt concentrations. Pull-out tests were performed to verify the material's performance, while potentiometric titration and X-ray diffraction (XRD) analysis were employed to analyze free chloride ion concentration (Cf) and corrosion products at the anchorage interface. Results indicate that after 1.5 years of corrosion in seawater with different salt concentrations, the bond strength stabilized at 1.05 MPa. No rust was observed on the anchor surfaces and the Cf exhibited a linear increase with salt concentration. The content of Friedel's salt increases with salt concentration, demonstrating adequate chloride ion binding capacity. The test results validate the exceptional seawater corrosion resistance and long-term stability of the novel grouting material.

Durability study of Portland cement blended with metakaolin from thermodynamic modeling

The addition of metakaolin to Portland cement (PC) can increase chloride binding but is vulnerable to carbonation. The durability issues against such a coupled effect remain uncertain although chloride ingress and carbonation can occur simultaneously. This study investigated the effect of carbonation on chloride resistance in PC blended with metakaolin through thermodynamic modeling. The results showed that chloride binding increased with metakaolin replacement in the absence of carbonation. However, as carbonation progressed, the chloride binding decreased with metakaolin replacement. It is because (1) metakaolin substitution decreases calcium content in the binder and (2) calcite formation also leads to the lack of calcium for the later formation of Friedel's salt. Therefore, metakaolin-blended cement certainly lost chloride binding capacity with carbonation which should be carefully considered in the design of durability performance of metakaolin blended cement.

Fillers to improve the ductility and impermeability of crumb rubber concrete

Crumb rubber concrete (CRC), a type of eco-friendly concrete materials, has attracted pronounced interest in the field of infrastructure construction and waste management. However, special attention should be paid to its permeability since it is an important indicator to determine its service life for building structures, especially in coastal regions. Present study discusses the feasibility of pozzolanic fillers to improve the ductility and impermeability of CRC. By incorporating fly ash (FA), silica fume (SF), and ground granulated blast furnace slag (GGBFS), fresh properties, mechanical strength and chloride resistance of CRC were evaluated. Results show that the fillers adopted enhance the toughness of CRC in terms of ductility index considering splitting-tensile and compressive strength, among which SF has the most positive effect. Micro-aggregate effect of FA particles, high active silica content of SF and more Friedel's salt formation are the main reasons for the improved impermeability of CRC. Radar charts are plotted to analyze the overall performance of CRC with fillers. Relative area values indicate that SF exhibits the most significant improvement in CRC performance, followed by GGBFS and FA.

Solidification/stabilization and optimization of municipal solid waste incineration fly ash with aluminosilicate solid wastes

Alkali-activation is an effective municipal solid waste incineration fly ash (MSWIFA) solidification/stabilization (S/S) technology. However, the characteristics of calcium-rich silica-poor aluminum phase in MSWIFA easily cause the structural instability and contamination of alkali activated MSWIFA S/S bodies. Therefore, the aluminosilicate solid wastes are used in this work to optimize the immobilization and structural properties. Results showed that incorporation of aluminosilicate solid wastes significantly improved the compressive strength and heavy metals pollution toxicity of MSWIFA S/S bodies. Compared to alkali activated MSWIFA, the compressive strength of S/S bodies with addition of coal fly ash, silica fume and granulated blast furnace slag improved by 31.0%, 47.6% and 50.8% when the curing time was 28 days, respectively. Leachability of Pb, Zn and Cd in these alkali activated MSWIFA S/S bodies was far below the threshold value specified in Standard GB16889. Aluminosilicate solid wastes provided abundant Si/Al structural units, and some new phases such as ettringite(AFt, 3CaO⋅Al2O3⋅3CaSO4⋅32H2O), calcium sulfoaluminate hydrate (3CaO⋅Al2O3⋅CaSO4⋅12H2O) and Friedel's salt (CaO⋅Al2O3⋅CaCl2⋅10H2O) can be detected in S/S matrix with aluminosilicate solid wastes, along comes increased the amount of the amorphous phases. Lower Ca/Si molar ratio tended to form the network structure gel similar to tobermorite with higher polymerization degree. Meanwhile, the silica tetrahedron of the gels changed from the oligomerization state like island to the hyperomerization state like chain, layer network or three-dimensional structure, and average molecular chain length increased. These findings provide theoretical basis for structural properties optimization and resource utilization of MSWIFA S/S matrices.

