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This study investigates the effect of the particle size and solid-to-liquid ratio on the dissolution rate of magnesium (Mg) and silicon (Si) in heat-activated lizardite. The investigation was conducted under specific conditions: without the presence of sodium bicarbonate (NaHCO3), at a moderate temperature (40 °C), and under elevated CO2 pressure (100 bar). The aim was to isolate the dissolution reactions and enhance comprehension of the factors constraining the overall yields in the Albany Research Center (ARC) mineral carbonation process. Our study disclosed two distinct dissolution regimes: an initial stage with a rapid initial rate of Mg extraction, resulting in the fraction of Mg extracted ranging from 30 to 65% during the first 20 min of the experiment, following which the dissolution rate decreases dramatically. The initial rapid dissolution stage is primarily driven by the low pH of the supernatant solution, resulting from CO2 dissolution, leading to a higher concentration of protons that extract Mg2+ cations. However, as the heat-activated lizardite dissolution progresses, the pH increases due to the high level of leached Mg2+, and a diffusion barrier forms due to the precipitation of amorphous silica. This phenomenon ultimately slows down the mineral's dissolution rate during the latter stages of particle dissolution. [ABSTRACT FROM AUTHOR]

Steel slag is the main by-product of the steel industry and can be used to produce steel slag fine aggregate (SSFA). SSFA can be used as a fine aggregate in mortar or concrete. However, SSFA contains f-CaO, which is the main reason for the expansion damage of mortar and concrete. In this study, the carbonation treatment of SSFA was adopted to reduce the f-CaO content; the influence of the carbonation time on the content of f-CaO in the SSFA was studied; and the effects of the carbonated SSFA replacement ratio on the expansion rate, mechanical properties and carbonation depth of mortar were investigated through tests. The results showed that as the carbonation time increased, the content of f-CaO in the SSFA gradually decreased. Compared to the mortar specimens with carbonated SSFA, the specimens with uncarbonated SSFA showed faster and more severe damage and a higher expansion rate. When the replacement ratio of carbonated SSFA was less than 45%, the carbonated SSFA had an inhibitory effect on the expansion development of the specimens. The compressive strengths of the specimens with a carbonated SSFA replacement ratio of 60% and 45% were 1.29% and 6.81% higher than those of the specimens with an uncarbonated SSFA replacement ratio of 60% and 45%, respectively. Carbonation treatment could improve the replacement ratio of SSFA while ensuring the compressive strength of specimens. Compared with mortar specimens with uncarbonated SSFA, the anti-carbonation performance of mortar specimens with carbonated SSFA was reduced. [ABSTRACT FROM AUTHOR]

The design processes for reinforced concrete are changing. More often, durability targets are being achieved by using modelling. This paper compares some of the models available and the precision undertaken to obtain the data that underpins the calculations, and it reflects on the change in the environment that is known to be occurring. In addition, a review of the sustainability implications of durability is considered. It is concluded that there may be more sustainable methods to achieve a long life than simply increasing cement contents and covers. [ABSTRACT FROM AUTHOR]

Carbonation curing has demonstrated potential of improving concrete performance while facilitating carbon dioxide utilization. However, reinforcement corrosion behavior in carbonation-cured concrete has not been documented. This paper presents a study on chloride-induced corrosion in reinforced concrete subjected to carbonation curing. A special carbonation curing process was developed for precast non-prestressed applications. Performance of carbonation curing was evaluated by concrete compressive strength, pH value, and carbon dioxide uptake, while corrosion resistance of the carbonation-cured concrete was assessed by reinforcing bar mass loss and concrete chloride content. To understand the mechanism, concrete and cement paste were further characterized using mercury intrusion porosimetry, absorption, and electrical resistivity tests. Micromorphology was assessed by scanning electron microscopy. It was found that apart from rapid early-age strength gain, carbonation curing could significantly reduce chloride permeation in concrete concerning both total and free chloride contents. It was attributed to the reduced pore size and pore volume by calcium carbonate precipitation. With subsequent 28-day hydration, the carbonation-cured concrete displayed a pH over 12.0 at the surface of steel reinforcing bars and a micromorphology similar to the non-carbonated reference. The direct corrosion tests showed that the corrosion-induced mass loss of steel reinforcing bar was lessened by 50% in concrete subjected to carbonation curing. [ABSTRACT FROM AUTHOR]

In this paper, the effects of carbonation and moisture on chloride and ionic transport into concrete are studied. Based on the concrete composition and porosity as well as the physicochemical equilibrium, a comprehensive macroscopic model is proposed for this problem. The model features the chemical activity of ions, the interactions between chloride ions and concrete; and the ion–ion interactions in the solution. The governing equations of moisture and ionic transport into nonsaturated concrete are thoroughly described and solved numerically by the finite difference method in time and in space. Applications of the numerical model are demonstrated by predicting the evolution of the porosity and the service-life of a concrete specimen exposed to an aggressive environment containing chloride, carbon dioxide and moisture. A comparison of the model with a set of experimental data is finally proposed. [ABSTRACT FROM AUTHOR]

Abstract Given the fast-paced world economy, increasing carbon dioxide (CO 2) emissions have caused serious concern regarding global warming. Many studies have examined the nexus between CO 2 emissions and economic growth and inconsistent results are provided. Limited evidence suggests that different time periods under investigation could be a source of inconsistency, but there lacks of a thorough analysis. This paper systematically focuses on the evolutionary characteristics of the nexus to explore the influence of time periods. The panel data are reorganized by considering different time periods in various countries, and an intelligent data-driven approach, symbolic regression, is applied to discover models for each period without predefined structure and parameters. Three issues (Does the time period matter? What are the trends in different time periods? What are the applicable scopes of the models?) have been addressed in this paper. The results reveal the evolutionary nature of the nexus and it is verified that an intrinsic time period distinction does generate extrinsic inconsistency. The detailed dynamics of models indicate that the optimal models tend to be more complex as the time periods are extended. For countries with different locations and income levels, the applicable scopes of the four superior models (monotonically increasing, inverted U-shaped, inverted N-shaped, and M-shaped) vary in different time periods. The findings highlight that the effect of time periods cannot be ignored and the models should not be applied unconditionally when conducting empirical research. Finally, several policy implications are provided, which are critical for the balance of economic growth and CO 2 emissions, particularly in some developing countries. Highlights • The inconsistency of nexus between CO 2 emissions and economic growth is examined. • The intrinsic time period difference does generate extrinsic nexus inconsistency. • The effect of time period is verified from geographic and socioeconomic aspects. • Rich evidence is revealed for the evolutionary nature of the relationship. [ABSTRACT FROM AUTHOR]

