Sodium Sulfate Solution Research Articles

Long-Term Performance of Concrete Made with Different Types of Cement under Severe Sulfate Exposure

Concrete sulfate attack is of great interest as it represents one of the main reasons of concrete deterioration and poor durability for concrete structures. In this research, the effect of different cement types on concrete sulfate resistance was investigated. This included three concrete classes, namely, low strength concrete, medium strength concrete, and high strength concrete. Blast furnace cement (BFC), sulfate resisting Portland cement (CEM I-SR5), and ordinary Portland cement (OPC) were used in a total of eighteen concrete mixes. Three binder contents of 250 kg/m3, 350 kg/m3, and 450 kg/m3 and a constant silica fume (SF) content were applied in this experimental study. The water/binder (w/b) ratio was varied between 0.4 and 0.8. Concrete specimens were immersed in highly severe effective sodium sulfate solutions (10,000 ppm) for 180 days after standard curing for 28 days. The fresh concrete performance was evaluated through a slump test to attain proper workability. Concrete compressive strength and mass change at 28 days and 180 days were measured before and after immersion in the solution to evaluate the long-term effect of sulfate attack on the proposed concrete durability. Scanning electron microscopy (SEM) analysis was conducted to study the concrete microstructure and its deterioration stages. The obtained results revealed that BFC cement has the best resistance to aggressive sulfate attacks. The strength deterioration of BFC cement was 3.5% with w/b of 0.4 and it increased to about 7.8% when increasing the w/b ratio to 0.6, which are comparable to other types of cement used. The findings of this research confirmed that the quality of concrete, specifically its composition of low permeability, is the best and recommended protection against sulfate attack.

Surface treatment with silicon-based nanoparticles in Portland cement specimens subjected to physical sulfate attack

The use of simple (SN) or functionalized (FN) silicon nanoparticles in three variants of surface treatments to mitigate the physical sulfate attack was evaluated in mortar specimens made with ordinary portland cement (OPC) or sulfate resistant cement (SR). The treatment variants were: T1 – with a 0.1% suspension of SN, applied by immersion, in the first 72 h of cure; T2 – with a 0.1% suspension of FN, applied by brushing after 28 days of hydration; and T3 – consecutive application of T1 and T2. Subsequently, the specimens were partially immersed in a 5% sodium sulfate solution and subjected to wetting and drying cycles. A physical-chemical characterization was carried out utilizing XRD, XRF, and SEM/EDS and the surface contact angle was determined regularly during the exposure. The results were complemented with a visual analysis of the specimens at the end of the exposure. The results showed that, although the specimens treated with T1, T2 and T3 show a certain degree of deterioration caused by the physical sulfate attack, the treatments have contributed to preventing the entry of the sulfate solution at the beginning of the exposure. Therefore, it is concluded that the treatments are effective in the initial stages of exposure to the physical sulfate attack.

The corrosion characteristics and electrochemical conservation treatment for an archaeological lead ossuary from Jordan

This study investigates the composition and corrosion of a lead ossuary excavated from the archaeological cemetery located on the site of the Queen Alia International Airport, Jordan. It also presents its conservation treatment using the potentiostatic reduction method. X-ray fluorescence (XRF) and scanning electron microscopy/ energy dispersive X–ray spectroscopy (SEM/EDX) analyses showed that the ossuary was made from a relatively pure lead metal. Examination of the corrosion products using X–ray diffraction (XRD) and SEM/EDX analyses indicated the presence oxides, carbonates and chlorides of lead as the main corrosion products on the ossuary. The metallographic examination showed a cast α-Pb dendritic microstructure. A linear sweep voltammetry (LSV) scan for a small fragment from the ossuary in a sodium sulfate solution allowed identifying the working conditions to reduce the corrosion products to lead metal. Chronoamperometry (CA) techniques helped to monitor the reduction process and determine its completion. The potentiostatic methods showed an excellent effectiveness in determining treatment parameters and monitoring the reduction process of the corrosion products on the ossuary and to prevent over-treatment. The reduced ossuary was finally coated with Paraloid B72 to prevent corrosion reoccurrence in the poor environmental conditions of the storage area.

