Dissolution Of Ca Research Articles

Degradation of alkali‐activated Fe‐rich slag in magnesium sulfate solution

AbstractThe potential of Fe‐rich non‐ferrous metallurgical slag (NFMS) in alkali‐activated materials has been demonstrated, exhibiting promising mechanical strength and fire resistance. However, the durability of alkali‐activated NFMS (AA‐NFMS) in terms of sulfate salt resistance remains unexplored. Additionally, the variability in the chemical composition of NFMS presents challenges in directly assessing the durability of AA‐NFMS. This study investigates the interplay between the Ca/Si and Mg/Ca ratios in Fe‐rich NFMS and the resulting MgSO4 resistance of the AA‐NFMS made thereof. To do so, mass change and compressive strength retention were monitored for six months on AA‐NFMS immersed in 5 wt% MgSO4, whereas microstructure was investigated using scanning electron microscope (SEM) and electron probe microanalyzer (EPMA). The results show that the MgSO4 solution attacked selectively the reacted binder matrix of AA‐NFMS rather than the unreacted slag. The corrosion primarily occurs at the sulfate exposure surface of AA‐NFMS, forming a dense, thin layer of Mg(OH)2, which acts as a barrier and impedes further attack. Dissolution of Ca is the most pronounced among the major elements (Ca, Fe, Al, and Si) in the NFMS. The mass of the specimens increased linearly with the initial Ca content. The medium Ca concentration (∼10%) retained the highest strength.

Reaction mechanism of Ca(OH)2-based carbon storage suitable for the production of building materials

The development of CaCO3 oxyfuel combustion and calcium looping techniques offers opportunities for Ca(OH)2 production with near-zero CO2 emissions. This study investigated the viability of Ca(OH)2-based CO2 storage for use in the production of building materials. To resolve the deficient strength gain of Ca(OH)2 carbonation, siliceous materials, using fly ash as a model compound, were introduced into the raw mixture, and hydrothermal curing was applied to promote the pozzolanic reaction. Two microstructural features, i.e., inner and outer CaCO3 formations, were identified during Ca(OH)2 carbonation via backscattered electron imaging. Inner CaCO3 formation is characterized by Ca(OH)2–CaCO3 conversion within the edge of Ca(OH)2 particles. Outer CaCO3 formation is characterized by the dissolution of Ca(OH)2 and the precipitation of CaCO3 in capillary pores, which generates hollow microstructures. The hollow space can be further filled by Al-tobermorite during hydrothermal curing. The prepared materials are capable of storing up to 16 % CO2 by the mass of the raw mixture. Furthermore, the pore structure influenced by inner and outer CaCO3 formation demonstrates the feasibility of adjusting the material's mechanical and CO2 storing properties by controlling the carbonation and hydrothermal curing schemes.

Study on the damage mechanism of hydraulic concrete under the alternating action of freeze-thaw and abrasion

This study investigates the damage mechanism of hydraulic concrete under the alternating action of freeze–thaw (F) and abrasion (W) in cold regions. To this end, three groups of working conditions were established with a constant water-binder ratio. The mass loss and relative dynamic elastic modulus (RDEM) of each cycle were evaluated using the fast-freezing method and underwater method. The phase composition and relative content change of hydration products, pore characteristic parameters and pore size distribution, morphology and microstructure of interfacial transition zone (ITZ) were analyzed by X-ray diffraction (XRD), mercury intrusion porosimeter(MIP), scanning electron microscopyn (SEM) and image recognition technology, respectively. The results indicate that the alternating effect will accelerate the mass loss of concrete and the reduction of the RDEM compared with the single factor. Following four consecutive alternating cycles, the mass loss rates of working conditions W and F group were found to be 4.61 % and 97.66 % lower, respectively, than those of working condition F-W group. The RDEM damage coefficient (γ < 1) was also observed. At the micro level, the freeze-thaw effect will result in the formation of a considerable number of micropores and the expansion of microcracks, with the abrasion effect continuing to promote this process. In the process of alternating action, the dissolution of Ca(OH)2 by external water through microcracks and interconnected pores is accelerated. Concurrently, the pores and microcracks in the ITZ expand, the morphology is roughened, and the microstructure is complicated. The interfacial bonding ability and compactness of different material components decrease. Under the action of external disturbance, cement paste and aggregate fall off, resulting in mass loss and a decreased RDEM.