This paper presents the results of investigations on reinforced concrete structures present in the former Auschwitz I and Auschwitz II-Birkenau concentration and extermination camp. Field inspection employing the non-destructive method of reinforcement potential measurement according to the ASTM-C 876-15 standard, followed by laboratory investigations performed on genuine historic reinforced concrete samples collected from the former Auschwitz camp, allowed assessment of the actual condition of this 70-year-old construction material. Electrochemical potential measurements allowed mapping of the regions characterized by enhanced reinforcement corrosion risk and the regions of its immunity. The composition of concrete, water absorbency, content of detrimental salts, and degree and profile of carbonation as well as mechanical properties were evaluated. Results revealed the most common and probable mechanism responsible for degradation of the reinforced concrete structures in the former Auschwitz camp. [ABSTRACT FROM AUTHOR]

Carbon capture and sequestration (CCS) has been employed to reduce global warming, which is one of the critical environmental issues gained the attention of scientific and industrial communities worldwide. Once implemented successfully, CCS can store at least 5 billion tons of CO2 per year as an effective and technologically safe method. However, there have been a few issues raised in recent years, indicating the potential leakages paths created during and after injection. One of the major issues might be the chemical interaction of supercritical CO2 with the cement, which may lead to the partial or total loss of the cement sheath. There have been many approaches presented to improve the physical and mechanical properties of the cement against CO2 attack such as changing the water-to-cement ratio, employing pozzolanic materials, and considering non-Portland cements. However, a limited success has been reported to the application of these approaches once implemented in a real-field condition. To date, only a few studies reported the application of nanoparticles as sophisticated additives which can reinforce oil well cements. This paper provides a review on the possible application of nanomaterials in the cement industry where physical and mechanical characteristics of the cement can be modified to have a better resistance against corrosive environments such as CO2 storage sites. The results obtained indicated that adding 0.5 wt% of Carbon NanoTubes (CNTs) and NanoGlass Flakes (NGFs) can reinforce the thermal stability and coating characteristics of the cement which are required to increase the chance of survival in a CO2 sequestrated site. Nanosilica can also be a good choice and added to the cement by as much as 3.0 wt% to improve pozzolanic reactivity and thermal stability as per the reports of recent studies. [ABSTRACT FROM AUTHOR]

The chlorinated tailing (CT) generated during titanium extraction from Ti‐bearing blast‐furnace slag contains 3–5 wt% chlorides, which are hazardous to the environment. In this paper, a process for simultaneous CO2 mineralization, dechlorination, and recovery of multiple value‐added products was proposed to fully utilize the CT. In this process, Ti, Al, Mg, and Ca are extracted in the form of their sulfates via roasting with recyclable (NH4)2SO4 followed by leaching with dilute H2SO4. Their extraction ratios are 83.1%, 87.4%, 89.4%, and 92.1%, respectively. The chlorides are simultaneously removed from the CT as gaseous HCl with a dechlorination ratio of 98.5%. Subsequently, 89.5% of the Al and 97.5% of the Ti in the leaching liquor are successively recovered as NH4Al(SO4)2·12H2O with a purity of 99 wt% and titanium‐rich material with TiO2 of 62.5 wt%. The CaSO4‐rich leaching residue and MgSO4‐rich leachate, after complete depletion of the Al and Ti, are separately carbonated with a mixed gas of CO2 and NH3 with total CO2 capacity up to 279.8 kg CO2 per tonne CT. The preliminary economic evaluation shows that income from selling products after the deduction of the cost of primary raw materials reaches 200 $/tonne CT. The proposed route therefore provides a cost‐efficient alternative for comprehensive utilization of the polluting CT. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of the coupling between cement hydration and drying of concretes with high amount of fly ash or blast-furnace slag and concretes with Portland cement or blended cements. The study, conducted with the MLS by FEMMASSE® code, aims at assessing the influence of drying on concrete properties governing carbonation kinetics, i.e. porosity, Portlandite content and liquid water saturation degree. These properties were found not uniform according to the depth within the concrete material. In the cover zone, high substitution concrete should have a higher carbonation risk than Portland cement concrete because of the slow hydration kinetics combined with the suspending effects of drying on hydration. Therefore, high substitution concrete should require a longer period of curing than Portland cement-based concretes. [ABSTRACT FROM AUTHOR]

Abstract Over the past few decades, climate change has become a significant problem, the ramifications of which are of concern around the world. Global warming is a long-term process caused by phenomena such as increased levels of carbon dioxide (CO 2) concentration in the atmosphere, increases in rainfall, changes in relative humidity and changes in temperature. Thus, this will aggravate the degradation process of reinforced concrete (RC) structures, leading to the decrease of durability, safety and serviceability. Regarding the durability of concrete structures, carbonation-induced corrosion is definitely a significant and costly source of degradation, directly related to climatic parameters such as CO 2 , temperature and relative humidity. In Paraguay, the lack of control during the construction of buildings often leads to weak structures highly vulnerable to the attack of harmful external agents. Therefore, this paper seeks to provide an analysis of the correlation between real degradation data and a numerical carbonation model for concrete structures in the country. According to the Intergovernmental Panel on Climate Change (IPCC) reports, climatic parameters will increase in the future. For this reason, an analysis of the decreasing performance of these structures caused by climate change will be addressed in this paper. This research aims to choose the most accurate mathematical model that can be applied in Paraguay for the prediction of the service life of concrete structures subjected to carbonation-induced corrosion. For that purpose, parameters and variables of the selected model have been defined and subsequently validated through carbonation tests results carried out in a set of RC structures built in the country. The results of this paper should be considered as a first stage for the further formulation of maintenance strategies, which is highly necessary for Paraguay. [ABSTRACT FROM AUTHOR]

Blended cements are preferred to Ordinary Portland Cement (OPC) in construction industry due to costs and technological and environmental benefits associated with them. Prevalence of significant quantities of carbon dioxide (CO2) in the atmosphere due to increased industrial emission is deleterious to hydrated cement materials due to carbonation. Recent research has shown that blended cements are more susceptible to degradation due to carbonation than OPC. The ingress of CO2 within the porous mortar matrix is a diffusion controlled process. Subsequent chemical reaction between CO2 and cement hydration products (mostly calcium hydroxide [CH] and calcium silicate hydrate [CSH]) results in degradation of cement based materials. CH offers the buffering capacity against carbonation in hydrated cements. Partial substitution of OPC with pozzolanic materials however decreases the amount of CH in hydrated blended cements. Therefore, low amounts of CH in hydrated blended cements make them more susceptible to degradation as a result of carbonation compared to OPC. The magnitude of carbonation affects the service life of cement based structures significantly. It is therefore apparent that sufficient attention is given to carbonation process in order to ensure resilient cementitious structures. In this paper, an indepth review of the recent advances on carbonation process, factors affecting carbonation resistance, and the effects of carbonation on hardened cement materials have been discussed. In conclusion, carbonation process is influenced by internal and external factors, and it has also been found to have both beneficial and deleterious effects on hardened cement matrix. [ABSTRACT FROM AUTHOR]