Protective effect of 7-hydroxyl-1-methylindole-3-acetonitrile on the intestinal mucosal damage response to inflammation in mice with DSS-induced colitis

Ulcerative colitis (UC), a pathological condition of inflammatory bowel disease, is a chronic inflammatory disorder that involves an abnormal immune response and epithelial barrier dysfunction. Although we have previously reported the anti-inflammatory effects of 7-hydroxyl-1-methylindole-3-acetonitrile (7-HMIA), a synthesized analog of arvelexin on macrophages and paw edema, its anti-colitis effect and its mechanism are not known. In this study, colitis was induced in mice model by 4% (w/v) dextran sodium sulfate (DSS) solution in drinking water for 9 days. At the same time, from the first day of administering drinking water containing DSS, the animals were treated with 5-aminosalicylic acid (5-ASA), 75 mg/kg/day, orally) or 7-HMIA (10 or 20 mg/kg/day, intraperitoneally), depending on the experimental group, respectively. The studies were terminated on the tenth day of the experiment. Our data showed that 7-HMIA reduced the disease activity index and spleen/body weight (S/B) ratio, and improved the shortened colon length comparable to the effects of 5-ASA observed in the DSS-exposed mice. 7-HMIA, like 5-ASA, inhibited the histological damage, such as a thickened colonic muscle layer and shortened crypt length in the colon of the mice with DSS-induced colitis. 7-HMIA restored the tight junction-related proteins (occludin, claudin-1, and claudin-2) and epithelial-mesenchymal transition-mediated proteins (E-cadherin, N-cadherin, and vimentin) in the colon tissue of mice with DSS-induced colitis. Additionally, 7-HMIA (20 mg/kg/day) showed the inhibitory effects similar to that of 5-ASA on the myeloperoxidase activity, interleukin (IL)-6 production, and expression levels of inducible nitric oxide synthase (iNOS), and even showed greater inhibition of IL-1β production in the DSS-induced mice. Furthermore, the DSS-induced activation of nuclear factor-kappa B (NF-κB) and signal transducer and activator of transcription 3 (STAT3) were effectively suppressed by 7-HMIA treatment like the effects of 5-ASA. Overall, our findings revealed that 7-HMIA decreased the severity of colitis by protecting the inflamed mucosal barrier by interfering with NF-κB and STAT3 activation.

Degradation of bisphenol A on SnO2-MWCNT electrode using electrochemical oxidation

Bisphenol A (BPA) is an organic pollutant that is widely spread in waterbody with effluents as a result of its extensive use in the plastics industry. It posses to the class of compounds that are endocrine disruptors and has a great negative impact on living organisms. Therefore, the progress in development of green technology for BPA removal from the aquatic media is crucial for environmental protection. This paper presents BPA removal by electrochemical oxidation on SnO2-MWCNT (tin dioxide-multi walled carbon nanotube) nanocomposite anode. The nanocomposite was structural and morphological characterized by X-ray powder diffraction and transmission electron microscopy. Electrochemical properties of the SnO2-MWCNT anode were investigated using cyclic voltammetry, linear sweep voltammetry and electrical impedance spectroscopy. The electrochemical behavior of anodes toward BPA in sodium sulfate solutions has shown that BPA is oxidized by indirectly mechanism via hydroxyl radicals (verified by quenching tests). Electrolysis of BPA has been done in galvanostatic regime at current densities 2.5, 5.0, 10.0, 15.0 and 20.0 mA cm−2 in 0.1 M sodium sulfate supported electrolyte pH 4.0. Complete degradation of BPA was accomplished after 5 h of electrolysis at a current density of 20 mA cm−2. The proposed electrochemical technology using a low-cost SnO2-MWCNT electrode is promising for wastewater BPA removal.

Effects of Alternate Wet and Dry Conditions on the Mechanical and Physical Performance of Limestone Calcined Clay Cement Mortars Immersed in Sodium Sulfate Media.