Influence of silica fume addition and content on the early hydration of calcium aluminate cement – The role of soluble silicon

Hydration of a commercial white calcium aluminate cement (CAC) at 23 °C was modified by silica fume (SF) addition in varying amounts. The process was followed by heat flow calorimetry, quantitative in-situ XRD analysis and Gillmore needle experiments supplemented by pore solution analysis, thermodynamic modelling, and 1H-TD-NMR measurements. Lower SF/cement ratios accelerate the hydration of CAC. Higher ratios trigger an intermediate heat flow event, which is correlated to increased setting rates. This intermediate event (IE) initiates an induction period of constant duration, which appears later with increasing SF/cement ratios. Results show the IE is caused by an initially hindered CA dissolution, in which dissolved silicon provided by SF plays a crucial role. Increasing the [Si] concentration in the pore solution leads to a further retardation of the IE and eventually prevents the entire hydration reaction if a critical amount is reached. A detailed model explaining the observed behavior is proposed.

Mechanism of Phosphate Desorption from Activated Red Mud Particle Adsorbents.

Herein, activated red mud particles are used as adsorbents for phosphorus adsorption. HCl solutions with different concentrations and deionized water are employed for desorption tests, and the desorption mechanism under the following optimal conditions is investigated: HCl concentration = 0.2 mol/L, desorbent dosage = 0.15 L/g, desorption temperature = 35 °C, and desorption time = 12 h. Under these conditions, the phosphate desorption rate and amount reach 99.11% and 11.29 mg/g, respectively. Notably, the Langmuir isothermal and pseudo-second-order kinetic linear models exhibit consistent results: monomolecular-layer surface desorption is dominant, and chemical desorption limits the rate of surface desorption. Thermodynamic analysis indicates that phosphorus desorption by the desorbents is spontaneous and that high temperatures promote such desorption. Moreover, an intraparticle diffusion model demonstrates that the removal of phosphorus in the form of precipitation from the surface of an activated hematite particle adsorbent primarily occurs via a chemical reaction, and surface micromorphological analysis indicates that desorption is primarily accompanied by Ca dissolution, followed by Al and Fe dissolutions. The desorbents react with the active elements in red mud, and the vibrations of the [SiO4]4- functional groups of calcium-iron garnet and calcite or aragonite disappear. Further, in Fourier-transform infrared spectra, the intensities of the peaks corresponding to the PO43- group considerably decrease. Thus, desorption primarily involves monomolecular-layer chemical desorption.

Effect of surface water on wollastonite carbonation: Activated dissolution and mass transfer

Carbonation of non-hydraulic calcium silicate to produce cement represents a crucial strategy for mitigating carbon emissions in the cement industry. However, the dissolution rate of Ca2+ is the main constraint on the reaction kinetics. In this study, the carbonation of wollastonite (CS) was investigated, with an emphasis on the role of water in the Ca2+ dissolution process. Firstly, we investigated carbonation performance of CS and its influencing factors. Results revealed a significant impact of the water-to-solid (w-s) ratio on carbonation. In the carbonate products, we observed that the dissolution of Ca2+ adsorbed on the particle surface, referred to as “pseudocrystalline replacement”, which can serve as supplementary cementitious material. Additionally, this observation signifies the accordance of Ca2+ dissolution with the electric double layer (EDL) theory. Subsequently, we quantitatively assessed the influence of w-s ratio on carbonation using kinetic models. Based on EDL theory and the kinetic model, we categorized water film effect into three stages and determined the influence of water film thickness on the rate-determining factors of CS carbonation. This provides theoretical support for preparation of CS-based cement. Interestingly, we found that reaction rate constant varies with changes in the w-s ratio, indicating a catalytic role of water. Further analysis using density functional theory demonstrated that the free water combined with the O/Ca sites on CS to form hydroxylated surfaces, thereby decreasing and increasing the abilities of the local and neighboring sites, respectively, to absorb H2CO3. These findings provide a theoretical foundation for the curing method of low-carbon cements and offer a methodology for the preparation of supplementary cementitious materials.