Highlights • The variations in binder/water ratio can have a counterproductive effect on the mortar carbonation process. • The carbonation velocity in lime mortar is not related to the type of lime. The carbonation process slows down in most mortars at 90 days. • The flexural and compression resistance of traditional lime mortars increases gradually and in relatively short periods of time (around 90 days). • The type of Liesegang patterns observed in the lime mortar is directly related to the characteristics of the carbonation process of binder. • SPL mortars have a higher volume of macropores than CL mortars that are characterized by a higher microporosity. Abstract In recent years, numerous studies have been published on the technology of industrial aerial lime mortars. These papers refer to the incidence of carbonation process and mechanical behavior in its properties as construction material, of interest for its application in Cultural Heritage conservation. Among other issues, the importance of the type of aggregate (composition and granulometry) and relation binder/water (B/W) in its properties are highlighted. There are fewer papers in which the type of lime is about, especially on the basis of its manufacturing process (traditional or industrial), on the presentation of the product (powder or putty), or referred to the different types of mortars (prepared in situ or pre-dosed). In this paper, the behavior of aerial lime mortars elaborated with traditional lime has been determined and a comparative study has been carried out with pre-dosed mortars of powder (mCL) and in putty (mSPL) lime for its application in render mortar (B) and plaster mortar (F). The control parameters have been mechanical resistances-elasticity, ultrasound velocity, carbonation process and distribution pore size and surface area by mercury porosimetry. It is concluded that the behavior of traditional lime mortars, both in putty and powder, depends on the B/W ratio, the type of aggregates and the lime content. The importance of the application of the product in the building as the behavior of the mortar is very sensitive to kneading water content it shows; aspects that define the plasticity of the mortar. It is also concluded that the carbonation of traditional lime mortars is not slow and that, under optimal dosages conditions, can be finished before 90 days. It is also verified that a higher carbonation rate does not imply better properties of the mortar in its long-term construction. The utility of ultrasound velocity test as a non-destructive test and easy to apply on building for the quality control of traditional lime mortars and its consistence values it highlights, as is a macroscopic property of easy control during the elaboration of the product. [ABSTRACT FROM AUTHOR]

Calcium‐based sorbents have been regarded as effective agents for capturing CO2 from industrial flue gas. Recent studies have shown that steam can enhance the carbonation performance of calcium‐based sorbents. In this paper, a CaO (001) surface was made to investigate the micro‐level mechanism of steam‐enhanced carbonation based on first principles calculations. Charge transfer and bond population were calculated to evaluate an interaction effect between adsorbates and the CaO (001) surface. Individual adsorption of CO2 and H2O was compared with binary adsorption and co‐adsorption of the two molecules on the CaO (001) surface, based on dispersion‐corrected density functional theory (DFT‐D) calculations. First, the predicted adsorption energies suggest the O‐top site is the best site. It forms carbonate‐like structure and hydroxyl‐like structure for the individual adsorption of CO2 and H2O. Binary adsorption calculations indicate that H2O is more easily adsorbed by the CaO (001) surface than CO2. The adsorption of H2O and CO2 adsorption are promoted in comparison with their individual adsorption on the CaO (001) surface. Moreover, the analysis of adsorption energies and partial density of states (PDOS) suggests that a H2O‐CaO (001) surface (CaO (001) surface that has already adsorbed H2O) is more reactive than the clean CaO (001) surface for CO2 adsorption, which further supports the idea that the steam‐enhanced mechanism is an Eley–Rideal (E–R) mechanism, which means H2O is adsorbed on the CaO surface, and then CO2 is adsorbed. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper addresses the effect of concrete carbonation on the propagation and dispersion of electromagnetic (EM) waves and the capability of two EM, non-destructive techniques to detect this pathology. A capacitive technique operating at low frequency (around 33 MHz) and a ground penetrating radar (GPR) with a 1.5 GHz antenna were tested for the monitoring of reinforced concrete structures. To better understand the phenomena involved in concrete carbonation, the results of two complementary experimental campaigns were analyzed for saturated concretes. First, the dispersion curves of complex permittivity were measured for both carbonated and non-carbonated samples by a cylindrical coaxial EM cell. Due to carbonation, the permittivity decreased and the level of dispersion reduced slightly. Second, using GPR (coupled at approximately 900 MHz) and capacitive measurements conducted on controlled slabs, it was confirmed that the real part of the relative permittivity decreased within a range of 2 at 33 MHz and a range of 1 to 900 MHz, while the radar signal amplitude increased. [ABSTRACT FROM AUTHOR]

In this paper we deal with a one-dimensional free boundary problem, which is a mathematical model for an adsorption phenomena appearing in concrete carbonation process. This model was proposed in line of previous studies of three dimensional concrete carbonation process. The main result in this paper is concerned with the existence and uniqueness of a time-local solution to the free boundary problem. This result will be obtained by means of the abstract theory of nonlinear evolution equations and Banach's fixed point theorem, and especially, the maximum principle applied to our problem will play a very important role to obtain the uniform estimate to approximate solutions. [ABSTRACT FROM AUTHOR]

Gypseous soils are known as problematic soils from an engineering point of view. Many studies deal with collapsible soils and how to reduce the sudden collapse when the soil is soaked in water. The objective of this paper is to investigate the collapse potential when the collapsible gypseous soil is mixed with different percentages of Magnesium Oxide (0, 5, 10, and 15%) and carbonated with different carbonation periods (0, 1, 3, and 24 hours). The adequate Magnesium Oxide percentage was found to be 10% because its effect is close to that of 15% and the optimum carbonation period time is 3 hours. The 10% Magnesium Oxide used in treating the gypseous soil of relative density 35% without carbonation resulted in a reduction in collapse potential by 76% as compared with the natural soil. When the natural soil is carbonated at the relative density of 35% without adding Magnesium Oxide, the collapse potential decreased about 65% from no carbonation to 3-hour carbonation, but it decreased only 9% between the carbonation period of 3 hours and 24 hours. As for samples prepared at the relative density of 75%, the collapse potential decreased more than 77% from no carbonation to 3-hour carbonation, but it decreased only 8% between 3 hours and 24 hours. [ABSTRACT FROM AUTHOR]

Reduction of CO2 emission is essential. In this paper, a study of the behaviour of modified Polish CaO-based sorbents during calcium looping cycles has been discussed. All the related tests were conducted using the Netzsch STA 409PG Luxx thermogravimetric analyser. Samples with a weight of ms=10.0±0.1 mg were placed in an Al2O3 crucible. The calcium looping processes were performed at the carbonation temperature of 650°C and the calcination temperature of 900°C. Additionally, calcination-carbonation cycles with different gas flows were examined. The authors investigated the influence of CO2 concentration and total gas flow on the carbonation conversion. Sorbents were studied by TG and porosimetry analysis. [ABSTRACT FROM AUTHOR]