Microstructure and chloride transport of aeolian sand concrete under long-term natural immersion

Abstract River sand was consumed in large quantities, and alternatives to river sand were urgently needed. There are a large number of natural resources of aeolian sand in western China. Aeolian sand was prepared into aeolian sand concrete (ASC). It can greatly reduce the consumption of river sand and inhibit the process of desertification to protect the environment. ASC is a new type of concrete material prepared by using aeolian sand as fine aggregate. To clarify the chloride ion transport behaviour in the ASC under long-term natural immersion, the aeolian sand was 100% substituted for the river sand to prepare the full ASC with three water–binder ratios. The ASC was naturally immersed in 3 and 6% NaCl solutions for a long time, and nuclear magnetic resonance and microscopic scanning electron microscopy techniques were used. The change rule of chloride ion content at different depths of the ASC was studied, and its microstructure characteristics under different erosion times were analysed. The results showed that the free chloride ion concentration at different depths of the ASC increased with increasing water–binder ratio, immersion time, and chloride concentration. After soaking in the salt solution, the hydration products in the ASC reacted with chloride ions to form Friedel salt, which filled the internal pores and microcracks of the ASC, improved its interface transition zone structure, and increased the compactness of the test piece. The porosity of the three groups of ASC with different water–binder ratios decreased by 0.95, 1.03, and 1.15% after soaking in 6% salt solution for 12 m. To study the diffusion law of chloride ions in ASC, combined with influencing factors such as temperature, humidity, D value, deterioration effect and chloride ion combination, Fick’s second law was modified, and a chloride ion diffusion model of ASC with high accuracy was established, with a fitting correlation number above 0.93, which provided a reference for the research and application of ASC in saline areas.

Deterioration of mechanical properties and damage mechanism of flue gas desulfurization gypsum backfill under long-term erosion of chloride salt solution

Flue gas desulfurization gypsum (FGDG), a solid waste material generated by power plants, offers a cost-effective solution for preparing backfill materials. Concurrently, given that most backfill materials are exposed to mine water erosion, an investigation into the mechanical performance degradation and damage mechanisms of FGDG backfill materials subjected to prolonged chloride salt erosion is crucial for ensuring the stability of goaf backfills (by goaf, we mean the area or space that remains underground after the extraction of valuable minerals). This study explores the mechanical properties and damage mechanisms of backfill samples treated with varying concentrations of chloride. The findings indicate that over time, the mass of the sample initially decreases, experiences a slight increase, and then gradually declines. Simultaneously, the strength and structure of the sample deteriorate rapidly, followed by a partial recovery. The rebound in sample mass is attributed to an incomplete hydration reaction that continuously transforms free water into bound water. Microstructure analysis techniques, such as X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), reveals that the degradation in mechanical strength and structure is a consequence of chloride salt erosion. Subsequent reuse is facilitated by the formation of Friedel's salt resulting from the reaction between hydration products and chloride ions, filling voids in the sample and impeding further chloride ion erosion into the deeper regions. Notably, FGDG exhibits outstanding reuse performance in prolonged and high-concentration chloride ion environments. This study serves as a reference for understanding the deterioration mechanism and practical application of backfills in chloride-rich environments.

The effects and solidification characteristics of municipal solid waste incineration fly ash-slag-tailing based backfill blocks in underground mine condition