Effect of Various Acid Solutions on the CO2 Dissolution Rate, Morphology, and Particle Size of Precipitated Calcium Carbonate Synthesized Using Seashells.

In this study, the influence of acid solutions on the production of precipitated calcium carbonate (PCC) using seashells was investigated. In terms of the Ca dissolution efficiency and atmosphere for dissolving CO32-, the results indicate that HCl, HNO3, CH3COOH, and HCOOH at 1.0 M were the most ideal among the acid solutions. The use of weak acids resulted in the low degree of dissolution of Al and Fe. These impurities could be mostly removed through the pH adjustment process, leading to PCC with a purity of 99% or more. Further, CH3COOH and HCOOH exhibited low CaCO3 carbonation efficiency owing to the hydrogen bonding of the carboxyl group and its hindering effect on the growth of CaCO3 particles. In addition, in the presence of the carboxyl group, the morphology tended to be oval, and the particle size was small. Particularly, when CH3COOH was used, the combined effect of the low initial Ca ion concentration and slow CO2 dissolution rate resulted in minimal changes during the carbonation time and the smallest particle size. However, variations in the degree of Ca concentration with a change in the acid solution concentration influenced the dominance of nucleation and particle growth, leading to variations in the particle size. The results of this study revealed that when manufacturing PCC using seashells, the appropriate acid solution must be selected to obtain the required PCC properties.

C-S-H decalcification in seawater: The view from the nanoscale

Decalcification of calcium silicate hydrates (C-S-H) is one of the most important issues for cement-based infrastructures concerning long-term safety performance. While enhanced decalcification of C-S-H in saline solution has been extensively characterized by experimental studies, the underlying microscale mechanisms are not well understood. Using molecular simulations with the metadynamics method, we compared the Ca dissolution free energy of amorphous C-S-H and C-S-H-like crystals in pure water and seawater. Previous experimental results were well-explained by our calculations, and we revealed an important mechanism of accelerated C-S-H decalcification in seawater that has not been emphasized before, i.e., the “ion exchange” between surface Ca and electrical double layers in seawater significantly reduces the Ca dissolution free energy through excess entropy gains. This provides a new and sound understanding of decalcification in contrast to previous empirical approaches stating that accelerated decalcification is mainly related to the pore solution's pH value and ionic solubility. It is also found that C-S-H with a larger Ca/Si ratio features lower dissolution free energy. Our finding enhances the assessment and prediction of cementitious materials' degradation and inspires relevant mitigation strategies. This work also enriches the understanding of mineral dissolution with a broader geoscience interest.

Bone regeneration property of tooth-derived bone substitute prepared chairside for periodontal bone defects: an experimental study

BackgroundPeriodontitis often leads to progressive destruction and loss of alveolar bone, the reconstruction of which remains difficult in periodontal therapy. As a novel bone graft material, tooth-derived bone substitute (TDBS) processed from extracted teeth has been previously reported about its osteoconductivity and promising results in bone regeneration. This study was to investigate the biological effects and bone regeneration properties of TDBS in vitro and in vivo using rat periodontal bone defect model.MethodsThree groups of materials were used in the experiments: TDBS, TDBS treated with ethylene diamine tetraacetic acid (EDTA) (TDBS-E), and allogeneic bone materials. Calcium (Ca) and phosphate (P) ion dissolutions were quantified by spectrophotometer for seven days. The releases of bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β1 (TGF-β1) were identified by enzyme-linked immunosorbent assay (ELISA). Human osteoblast proliferation, migration, and differentiation were detected by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, cell counting, alkaline phosphatase activity (ALP), and alizarin red staining (ARS), respectively. Furthermore, the osteogenic effects of TDBS on periodontal furcation bone defects were evaluated at eight weeks postoperatively using micro-computed tomography (Micro-CT) and histological analysis.ResultsThe dissolution of both Ca and P ions in TDBS increased over time. The BMP-2 released from TDBS was significantly higher than that from TDBS-E and allografts, while the TGF-β1 release from TDBS and TDBS-E groups was higher than that in the allografts. The TDBS-E group could induce the highest level of osteoblast proliferation compared to other groups. Cell migration with allografts co-culture was significantly induced compared to the blank control. However, all groups demonstrated similar positive effects on osteoblast differentiation. Furthermore, in the periodontal model, all materials could effectively enhance bone regeneration in the furcation defect.ConclusionsThe TDBS prepared chairside as an autogenous bone graft, demonstrating osteoinductivity, which enhances the osteogenic biological characteristics. Therefore, TDBS is suggested as an economical and biocompatible material for periodontal bone regeneration.