This paper reviews the long-term properties of recycled aggregate concrete (RAC), including long-term strength, shrinkage, creep, carbonation resistance, antifreeze resistance, impermeability, abrasion resistance, alkaline aggregate reactions, sulphate corrosion and fatigue behaviour. Most studies have shown that the long-term properties of RAC are inferior to those of natural aggregate concrete (NAC), and some researchers have observed that the long-term properties are better than those of NAC. RAC's long-term properties are affected by many factors such as recycled coarse aggregate (RCA) replacement percentage, water-cement ratio, mineral admixtures and mix proportions. The long-term properties of RAC can be improved through better control of these factors. This paper will be helpful for a comprehensive understanding of and further research on RAC, and provides an important basis and references for the engineering applications of RAC. [ABSTRACT FROM AUTHOR]

The article presents an application of X-ray microtomography for identification of the carbonation zone in concrete material. A concrete specimen subjected earlier to harsh environmental conditions is investigated. The material is firstly checked with the use of chemical corrosion indicators and then is subjected to microstructural analysis performed with the use of X-ray microtomography. Two different settings of scanning parameters are applied implying the image resolutions of approximately 14 μm per 1 pixel and about 7 μm per 1 pixel, respectively. The results obtained are then compared and analyzed. The depth of the carbonation zone is evaluated based on the attenuation curve. The paper highlights also the significance of the corrosion phenomenon in concrete structures. Details of the deterioration mechanisms in concrete are shortly presented. [ABSTRACT FROM AUTHOR]

This paper is part of a study on earthen mixtures for the 3D printing of buildings. To meet the ever increasing environmental needs, the focus of the paper is on a particular type of biocomposite for the stabilization of earthen mixtures—the rice-husk–lime biocomposite—and on how to enhance its effect on the long-term mechanical properties of the hardened product. Assuming that the shredding of the vegetable fiber is precisely one of the possible ways to improve the mechanical properties, we compared the results of uniaxial compression tests performed on cubic specimens, made with both shredded and unaltered vegetable fiber, for three curing periods. The results show that the hardened earthen mixture is not a brittle material, in the strict sense, because it exhibits some peculiar behaviors that are anomalous for a brittle material. However, being a "designable" material, its properties can be varied with a certain flexibility in order to become as close as possible to the desired ones. One of the peculiar properties of the hardened earthen mixture deserves further investigation, rather than corrections. This is the vulcanization that occurs (in a completely natural way) over the long term, thanks to the mineralization of the vegetable fiber by the carbonation of the lime. [ABSTRACT FROM AUTHOR]

In the present paper, spherical porous N-doped carbon nanomaterials were synthesized using polyaniline nanofiber as nitrogen-carbon precursor through spray drying and high temperature carbonation. In the process of spray drying, polyaniline fibers were entangled to form spherical nanomaterials with pore structures. The average diameter of the carbon spheres prepared by spray drying was about 3 μm. After high temperature carbonation, the diameter shrunk to 2 μm. The catalyst (CSDP-900) which was fabricated by spray drying and high temperature carbonation at 900 °C exhibited a high onset potential and a nearly four-electron pathway for oxygen reduction reaction (ORR) in 0.1M KOH solution as well as stronger methanol tolerance and better long-term durability than commercial Pt/C. This paper provides a novel, easy and scalable method to prepare N-doped porous carbon spheres with excellent ORR catalytic properties. [ABSTRACT FROM AUTHOR]

The paper presents the results of carbonation resistance of Self-Compacting Concrete (SCC) made with coarse recycled concrete aggregates and fine recycled concrete aggregates. To evaluate the carbonation resistance of SCC mixes, two different groups consisting of nine SCC mixes were prepared. The impact of coarse recycled concrete aggregates and fine recycled concrete aggregates has been quantified based on the observed performance of SCC mixes of Group-I whereas, the influence of Silica Fume (SF) has been observed on the SCC mixes of Group-II. The incorporation of SF in various SCC mixes not only improves the compressive strength, the carbonation resistance is significantly increased. It has also been observed with the scanning electron microscopy analysis that the incorporation of SF in SCC mixes containing different contents of recycled concrete aggregates, resulted in the production of SCC mixes, with higher/comparable performance to that of control SCC mix. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of magnesium oxide (MgO) on the carbonation resistance of alkali-activated slag-fly ash blend containing 75% ground granulated blast furnace slag (GGBS) and 25% low-calcium fly ash. Two types of GGBS were used with different magnesium oxide content. Phenolphthalein indicator and pH profiles showed that the GGBS with higher levels of magnesium oxide offered no significant improvement in resistance against natural and 1% accelerated carbonation. X-ray diffraction confirmed no hydrotalcite formation, although the magnesium oxide content was 9·1%. A very small amount of free magnesium ions (Mg2+) was available in the pore solution, which was deemed insufficient to form hydrotalcite. Lack of its formation was the major reason for the lower carbonation resistance. Excessive silicate in the system reduces the calcium oxide/silicon dioxide ratio, which leads to the incorporation of magnesium ions in the calcium silicate hydrate structure. Hydrotalcite was observed when the activator concentration was reduced. The results suggest that in addition to magnesium and aluminium ion (Al3+) availability, silicate concentration also plays a strong role in deciding the hydrotalcite formation in alkali-activated GGBS concrete. [ABSTRACT FROM AUTHOR]

The heterogeneity of a cement-based material results in a random spatial distribution of carbonation depth. Currently, there is a lack of both experimental and numerical investigations aiming at a statistical understanding of this important phenomenon. This paper presents both experimental and numerical supercritical carbonation test results of cement mortar blocks. The carbonation depths are measured along the carbonation boundary by the proposed rapid image processing technique, which are then statistically studied by calculating, e.g., their probability density and power spectral density (PSD). The results indicate that the distribution of the carbonation depth can be approximately represented by a lognormal distribution function and the PSD has quantitative correlation with some of the statistic parameters used in the simulations. In particular, the effects of the autocorrelation lengths and the coefficient of variation of porosity, which are used to define the random porosity field, on the irregularity of carbonation depth are analyzed numerically in details and validated by experimental results. The study has shown that using a random field of porosity with due consideration of spatial correlation and variance, the irregularity of carbonation depth can be realistically captured by the numerical model. The numerical results confirm that lognormal distributions represent the random nature of carbonation depth well and the average and variance of the irregular carbonation depth increase with the increase of carbonation time, autocorrelation length and coefficient of variation of porosity. The study also offers a potential method to numerically calibrate some of the statistic parameters required by a numerical carbonation model through comparing the PSD with that from experimental tests. Overall the methodology adopted in the paper can provide a foundation for future investigations on probability analysis of carbonation depth and other similar work based on multi-scale and -physics modelling. [ABSTRACT FROM AUTHOR]