Currently, there is a limited understanding of the material performance effects and hydration mechanisms of solidified waste-based binder filling materials containing municipal solid waste incineration fly ash in the actual underground filling environment. This study utilized the Circulating Fluidized Bed (CFB) and Grate Furnace (GF) techniques to produce quadri-component uncalcined binder fill materials from municipal solid waste incineration fly ash (MSWI FA), blast furnace slag (BFS), flue gas desulfurization (FGD) gypsum, and tailings. The solidified test blocks were then cured in-situ in a void space of a certain iron underground mine in Hebei with conditions (temperature: 25-30℃, humidity: 18-19%, mine water influx: annual average of 150m3/h). The research results indicate that the mechanical performance and heavy metal solidification outcomes of these blocks surpassed those samples cured under laboratory conditions (temperature: (20±1)℃, humidity: not less than 90%), demonstrating the feasibility of using MSWI FA for underground binding and backfilling in mines. Building upon preliminary research, a binder mix proportion was established: 20wt.% MSWI FA, 70wt.% BFS, and 10wt.% FGD gypsum. Test blocks were fabricated using lead-zinc tailings sand and iron mine tailings as aggregates. All blocks met the 28-day strength requirements for typical mine backfills ranging from 1 to 6.5MPa, with the strength of the XB-M group reaching 24MPa (when cured underground). The leaching concentrations of heavy metals were all below the limits set for class III groundwater standards, with Pb, Zn, Cr, and As achieving a solidification rate of over 98% at 28 days. Microscopic hydration mechanisms indicate that the primary hydration products in the solidified body were ettringite, Friedel's salt, and C-S-H gel. The high content of reactive Al2O3 in the CFB fly ash enables the [AlO4]5- to substitute [SiO4]4-, leading to the formation of a long-chain, high polymerization C-A-S-H phase. Additionally, the abundant Cl- in GF fly ash readily reacted with [Al(OH)6]3−, forming Friedel's salt. The hydration products in the blocks cured under mine conditions significantly increased in their degree of polymerization. This provides a practical application approach for the future treatment and reuse of MSWI FA. Environmental implicationMSWI FA is classified as hazardous waste (HW18) due to its content of harmful components such as chlorides, dioxins, and soluble heavy metals, which pose potential environmental pollution risks. This study, for the first time, solidified test specimens were sent to an underground mining void in a certain iron mine in Hebei, China, for on-site curing (temperature range: 25-30°C, humidity: 18-19%, annual average mine water inflow rate: 150m3/h). The mechanical properties and the solidification performance of heavy metals in the test specimens were superior to those under laboratory curing conditions (temperature: (20±1)°C, humidity not less than 90%).

Factors affecting free chloride contents of hardened cement paste with a low water-to-binder ratio determined by rapid water extraction

This paper assesses the free chloride content in hardened cement paste with a low water to binder (w/b) ratio by a proposed water extraction method. Key parameters including extraction time, particle size of the sample, types and concentrations of chloride salts are thoroughly investigated to understand their effects on the extraction results. The findings reveal that the increase of extraction time from 1 to 20 min and the decrease of particle size from 0.315-1 mm to 0.045–0.08 mm exhibits limited influence on the hydration degree of cement paste with low w/b ratio during the extraction process. With the increase of extraction time, the extracted chloride content gradually increases. Beyond the 10-min mark, the extracted chloride content stabilizes gradually, indicating a stable status in the extraction process. The content of Friedel's salt remains relatively unchanged throughout the 10min water extraction, while physically bound chloride gradually releases into the extraction water. Almost all physically bound chloride in NaCl and CaCl2 mixed specimens is desorbed within the 10min of extraction time when the particle size is set at 0.045–0.08 mm. However, the physically bound chloride in MgCl2 blended specimens keeps relatively stable when the particle size varies, which is due to the improved chloride binding capacity by M-S-H. The rapid water extraction method is proved suitable for free chloride testing of hardened cement paste with low w/b ratio by controlling suitable parameters. Consequently, the particle size of 0.08–0.16 mm and extraction time of 10 min are recommended for conservative evaluation of free chloride content in hardened cement paste with low w/b ratio.

Effect of maintenance environment on the mechanical property and pore structure of cement paste mixed by seawater

This study aims to examine the effects of macroscopic mechanical properties and pore structure of cement paste mixed with seawater and explore the relationship between pore structure and macroscopic properties. The changeable law of mechanical properties and pore microstructure of hardened cement paste under seawater mixing is methodically investigated based on the compressive and flexural strengths using X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) tests. The relationship between the fractal dimension of the pore structure of cementitious materials and the concentration of chloride ions in simulated seawater, the curing age, and the fly ash dose was obtained based on the thermodynamic fractal model. The obtained results reveal that with the increase in chloride ion concentration of the simulated seawater, the compressive and flexural strengths of the samples substantially grow in the pre-maintenance period, and the strength of the samples tends to stabilize at the age of 28 days. For the case of the chloride ion concentration of 1.3%, the compressive strength of the cement slurry exhibits a growth of 24.08%, 24.83%, and 28.81% compared to the control cement sample after curing at 5 °C, 20 °C, and 50 °C for 3 days, respectively. Additionally, the flexural strengths in order are 41.86%, 46.57%, and 38.89% higher than those of the control group. The interaction of Cl- with C3A, C4AF, or AFm in seawater mixed at high-temperature to generate Friedel's salt stimulated the volcanic ash reaction of fly ash, promoted the generation of C-S-H gels, and improved the internal pore structure of cementitious materials. The results indicate that at 50 °C curing, the most probable pore diameter of cement paste with 1.3% chloride ion concentration is 39.51% less than that of the control cement sample, and its harmless pores are 23% higher than that of the control group. At 5 °C curing, the total specific pore volume of cement paste with 1.3% chloride ion concentration is 30.18% less than that of the control cement sample, and the total specific pore volume is 22.9% less than the control group for the fly ash content of 25%. The obtained results reveal that there exists a fairly low correlation between the fractal dimension of the pore surface area of the cement paste and porosity, compressive strength, and flexural strength. However, the achieved correlation with the mean pore diameter, median pore diameter, and pore surface area is good, which are all above 75%. The fractal dimension fitting curve of high-temperature curing possesses a range of singular points.