Phosphorus removal by porous aggregate from aerated concrete waste residues：Simulating porous structure, performance optimisation, adsorption mechanisms

In order to reveal the action mechanism of porous structure of aerated concrete waste residues as aggregate on its working performance and phosphorus removal performance, the porous aggregate is prepared by adjusting the of hydrogen peroxide (H2O2) content to simulate the pore structure of aerated concrete. The results show that with the increase of H2O2 content, the porosity and pore diameter of porous aggregate increases, which causes that the loose bulk density, cylinder compressive strength and softening coefficient decrease and the water absorption through time increases. The correlation coefficients between the four performances and total porosity exceeds 0.95. With the increase of H2O2 content, the pH value decreases and TP removal rate increases. The better fitting of the pseudo-second-order model and Elovich model confirms the key existence of chemical adsorption. In the fitting curve of Intraparticle diffusion model the second stage of rapid absorption becomes the main control step due to calcium and aluminum ions from capillary pores and gel pores and the hydration of unreacted particles. The Ca/Si and Al/Si decreases by 57.7% and 46.6% in EDS maps after phosphorus removal, and CaO/SiO2 and Al2O3/SiO2 in XRF results decrease. The hydration of unreacted C4A3Š and dissolution of Ca2+ and Al3+ during phosphorus removal promote the chemical absorption of porous aggregate. P/Si increases by 631.9% and P2O5/SiO2 increases from 0.01 to 0.11. The fluffy substances appears on the smooth inner wall of the porous aggregate in SEM morphology is phosphate mineral.

Geochemistry of Weathering Cover and the Main Influencing Factors in Karst Area of Guilin, Southwest China

The chemical weathering of carbonate rocks is the main form of interaction between earth spheres in the karst critical zone. The karst weathering cover, which is composed by residua from carbonate rocks weathering, contains important information about the climate environment and material cycle. We present the chemical composition of weathering covers in karst area of Guilin, Guangxi province, analyze their weathering process and strength, and compare them with the other weathering covers in other karst area in China, including Yunnan, Guizhou, Hunan, and Qinghai Tibet Plateau. The results showed: (1) the chemical composition of Guilin weathering covers were similar to that of carbonate weathering covers in other areas of China, and had the common characteristics of uniform distribution of chemical composition in the profile and obvious enrichment of Fe, Al and trace elements. During the formation of carbonate weathering cover and the residua, the rapid dissolution of Ca and Mg had an important impact on the migration and enrichment of other elements. (2) The chemical index of alteration (CIA) of carbonate weathering covers in Guilin and other karst areas of China was much larger than that in the upper crust (UCC) (60.13), which showed strong chemical weathering characteristics of the humid and hot climate. (3) The weathering process of carbonate rock was different from that of silicate and loess. In the early stage of carbonate rock weathering, soluble components (calcite and dolomite) had been rapidly dissolved and leached. All the carbonate weathering residua was measured to be in the stage of K2O weathering and Al2O3 increasing. Therefore, the weathering degree of carbonate weathering cover was mainly determined by the leaching of K2O and the increase of Al2O3. As a result, there was no correlation between CIA and Na/K (molar ratio), but was a significant negative correlation between CIA and K/Al (molar ratio) in the carbonate weathering cover. (4) The CIA values of weathering residua and carbonate weathering cover in southern China were negatively correlated with latitude, reflecting the influence of climate factors. From the influence of lithology, the chemical compositions of parent rock can affect the CIA of weathering cover, and the content of insoluble matter in carbonate rock was negatively correlated with CIA. From the influence of topography, the CIA value of weathering residua decreased from the high to the low position and from the shallow to the deep part of the profile.

Application of oyster shell powder reduces cadmium accumulation by inhibiting the expression of genes responsible for cadmium uptake and translocation in rice.