Abstract: Two heptanuclear discs, [Co7(L1)6(CH3O)6]2+ (Co7‐L1) and [Co7(L2)12](ClO4)2 (Co7‐L2), have been prepared by the reaction of [Co(H2O)6](ClO4)2 with Schiff base HL1 (HL1=2‐methoxy‐6‐[(methylimino)methyl]phenolate) or benzimidazole derivative HL2 (HL2=(1H‐benzo[d]imidazol‐2‐yl)methanolate), respectively, at room temperature. In contrast, the reaction of [Co(H2O)6](ClO4)2 with a mixture of the two ligands, having similar coordination ability but different shapes, resulted in the butterfly‐structured tetranuclear cobalt cluster [Co4(L1)2(L2)4](ClO4)2⋅2H2O (Co4). Electrospray ionization mass spectrometry (ESI‐MS) analysis in real time revealed the {Co1}→{Co2}→{Co4}→{Co7} stepwise assembly when a single ligand was employed, but when both ligands were used only {Co4} was obtained, an intermediate of the former reactions. Interestingly, ligand competition was evidenced in the assembly process of the reaction with two ligands; for example, [Co4(L1)4(L2)2]2+ and [Co4(L1)3(L2)3]2+ were the primary clusters observed at the beginning of the reaction, but [Co4(L1)2(L2)4]2+ was the final product. This observation suggests ligand exchange is taking place, and DFT calculations confirmed that these transformations were possible. By varying the ratio of ligands, the abundance of a certain species in the competitive reaction could be controlled. [ABSTRACT FROM AUTHOR]

Recognition of the rising amount of atmospheric CO 2 has brought renewed interest in understanding the effects of carbonation on reinforced concrete performance. In laboratory testing, the specimens must be preconditioned to effectively study carbonation. This paper studied the influence of several preconditioning schemes on the carbonation profiles of cement paste specimens subjected to accelerated carbonation tests. The evolution of microstructure and moisture during carbonation were investigated accordingly. Bulk of the work was based on an extended X-ray attenuation method (XRAM), which relied on X-ray computed tomography (CT). A novel method was introduced to evaluate the extent of damage due to drying. Based on extent of damage, the paper recommends standard-cured specimens for carbonation tests as compared to water-cured specimens. Also, when comparing between oven drying and mass balancing, the latter was shown to be more suitable, as the inner moisture distribution becomes more uniform after this drying protocol, and less fluctuation of humidity will occur during carbonation. [ABSTRACT FROM AUTHOR]

Abstract: An alternative of wellbore cement system, formed through alkali‐activation of fly ashes also knows as fly ash geopolymers, provides attractive properties due to its high compressive strength and durability against acidic conditions. However, changes in microstructure and mineralogical during accelerated carbonation affected by CO2 alteration under elevated temperature and pressure have not been fully understood. This paper investigates and characterizes the alkali‐activated binders fly ash based in the acidic environment under elevated temperature and pressure. The results found that scales such a layer was formed in the gel phase at the spherical surface of fly ash after carbonation process. The effect is that the surface of the gel phase became rough and porous. Carbonate and/or bicarbonate salts were identified as major phase in the sample after immersion test. In the activated fly ash cement, both calcium aluminate silicate hydrate (C–A–S–H) and sodium aluminosilicate hydrate (N–A–S–H) gels were formed; C–A–S–H gel was decalcified, forming calcium carbonate phase in the form of calcite, aragonite or vaterite. These findings were confirmed by the analysis and identification of Fourier‐transformed infrared spectroscopy, X‐ray diffraction and scanning electron microscopy tests on the changes in the cement system. [ABSTRACT FROM AUTHOR]

Accurate prediction of concrete carbonation is important for the correct assessment of both durability and environmental impact of reinforced concrete (RC) structures. Loading applied to a RC structure and concrete cracking caused by this loading may significantly affect the concrete carbonation process. However, so far these factors have received little attention of researchers, especially this concerns ‘green’ concretes, i.e. concretes in which Portland cement (PC) is partially replaced by supplementary cementitious materials such as fly ash (FA) and ground granulated blast-furnace slag (GGBS). Thus, the aim of the study presented in the paper was to experimentally investigate the influence of static loading and associated concrete cracking on carbonation of RC elements made of PC concretes and ‘green’ concretes containing significant amounts FA and GGBS. For this purpose, six concrete mixes with two water/binder (w/b) ratios (0.40 and 0.55) and different proportions of PC, FA and GGBS were prepared. The mixes were used to cast twelve RC beams (100 × 120 × 900-mm) and a larger number of 100-mm concrete cubes. The beam specimens were loaded in four-point bending to produce flexural cracks of maximum width of either 0.1 mm or 0.3 mm. The loaded beam specimens along with unloaded cube specimens were then placed into a carbonation chamber and subject to accelerated carbonation for 120 days. After that the carbonation depths in the beams and cubes were measured. Results of the tests show a significant effect of load induced stresses (both tensile and compressive) on the carbonation resistance of the concretes, especially of ‘green’ concretes. The influence of cracking on concrete carbonation was also observed and discussed in the paper. [ABSTRACT FROM AUTHOR]

In this paper we define and study a finite volume scheme for a concrete carbonation model proposed by Aiki and Muntean in [Adv. Math. Sci. Appl.19 (2009) 109–129]. The model consists in a system of two weakly coupled parabolic equations in a varying domain whose length is governed by an ordinary differential equation. The numerical sheme is obtained by a Euler discretisation in time and a Scharfetter-Gummel discretisation in space. We establish the convergence of the scheme. As a by-product, we obtain existence of a solution to the model. Finally, some numerical experiments show the efficiency of the scheme. [ABSTRACT FROM AUTHOR]

This paper studies hydration stages and phase transformation mechanism of the Sr2+-doped calcium zirconium aluminate cement at room temperature. Features of different hydration stages of this cement paste are identified by X-ray diffraction, scanning electron microscopy, differential thermal analysis, thermogravimetric analysis, evolved gas analysis and heat evolution test. It was found that the partial isovalent substitution of Ca2+ for Sr2+ such as Ca7−xSrxZrAl6O18, with x = 0.3, 0.6, 1.0, reduced the hydraulic reactivity of Ca7ZrAl6O18 phase. The course of hydration of mixed oxides of the type 6CaO·SrO·3Al2O3·ZrO2 documented using microcalorimetry was supported by investigations of the solid hydration products. Research showed that the hexagonal hydrates were stable at early and middle curing ages, an indication that including Sr in solid solution could effectively inhibit the conversion from both C4AH13–19 and C2AH8 to C3AH6 and AH3. Calcium monocarboaluminate C4AC¯H11 phases were also found in this system due to the carbonation process of C4AH13 phase. Substitution of Ca2+ ions by Sr2+ ions in hexagonal calcium aluminate hydrates causes structural disorder and then contributes to the broadening of lines in the powder X-ray diffraction patterns of layered structures of metastable hydrates. At later curing ages, the formation of two hydrogarnet phases, one Sr-rich (C,Sr)3AH6 and the other Ca-rich (C,Sr)3AH6, was proved. [ABSTRACT FROM AUTHOR]