Effects of a novel carbon nanomaterial on hydration, mechanics, and chloride binding of cement composites

Nanomaterials have made significant progress in the field of cementitious materials, but suffer from primary problems of high cost and poor dispersity. Carbon dots (CDs) as a novel low-cost and high-dispersity nanomaterial preliminarily demonstrate their potential for cement. Nevertheless, it lacks systematic research on their effects on cement properties. For the first time, this work comprehensively investigates the effects of CDs on the hydration, mechanics, and chloride binding of cement composites. More importantly, the relevant mechanisms are revealed in depth. Specifically, cost-efficient ($0.013/g) and well-dispersed (dispersed in simulated concrete pore solution for 7 d) CDs are directly prepared by a facile microwave approach. The CDs dramatically retard the early hydration, improve later mechanical performance, and obviously enhance chloride binding. In detail, 0.2 wt% CDs reduce the second exothermic peak value by 75%, but improve the chloride binding capacity of paste by 51%. And 0.1 wt% CDs enhance the compressive/flexural strength of mortar by 17%/21%. Crucially, related influence mechanisms are uncovered. Namely, CDs create a protective barrier to retard hydration through the complexation of their abundant surface groups with Ca2+; While CDs exert the nucleation effect to boost the formation of C–S–H and Friedel's salt, thereby playing a positive impact on chloride binding; Meanwhile, CDs with suitable dosages carry out nucleation effect to counteract their air-entraining effect, thus improving later mechanical properties of cement composites. This study is expected to provide a novel low-cost and high-dispersity CDs nanomaterial to advance the development of high-performance cementitious materials.

Mikrostruktur, Porositas Dan Sifat Mekanik Beton Curing Air Laut

When constructing buildings in coastal areas, contact between the materials used and sea air is sometimes unavoidable. Before the concrete reaches its maximum strength at the age of 28 days, it can be exposed to contamination from sea air. Consequently, chemical reactions involving cement, air, and sea air become inevitable. The research aims to ascertain the microstructure, porosity, and mechanical properties (compressive strength and tensile strength) of high-performance concrete utilizing the Self Compacting Concrete method. Two variations of concrete samples are considered, involving differences in the curing water used and the age of the concrete. There are two curing variations: concrete cured with fresh water and concrete cured with seawater. The concrete's age is monitored at 1, 3, 7, 28, and 90 days. In the microstructure analysis of seawater-cured concrete, chloride compounds, ettringite, and Friedel's salt were identified. Both compressive strength and splitting tensile strength show an increase with the concrete curing age, and they are inversely proportional to the concrete's porosity. The mechanical properties of freshwater-cured concrete are slightly lower than those of seawater-cured concrete. However, the difference is negligible, meaning they are nearly identical at both 28 days and 90 days.

Strength development and self-desiccation of saline cemented paste backfill.