The application of waste oyster shell in agriculture is of extensive concern due to its benefits on improving yields and inhibiting cadmium (Cd) accumulation in edible parts of crops. However, the underlying mechanisms responsible for oyster shell powder (OSP) that decreases Cd accumulation in crops remain poorly understood. This study explored the effects of OSP on growth and Cd accumulation in rice via pot experiments and hydroponics. Pot experiments showed that the application of 1g·kg-1 OSP improved rice yields and decreased Cd concentrations in all tissues of rice, especially in grains, which was reduced by 43.5%. The pH was increased and the phytoavailability of Cd in soil was reduced by OSP supplementation. In addition, OSP also exhibited high dissolution of Ca, Fe, Zn, and Se. In hydroponics, OSP supply also suppressed Cd accumulation in rice and increased plant growth. Pretreatment with OSP inhibited the accumulation of Cd in the roots and shoots. Simultaneously, OSP reduced the content of Cd in the root cell sap, cell wall, and xylem sap, and downregulated the expression of OsNramp5, OsNramp1, OsIRT1, and OsHMA2. These findings suggested that the application of OSP could reduce Cd accumulation by inhibiting the expression of genes responsible for Cd absorption and xylem loading in rice.

Understanding the adsorption of calcium sulfate on tetracalcium aluminoferrite and tricalcium aluminate surfaces

AbstractCalcium sulfate (CaSO4), an essential retarder in cement, retards the hydration of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO4f on C4AF and C3A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO4 on C4AF was stronger than that on C3A due to the slower CaSO4 consumption rate, lower driving force for CaSO4 adsorption, and surface coverage of Fe(OH)3 gel. The adsorption of CaSO4 hindered Ca dissolution more markedly on C4AF than C3A, which was pronounced on Fe‐free C4AF surfaces. The adsorption of CaSO4 weakened the affinity of water on C4AF and C3A surfaces, lowering the driving force for H2O adsorption. The adsorption of H2O and CaSO4 promoted the dissolution of Al on the [AlO6] octahedral surface of C4AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO4 on initial C4AF and C3A hydration was explained.

Deterioration of Cement-Based Materials in Low-Temperature Seawater.

Cementitious materials have potential for infrastructure development in low-temperature marine environments, including in seawater at high latitudes and in deep-sea environments (water depths of >1000 m). Although the marine deterioration of cementitious materials has been widely investigated, the influence of seawater temperature has not been elucidated. In this study, to determine the effects of low-temperature seawater on the durability of cementitious materials, cement paste specimens were immersed in a seawater tank at room temperature and 2 °C for 433 days. The specimen immersed in low-temperature seawater exhibited significant deterioration with a partially collapsed surface, whereas the specimen immersed in room-temperature seawater maintained its original shape. Following low-temperature immersion, Ca dissolution was more pronounced and dissolved portlandite, decalcified calcium (alumino)silicate hydrate (C-(A-)S-H), magnesium (alumino)silicate hydrate (M-(A-)S-H), and thaumasite were observed on the collapsed surface. Such significant deterioration can be attributed to the increased solubility of portlandite under low-temperature conditions, which could promote Ca dissolution and subsequently lead to C-(A-)S-H decalcification and the formation of M-(A-)S-H and thaumasite. These insights are expected to contribute to the successful construction and maintenance of cementitious structures in low-temperature seawater.

First principal observation documenting the three-dimensional uptake of cadmium and spatial distribution of cadmium hydroxyapatite mineral in bone char