The knowledge of the soil variability is essential for its hydrologic characterization, for irrigation management and planning and especially for water movement modeling. Nevertheless, the scarcity of soil information in Spain precludes its use in irrigation management and, particularly, in the on-going irrigation modernization process. The Violada Irrigation District (VID) comprises 5234 ha and is located in a semi-arid gypsum-rich region in northeast Spain. VID has been under flood irrigation since the mid-20th Century and has been modernized recently to sprinkler irrigation. Since the early 1980s water balances in VID have been based on average soil properties and bulk irrigation data. The current irrigation system provides detailed irrigation data at plot level, thus allowing for performing distributed balances (in hydrologic homogenous zones) that require detailed or semi-detailed soil information in the form of a soil map. The objective of this two-part study is to define homogeneous zones in relation to hydrological properties relevant for irrigation management, first defining the soil variability within the VID and then analyzing the soils hydrologic and salinity features (in a companion paper). The specific objectives of this paper were (i) to draw the VID semi-detailed soil map; (ii) to explain the variability of soil properties; and (iii) to map the main soil characteristics related to irrigation management. Thirty four soil pits, located on all the different geomorphological areas, were opened and described. Samples were taken from all the horizons and analyzed for chemical and physical properties. The limits of soil units were drawn with the aid of 32 additional auger holes, the data from 72 pits opened during the irrigation transformation works, and all the previous soil information available. Finally the soils were classified down to family level. The main soil formation processes in VID were carbonation and gypsification. Thirteen soil units were defined, included in five soil subgroups and six particle size families. “Fine” particle sizes were dominant along the gypsum-rich valley bottoms. Coarser textures were found on the heights and showed higher CaCO 3 equivalent content and no presence of gypsum accumulations. A priori, the main limitation for irrigation in the valley soils was the limited drainage (due to fine textures and reduced slope) while in the highland soils it might be constrained by their low retention capacity. The high gypsum and carbonate content of the VID soils may prevent the development of infiltration problems. The soil properties map in this work will be the basis for a companion paper delineating the hydrologic properties of the soils, the basis for improved soil water balances. [ABSTRACT FROM AUTHOR]

We consider a new two-scale problem which is given as a mathematical model for moisture transport arising in a concrete carbonation process. In research for moisture transport, it is a crucial step how to describe the relationship between the relative humidity and the degree of saturation, mathematically. Here, we have proposed the two-scale model consisting of the diffusion equation for the relative humidity in a macro domain and the free boundary problems describing the relationship in infinitely micro domains. Accordingly, the structures of the micro domains are unknown in our model. This is a significant feature of our new model to emphasize. The aim of this paper is to establish local existence in time and uniqueness of a solution to the model. [ABSTRACT FROM AUTHOR]

This paper presents an in-situ inspection and experimental investigation on the pore solution pH of concrete cover for the corrosion initiation of rebars under a long-term natural carbonation reaction. A 77-year-old steel mill was inspected, and 35 concrete column cover samples were collected to test the pH of the pore solution and phase compositions layer by layer. The variation in pH and phase compositions of the concrete along the cover depth was studied. The in-situ inspection results indicate that the rebar embedded in concrete had begun to corrode when the carbonation depth was almost less than one-third of the cover depth. The corrosion initiation of rebars embedded in concrete can occur when the pH is between 11.3 and 12.1. The pore solution pH test results and X-ray diffraction (XRD) analysis indicated that there is a semi-carbonated zone between the fully carbonated zone and the rebar. The pH of a fully carbonated zone is in a range of 8.0-9.5, and the pH of a semi-carbonated zone is between 9.5 and 12.1. [ABSTRACT FROM AUTHOR]

Determining the carbonation depth is of paramount importance in assessing the durability of cementitious materials as the carbonation process alters their near-surface physicochemical properties. This paper introduces a novel quantitative ultrasound (QUS) technique capable of nondestructively identifying the carbonation depth, regardless of the tortuosity of the carbonation front. A 2.5 MHz phased array transducer was employed to construct beamformed radio frequency images and measure two microstructure-dependent parameters – spectral slope (SS) and spectral intercept (SI) – based on backscatter and attenuation coefficients. The change in obtained SS and SI were then visualized as QUS images, which displayed the geometry of the carbonation front, correlating with images constructed using a phenolphthalein solution (1.6 mm difference in depth). Mineral compositions and porosities were analyzed to account for microstructural changes in each layer. This study opens an opportunity for nondestructive detection of carbonation depth, overcoming the limitations in conventional methods or regression-based approaches. • Quantitative ultrasound (QUS) technique is developed for detecting carbonation depth. • QUS images achieved based on scattering and attenuation phenomena confirm densification in carbonated layer. • Upon the QUS images, the measured carbonation depth shows great agreement with results from phenolphthalein solution. • Mineral compositions and porosities were evidenced by TG, MIP, XRD, and QXRD analyses. • The proposed method offers potential for visualizing ambiguous geometry of inclusions in cementitious materials. [ABSTRACT FROM AUTHOR]

Concrete carbonation has an important influence on chloride penetration. The chloride distribution profiles found in carbonated concrete are often different from those found in uncarbonated concrete even when they have the same exposure conditions. In this paper we present an experimental study on chloride penetration in carbonated concrete specimens when they are subjected to stationary, splash, submerged, and pressured exposure conditions. Free chloride concentration profiles were measured in the specimens to examine the effect of carbonation and exposure condition on chloride diffusion. In addition, material characterisations were carried out using x-ray radiography imaging tests and thermogravimetric analysis to understand the influence mechanism of concrete carbonation on chloride penetration in concrete. The experimental results showed that the chloride distribution curve in carbonated concrete exhibits a second peak under marine exposure conditions. The exposure condition can significantly affect the position and value of the second peak point in the profile. It is also shown that concrete carbonation can drastically change the distribution of pores in concrete, leading to a sudden change in porosity, which is the reason for the carbonated concrete to have the zone-dependent chloride variation profiles. • The chloride distribution curve in carbonated concrete exhibits a second peak under marine exposure conditions. • The exposure condition can significantly affect the position and value of the second peak point in the profile. • The different carbonation zones can be identified based on the distribution curve of the calcium carbonate. • Change in porosity caused by carbonation leads to a zone-dependent chloride variation profiles. [ABSTRACT FROM AUTHOR]