Given that many mines around the world are located in areas where fresh water is scarce, and companies are being held to increasingly stringent sustainability and environmental responsibility standards, many mines are looking to use locally available saline groundwater or seawater as mixing water in cemented paste backfill (CPB). However, the impacts of this decision on key engineering properties of CPB (e.g. strength and self-desiccation) that affect its mechanical stability need to be better understood to allow confident selection of this practical and more sustainable solution. Thus, the effect of mixing water salinity and binder type on the strength (unconfined compressive strength, UCS) development and self-desiccation (measured by suction and volumetric water content) of CPB is explored in this research. NaCl concentrations from 0 to 300g/L were used in CPB made with silica tailings and Portland cement type I (PC). Concentrations of 10 and 35g/L were found to moderately increase UCS, while a concentration of 100g/L had comparable UCS to non-saline CPB and a concentration of 300g/L was found to significantly decrease UCS over all curing times. The overall trend is 10g/L > 35g/L > 0g/L > 100g/L > 300g/L. The UCS of the 60-day-old CPB with a NaCl of 300g/L is significantly lower, registering a 26% decrease compared to the UCS of the 60-day-old CPB without salt. In contrast, the UCS of the 60-day-old CPBs containing 10g/L and 35g/L of salt exhibits a notable improvement, being 15% and 10% higher, respectively, than the UCS of the 60-day-old CPB without salt. Water content and suction monitoring were conducted up to 28days of curing time, and it was found that suction only slightly contributed to UCS gain of the saline CPB, and high salt contents (100 and 300g/L) significantly inhibited the self-desiccation ability of CPB due to inhibition of cement hydration by the excessive amount of salt. The increase in strength of both saline and non-saline samples was attributed primarily to the increase in cement hydration products, while the increased strength of the samples with salinities of 10 and 35g/L was mainly attributed to the enhancement of the binder hydration due to the low amount of salt and the presence of Friedel's salt in the pores. The effect of PC replacement by 25 to 75% with slag on CPB with 35g/L mixing water salinity was also studied. Slag replacement of 50% and higher resulted in significantly higher UCS over most curing times. Suction likely moderately contributed to UCS of the saline CPB with slag, in addition to the presence of Friedel's salt in the pores and the acceleration of cement and slag hydration by the presence of NaCl.

Residual impact resistance behavior of PVA fiber reinforced cement mortar containing Nano-SiO2 after exposure to chloride erosion

High-performance cement-based composite materials are commonly employed in marine structural engineering. However, the deterioration of mechanical and durability properties of composite structures caused by chloride erosion in marine environments is a typical engineering problem. This study aims to assess the impact of NS and PVA fibers on the strength, chloride resistance, and impact resistance of NS-PVA fiber mortar. The impact frequency distribution of NS-PVA fiber mortar was analyzed under two conditions: natural environment and after chloride erosion, utilizing both log-normal and two-parameter Weibull distribution models. The findings indicate that the impact energy consumption of NS-PVA fiber mortar increases after 28 days of chloride erosion compared to the natural environment, primarily due to the formation of a compact pore structure attributed to the reaction product Friedel's salt. The optimal composite dosage is determined to be 1.8% for NS and 1 vol% for PVA fiber. At this dosage, the PS18 mortar exhibits a 44.8% and 53.7% increase in initial cracking and failure impact energy, respectively, compared to conventional mortar in the natural environment. Furthermore, these enhancements are further amplified to 53.4% and 64.6% after chloride erosion. Concurrently, a negative correlation is observed between accelerated impact energy consumption and chloride diffusion depth in NS-PVA fiber mortar. A predictive model for impact energy consumption of NS-PVA fiber mortar following chloride erosion, considering various failure probabilities, is established, offering valuable insights for the practical application of NS-PVA fiber mortar in marine engineering.

A Study on Chloride Corrosion Resistance of Reactive Powder Concrete (RPC) with Copper Slag Replacing Quartz Sand under Freeze-Thaw Conditions.

In order to study the influence of freeze-thaw cycles on chloride ion corrosion resistance of RPC with copper slag (CS) instead of quartz sand (QS), the 28d uniaxial compressive strength (UCS) of CSRPC with a different CS substitution rate was investigated by unconfined compression tests. The electric flux test method was used to study the chloride ion diffusion resistance of CSRPC after freeze-thaw cycles, and the pore size distribution was obtained through the nuclear magnetic resonance (NMR) method. Then, a mathematical relationship between the chloride ion diffusion coefficient and the pore fractal characteristic parameter T was established to study the effect of freeze-thaw cycles on chloride ion diffusion. Finally, SEM/EDS, XRD, and DTG methods were combined to study the influence of the distribution of Friedel's salts generated after freeze-thaw cycles on chloride ion diffusion in CSRPC. The results indicate that CS has a micro aggregate effect and pozzolanic activity, which can effectively improve the chloride ion diffusion resistance of CSRPC after freeze-thaw cycles. In addition, the electric flux of CSRPC decreases with the increase in freeze-thaw cycles, and the chloride diffusion coefficient is closely related to the pore fractal dimension.