The 3-D matrix scale ion-exchange mechanism was explored for high-capacity cadmium (Cd) removal using bone chars (BC) chunks (1–2 mm) made at 500 °C (500BC) and 700 °C (700BC) in aqueous solutions. The Cd incorporation into the carbonated hydroxyapatite (CHAp) mineral of BC was examined using a set of synchrotron-based techniques. The Cd removal from solution and incorporation into mineral lattice were higher in 500BC than 700BC, and the diffusion depth was modulated by the initial Cd concentration and charring temperature. A higher carbonate level of BC, more pre-leached Ca sites, and external phosphorus input enhanced Cd removal. The 500BC showed a higher CO32-/PO43- ratio and specific surface area (SSA) than the 700BC, providing more vacant sites by dissolution of Ca2+. In situ observations revealed the refilling of sub-micron pore space in the mineral matrix because of Cd incorporation.The X-ray nanodiffraction (XND) analyses revealed that Cd was mainly removed from water by incorporation into the mineral lattice of 500BC via ion exchange, rather than surface sorption and precipitation, and the mineral phase was transformed from hydroxyapatite (HAp) to cadmium hydroxyapatite (Cd-HAp). The Rietveld's refinement of X-ray diffraction (XRD) data resolved up to 91% of the crystal displacement of Ca2+ by Cd2+. The specific phase and stoichiometry of the new Cd-HAp mineral was dependent on the level of ion exchange. This mechanistic study confirmed that 3-D ion exchange was the most important path for heavy metal removal from aqueous solution and immobilization in BC mineral matrix, and put forward a novel and sustainable remediation strategy for Cd removal in wastewater and soil clean-up.

Dissolution of Calcium Hydroxide in Water: A Guided Inquiry in University and High School Chemistry Laboratories

In this laboratory experiment, a guided inquiry exploring the physicochemical principles of the dissolution of Ca(OH)2(s) in water is proposed for laboratory classes in university and high school. As part of students’ inquiry, two experimental approaches are used. One is the change in solubility with temperature revealed by measuring the pH values of the suspended solution of Ca(OH)2(s) at various temperatures, which is then extended to its thermodynamic relationship via examining the temperature dependence of the solubility constant. The other method is to determine the enthalpy of solution using the calorimetric measurements. Due to the poor solubility of Ca(OH)2(s), for determining the enthalpy of the Ca(OH)2(s) solution, development of an energy diagram composed of several paths of a reaction that involve the dissolution of Ca(OH)2(s) as a component process and using Hess’s law are essential. A combination of these experimental approaches yields a stepwise students’ inquiry for revealing the source of the changes in Ca(OH)2(s) solubility with temperature, which may be flexibly adapted as an appropriate program depending on the targeted students. The two experimental procedures are presented by critically examining the experimental results. Based on the results of educational practices, typical guided inquiry constructions suited for the university and high school chemistry courses are proposed.

The mechanisms and drivers of lithification in slag‐dominated artificial ground

AbstractUnconsolidated artificial ground is an ever‐increasing feature on the Earth's surface but it poses various challenges such as pollutant release and ground instability. The process of lithification could be an important factor in changing the properties of artificial ground and ameliorating these challenges. In this study, a lithified deposit of a furnace slag associated with a former iron and steel works in Scotland was analysed to determine the mechanisms and drivers of lithification. Scanning electron microscope analysis showed that Ca leached from around the edges of clasts of slag through reaction of the chemically unstable slag with water from an adjacent water body. Dissolution of Ca (and OH−) from the slag caused the water in contact with the slag to become hyperalkaline, facilitating ingassing and hydroxylation of CO2 from the atmosphere (fingerprinted through carbon isotope analysis). Reaction of the dissolved Ca and CO2 led to precipitation of calcite. Scanning electron microscope analysis showed the calcite is distributed between slag clasts, forming rims around clasts and cementing clasts together into a solid rock‐like mass. Understanding the mechanisms and drivers of lithification in artificial ground will be important, given its widespread nature particularly in urban areas where artificial ground is the substrate of most development.

Influence of Peat Soil Environment on Mechanical Properties of Cement-Soil and Its Mechanism