Exploring the mineral compositions of steel slag and the microstructure evolutions of its carbonation products is very important for deeply understanding the carbonation mechanism of steel slag, which can promote the application of steel slag in carbon capture and storage. In this paper, the carbonation behaviors of various mineral phases in steel slag were investigated by using an ingenious in-situ observation method through combing laser scanning confocal microscopy (LSCM), backscattered scanning electron microscopy (BSEM) and focused ion beam-transmission electron microscope (FIB-TEM). It is shown that dicalcium silicate (C 2 S) and free lime (f-CaO) had high carbonation reactivity, while the calcium aluminoferrite with different Al/Fe ratios and mayenite (C 12 A 7) exhibited extremely poor carbonation reactivity. The carbonation reactivity of RO phase was positively correlated with its Mg/(Fe + Mn) ratio. RO phase with a Mg/(Fe + Mn) ratio below 0.5 owned almost no carbonation reactivity, while the Mg2+ would leach from RO phase with a Mg/(Fe + Mn) ratio higher than 2.0 during the carbonation process, inhibiting the precipitation of CaCO 3 , changing the micro morphology of CaCO 3 , and generating spheroidal aragonite. The ingenious in-situ observation method developed in this study provides a very visual information, which can be further extended to more fundamental research. • In-situ investigation on the carbonation behaviors of various mineral phases in steel slag was studied. • Calcium aluminoferrite with different Al/Fe ratio and C 12 A 7 exhibited extremely poor carbonation reactivity. • The carbonation reactivity of RO phase was positively correlated with its Mg/(Fe + Mn) ratio. • Mg2+ would leach from high Mg RO phase during the carbonation process, changing the micro morphology of CaCO 3. [ABSTRACT FROM AUTHOR]

Due to the depletion of river sand, the construction industry is eager to develop upcycling techniques for transforming secondary by-products derived from construction and demolition (C&D) waste into quality fine aggregates. This paper presents a study of replacing river sand with enhanced recycled fine aggregate through a wet carbonation process developed by the authors previously. The fine recycled concrete aggregate (FRCA) ranging from 0.15 to 5 mm was prepared by demolishing a concrete with a known mixture design. After wet carbonation, the particle size, water absorption, and density of the FRCA were tested and compared with the original samples. The chemical characteristics of the original and carbonated FRCA (C-FRCA) were analyzed by a series of experiments. The results showed that (1) an increase of carbonation products and a significant reduction of hydration products; (2) microscopic observation of the C-FRCA showed a surface layer densified by calcite after wet carbonation; and (3) no significant strength loss were observed when replacing up to 50% river sand by C-FRCA in mortar specimens. The potential environmental and economic impacts were also analyzed. [Display omitted] • Wet carbonation significantly improved the quality of FRCA. • The carbonation efficiency of wet carbonation excelled conventional carbonation methods. • Up to 50% river sand replacement ratio was achieved by carbonated FRCA. [ABSTRACT FROM AUTHOR]

Ammonium carbonate was used as the carbonation accelerator for air-hardening lime mortars in this study. Various analytical methods, including XRD, IR, TGA-DSC and SEM were employed to access the phase transformation, degree of carbonation and microstructural features of the lime-based mortars. The paper explores the correlation between the mechanical strength, microstructure of cured lime mortars and the composition/carbonation degree with the crystallization behaviors in Ca(OH)2/CaCO3 binders. The results indicate that ammonium carbonate significantly accelerates the carbonation reaction and increases the early mechanical strength of air-hardening lime mortars. The mechanical strength of these modified lime mortars can be comparable to that of natural hydraulic lime (NHL). The aggregation behaviors of nano-CaCO3 under kinetic control, are believed to contribute much to the compact pore-structures and robust mechanical strength of lime mortars. This viable accelerating carbonation method holds promise for enhancing the properties of various lime-containing building materials in conservation and modern buildings. • Ammonium carbonate is effective in accelerating the carbonation of aerial lime. • Lime mortars modified by ammonium carbonate rival natural hydraulic lime (NHL). • The mechanical strength is highly related to carbonation degree and pore structure. • Nano-CaCO 3 play pivotal role in compacting the pore structure of lime mortar. [ABSTRACT FROM AUTHOR]

[Display omitted] • This study used synthetic pellets to amplify the fly ash carbonation reaction. • Physical encapsulation is the main mechanism for solidifying heavy metals. • Carbonation depth determined heavy metal solidification rate. • Shrinking core model can be applied to carbonation and leaching. • Leaching of heavy metals from carbonized fly ash can be divided into three stages. The physical encapsulation mechanism prevents the leaching of heavy metals by forming calcium carbonate-encapsulated fly ash through the reaction of carbon dioxide and calcium oxide. However, current research in this area primarily involves qualitative analysis, focusing on various methods to enhance the fixation effect of heavy metals. There is a lack of quantitative description regarding how physical encapsulation solidifies heavy metals, and the solidification mechanism of heavy metals is challenging to verify through macroscopic experiments. In this paper, a method for synthesizing small fly ash particles into pellets is proposed to quantitatively study the mechanism of physically encapsulating heavy metals. The data from the carbonation reaction and leaching reaction are fitted using the shrinking core model. The depth of carbonation increases with the lengthening of carbonation time, and the physical encapsulation strengthens the immobilization of heavy metals. The leaching of heavy metals after carbonation can be categorized into three stages based on the shrinking core model: external diffusion, internal diffusion, and chemical reaction control. Synthetic pellets enhance all three stages, especially the external diffusion, which is not detectable with fly ash powder. The fitting results of the experimental data for the carbonation reaction of the enlarged synthetic pellets and the shrinking core model are both above 0.9. This indicates that the primary barrier to leaching of heavy metals is the physical encapsulation formed by the carbonation reaction following the shrinking core model. The physical encapsulation serves as the primary mechanism for the solidification of heavy metals during the carbonation process. [ABSTRACT FROM AUTHOR]

Accelerated carbonation of recycled aggregates (RAs) enables fast carbon sequestration while improving engineering properties of prepared concrete. This paper presents a systematic experimental study on the fracture behaviour of concrete with carbonated recycled coarse and fine aggregates (RCA and RFA), in terms of fracture parameters and fracture process zone (FPZ). The relevant influencing mechanism was investigated by relating the macroscopic fracture responses to microstructural features. Results indicate that carbonated RFA mainly enhanced the initial fracture toughness of concrete, while carbonated RCA primarily increased its unstable fracture toughness. The enhancement of fracture energy was closely associated with carbonated RCA and RFA. The utilization of RFA resulted in the looseness and higher porosity of mortar matrix, which reduced the crack initiation resistance of concrete, mainly affecting its initial cracking stage. The incorporation of RCA reduced crack bifurcation and deflection of concrete, leading to a smaller width of FPZ, which made the initial descending segment of the tension softening curve (TSC) steeper and its tail segment shorter. The carbonation treatment of RAs improved the densification of the adhered old mortar and reduced the porosity of prepared concrete, rising the cracking resistance and inducing a wider FPZ. As a result, the initial descending segment of TSC became flatter and its tail segment tended to be longer. • Roles of carbonated recycled fine and coarse aggregates in concrete fracture are explored. • Fracture process zone is systematically characterized based on DIC and inverse analysis. • Fracture mechanism is discussed from both microscopic and macroscopic viewpoints. • Recycled fine and coarse aggregates respectively affect the crack initiation and propagation. [ABSTRACT FROM AUTHOR]

This study describes the application of wollasonite microfibres for stabilising soil with the additional function of sequestering CO2. The high aspect ratio, needle-like structure of wollasonite imparted a microfibre mechanical reinforcement whilst the associated high surface area promoted carbonation. The originality of this paper lies in two unique aspects: the first stage assessed the efficacy of incorporating wollastonite microfibres inside the soil mass, while the second stage addressed the mechanical performance of the fibre-reinforced soil after different CO2 pressures and carbonation times. In these two stages, the unconfined compressive strength (UCS), indirect tensile strength (ITS) and flexural strength (FS) were determined. The test results indicated that the inclusion of the fibres increased the peak and post-peak response during unconfined compressive strength (UCS) tests, while also improving the ITS and FS. The UCS peak stress was further improved when the fibre-reinforced soil was subjected to the carbonation process. This work impacts the soil stabilisation industry through a novel soil strengthening process that also promises an effective route to combat climate change through sequestration of CO2. [ABSTRACT FROM AUTHOR]

This paper investigates the mineral sequestration of carbon dioxide in circulating fluidized bed combustion (CFBC) boiler bottom ash. CFBC bottom ash, which originated from two sources, was prepared along with pulverized coal-fired (PC) boiler bottom ash as a control. These ashes were exposed to accelerated carbonation conditions at a relative humidity of 40% and 100%, in order to investigate the effects of humidity on the carbonation kinetics of the bottom ash. The obtained results showed that not only lime but other calcium-bearing phases (gehlenite, wollastonite, and brownmillerite) in CFBC bottom ash participated in the mineral carbonation reaction. In particular, these phases underwent hydration in a wet carbonation environment, whereby the carbon dioxide uptake and capacity of CFBC bottom ash are significantly enhanced. This study may have important implications, demonstrating the feasibility of carbon dioxide sequestration and recycling of CFBC boiler bottom ash. [ABSTRACT FROM AUTHOR]

The paper is focused on the deterioration of industrial reinforced concrete chimneys caused by carbonation. It is considered that a chimney has to be repaired when more than 30% of its surface is affected by visible corrosion-induced cracks. An optimal maintenance strategy aims at the postponement of this state beyond the intended service life of the chimney with minimum maintenance costs. [ABSTRACT FROM AUTHOR]

This paper presents a critical review of existing studies on the effects of using sea-sand and/or seawater as raw materials of concrete on the properties of the resulting concrete, including its workability, short- and long-term strength as well as durability. It has been shown by existing research that concrete made with sea-sand and seawater develops its early strength faster than that of ordinary concrete, but the former achieves a similar long-term strength to the latter. Existing studies have also shown that the use of sea-sand and seawater may have a significant effect on chloride-induced steel corrosion but has only a negligible effect on the carbonation process of concrete. Strong evidence exists that a combination of mineral admixtures for the concrete and reinforcement with fiber reinforced polymer (FRP) can effectively solve the durability problem associated with the abundance of chloride ions in sea-sand seawater concrete (SSC). Such use of SSC also offers a good opportunity for the incorporation of recycled coarse aggregate (RCA) in concrete, particularly those that have been chloride-contaminated, as has been demonstrated by some preliminary research. The current understanding of the behavior of SSC, as summarized in the present paper, provides a solid basis for further research in the area to enable the wide use of SSC in concrete construction worldwide, particularly when combined with FRP as the reinforcing material. [ABSTRACT FROM AUTHOR]

Concrete durability is a key aspect for forecasting the expected life time of concrete structures. In this paper, the effect of compressive strength and durability of concrete containing metakaolin developed from a local natural material (Soorh of Thatta Distict of Sindh, Pakistan) is investigated. Soorh is calcined by an electric furnace at 800°C for 2 hours to produce metakaolin. One mix of ordinary concrete and five mixes of metakaolin concrete were prepared, where cement is replaced by developed metakaolin from 5% to 25% by weight, with 5% increment step. The concrete durability was tested for water penetration, carbonation depth and corrosion resistance. The obtained outcomes demonstrated that, 15% replacement level of local developed metakaolin presents considerable improvements in concrete properties. Moreover, a considerable linear relationship was established between compressive strength and concrete durability indicators like water penetration, carbonation depth and corrosion resistance. [ABSTRACT FROM AUTHOR]

It was previously revealed that the total CO concentration in seawater decreased during the Late Archean. In this paper, to assess the secular change of total CO concentration in seawater, we focused on the Paleoproterozoic era when the Earth experienced its first recorded global glaciation. The 2.4 Ga Ongeluk Formation outcrops in the Kaapvaal Craton, South Africa. The formation consists mainly of submarine volcanic rocks that have erupted during the global glaciation. The undeformed lavas are mostly carbonate-free but contain rare disseminated calcites. The carbon isotope ratio of the disseminated calcite (δC vs. VPDB) ranges from − 31.9 to − 13.2 ‰. The relatively low δC values clearly indicate that the carbonation was partially contributed by C-depleted CO derived from decomposition of organic matter beneath the seafloor. The absence of δC higher than − 13.2‰ is consistent with the exceptionally C-depleted CO in the Ongeluk seawater during glaciation. The results suggest that carbonation occurred during subseafloor hydrothermal circulation just after the eruption of the lavas. Previously, it was reported that the carbonate content in the uppermost subseafloor crust decreased from 3.2 to 2.6 Ga, indicating a decrease in total CO concentration in seawater during that time. However, the average CO (as carbonate) content in the Ongeluk lavas (< 0.001 wt%) is much lower than those of 2.6 Ga representatives and even of modern equivalents. This finding suggests that the total CO concentration in seawater further decreased during the period between 2.6 and 2.4 Ga. Thus, the very low content of carbonate in the Ongeluk lavas is probable evidence for the extremely low CO concentration in seawater during the global glaciation. Considering that the carbonate content of the subseafloor crusts also shows a good correlation with independently estimated atmospheric pCO levels through the Earth history, it seem highly likely that the low carbonate content in the Ongeluk lavas reflects the low atmospheric pCO at that time. We conclude that the continuous decrease in CO concentration of seawater/atm. from 3.2 Ga was one of the contributing factors to the Paleoproterozoic global glaciation. [ABSTRACT FROM AUTHOR]