The influence of peat soil environment (PSE) on the mechanical properties of cement-soil in the area around Dianchi Lake and Erhai Lake in Yunnan Province has attracted much attention. This study explores the change law of cement-soil UCS in the PSE, and provides guidance for the development and sustained usage of peat soil foundation. The paper discusses the preparation of cement-soil samples by adding humic acid (HA) and cement to cohesive soil with low organic matter content (blending method) and soaking it in fulvic acid (FA) solution and deionized water (steeping method) to simulate the actual working environment of cement-soil. Unconfined compressive strength (UCS), acid consumption, ion leaching, scanning electron microscope (SEM), and X-ray diffraction (XRD) tests are carried out on cement-soil samples soaked for 90 days. The results show that HA can significantly reduce the UCS of cement-soil. FA can reduce the UCS of cement-soil when the content of HA is less than 18%. However, when the amount of HA is more than 18%, the UCS of cement-soil increases slightly. FA makes the deformation and failure type of cement-soil gradually change from brittle shear failure to plastic shear failure. FA reacts with the cement hydration products in the sample so that the cumulative acid consumption of the cement-soil sample continues to increase, and the dissolution of Ca2+, Mg2+, Al3+, and Fe3+ in the sample increases the ion concentration of the soaking solution. In addition, SEM and XRD show that HA can increase the macropores and connectivity of cement-soil, while FA fills part of the pores of the wetting layer. In the PSE, FA can strengthen the inner structure of HA particles and fill and cement the layers of cohesive particles, enhancing the construction of cement-soil with HA content greater than 18%, so that its UCS is relatively improved. However, when the amount of HA is less than 18%, there are more small pores in the cement-soil. The interaction between FA and HA in the cement-soil is weak. The influence of FA on cement-soil is mainly a weakening effect, and its UCS is relatively reduced.

Wildfire effects on the hydrogeochemistry of a river severely polluted by acid mine drainage

This study evaluates for the first time the impact of a large wildfire on the hydrogeochemistry of a deeply AMD-affected river at the beginning of the wet season. To accomplish this, a high-resolution water monitoring campaign was performed within the basin coinciding with the first rainfalls after summer. Unlike similar events recorded in AMD-affected areas, where dramatic increases in most dissolved element concentrations, and decreases in pH values are observed as a result of evaporitic salts flushing and the transport of sulfide oxidation products from mine sites, a slight increase in pH values (from 2.32 to 2.88) and decrease in element concentrations (e.g.; Fe: 443 to 205 mg/L; Al: 1805 to 1059 mg/L; sulfate: 22.8 to 13.3 g/L) was observed with the first rainfalls after the fire. The washout of wildfire-ash deposited in the riverbanks and the drainage area, constituted by alkaline mineral phases, seems to have counterbalanced the usual behavior and patterns of the river hydrogeochemistry during autumn. Geochemical results indicate that a preferential dissolution occurs during ash washout (K > Ca > Na), with a quick release of K followed by an intense dissolution of Ca and Na. On the other hand, in unburnt zones parameters and concentrations vary to a lesser extent than burnt areas, being the washout of evaporitic salts the dominant process. With subsequent rainfalls ash plays a minor role on the river hydrochemistry. Elemental ratios (Fe/SO4 and Ca/Mg) and geochemical tracers in both ash (K, Ca and Na) and AMD (S) were used to prove the importance of ash washout as the dominant geochemical process during the study period. Geochemical and mineralogical evidences point to intense schwertmannite precipitation as the main driver of reduction in metal pollution. The results of this study shed light on the response of AMD-polluted rivers to certain climate change effects, since climate models predict an increase in the number and intensity of wildfires and torrential rain events, especially in Mediterranean climates.

Tunability of mechanical and biodegradation properties of zinc-based biomaterial with calcium Micronutrient alloying

Biodegradable metals are being investigated as temporary implants that dissolve safely in the body after bone regeneration. Zinc (Zn) has an intermediate biodegradation rate between magnesium and stainless steels, yet its degradation rate is too slow to function as a temporary orthopedic implant. Alloying with nutrient elements is considered a strategy to tune its mechanical properties and in vivo biodegradability. Zn/calcium (Zn/Ca) alloys (with 0.5, 1, and 2 wt% Ca) were processed by spark plasma sintering and their microstructure, mechanical, and biodegradation properties were investigated. Ca was distributed in the grain boundary regions of Zn due to its low miscibility in Zn. Furthermore, the corrosion rates of Zn/Ca alloys determined from linear polarization measurements (0.164–0.325 mm/yr) accelerated by at least 10% compared with pure sintered Zn (0.149 mm/yr) with simultaneous dissolution of Zn and Ca, as verified from X-ray diffraction analysis of the corrosion products. The alloy specimens exhibited hardness (52–58 HV) and compressive strength (93–119 MPa) comparable with those of human cortical and cancellous bones (49 HV; 90–209 MPa). This study demonstrated the tunability of the mechanical and biodegradation properties of Zn-based materials by alloying them with a nutrient element for potential application as temporary orthopedic implants.