Sulfate attack in concrete structures significantly reduces their durability. This article reports the experimental findings on the effects of sodium sulfate on limestone calcined clay cement (LC3) in an alternate wet and dry media. The samples underwent wet-dry conditions of 28 cycles. Two types of LC3 were studied, one made from clay (LC3-CL) and the other made from fired rejected clay bricks (LC3-FR). The composition of each LC3 blend by weight was 50% clinker, 30% calcined clay, 15% limestone, and 5% gypsum. The reference compressive strength was evaluated at 2, 7, and 28 days of age. Then, ordinary Portland cement (OPC) and LC3-CL blends were subjected to alternate wet-dry cycle tests, immersion in a 5% sodium sulfate solution, or in water. For all exposed samples, sorptivity tests and compressive strength were done. The results showed that LC3 blends met the requirements for KS-EAS 18-1:2017 standard, which specifies the composition and conformity criteria for common cements in Kenya. The LC3 blend also had a lower rate of initial absorption compared to OPC. Additionally, LC3 blend also showed good resistance to sodium sulfate when exposed to alternating wetting and drying environment. OPC showed higher compressive strength than LC3 blends for testing ages of 2, 7, and 28 days. However, the LC3 samples utilized in the sodium sulfate attack experiment, which were later tested after 84 days, exhibited higher compressive strengths than OPC tested after the same period.

Development of top-level software for the process control system for bentonite-containing bipolar membranes

An approach to automating the technological process of obtaining bentonite-containing ion-exchange bipolar membranes is considered. The structures of automated systems are given, both with the use of local automation tools, and with the use of industrial controllers and workstations. The advantages and disadvantages of each of the structures are described. A two-level control system with the use of sensors, actuators, and an industrial controller at the lower level and a workstation at the upper level is suggested. The scheme of the complex of technical means is implemented. The information model of the upper level software of the control system taking into account the peculiarities of the technological process is developed. The data flow diagram is described and functioning principles of the upper level software are proposed. A software module, that is designed both to calculate control actions for the lower control level and to simulate periodic laboratory bipolar electrodialysis of a sodium sulfate solution with experimental bentonite-containing bipolar membranes to produce acid and alkali, has been developed in C++. During the operation of the software module, it is possible to trace the dependence of technological parameters of electrodialysis (ion flux, current efficiency, energy costs for the production of the target product) on the type of bentonite modifier used (alkyldimethylbenzylammonium chloride (surfactant) and stearic acid) and its quantity in membranes (1, 2, 3% wt.) at different current densities. The software module can be used in existing industries within the framework of functioning process control systems in order to optimize operating parameters during control of electrodialysis processe.

Protective Effects of Protopanaxatriol Saponins on Ulcerative Colitis in Mouse Based on UPLC-Q/TOF-MS Serum and Colon Metabolomics.

Ulcerative colitis (UC) is a chronic, nonspecific inflammation of the bowel that mainly affects the mucosa and submucosa of the rectum and colon. Ginsenosides are the main active ingredients in ginseng and show many therapeutic effects in anti-inflammatory diseases, cancer, and nervous system regulation. Protopanaxatriol saponin (PTS) is an important part of saponins, and there is no research on its pharmacological effects on colitis. In this study, a model of ulcerative colitis in mice was induced by having mice freely drink 3.5% dextran sodium sulfate (DSS) solution, and UPLC-Q-TOF-MS-based metabolomics methods were applied to explore the therapeutic effect and protective mechanism of PTS for treating UC. The results showed that PTS could significantly prevent colon shortening and pathological damage and alleviate abnormal changes in UC mouse physiological and biochemical parameters. Moreover, PTS intervention regulated proinflammatory cytokines such as TNF-α, IL-6, and IL-1 in serum, and MPO and NO in colon. Interestingly, PTS could significantly inhibit UC mouse metabolic dysfunction by reversing abnormal changes in 29 metabolites and regulating eleven metabolic pathways. PTS has potential application in the treatment of UC and could alleviate UC in mice by affecting riboflavin metabolism, arachidonic acid metabolism, glycerophospholipid metabolism, retinol metabolism, and steroid hormone biosynthesis and by regulating pentose and glucuronate conversion, linoleic acid metabolism, phenylalanine metabolism, ether lipid metabolism, sphingolipid metabolism, and tyrosine metabolism, which points at a direction for further research and for the development of PTS as a novel natural agent.

The effect of tuina on ulcerative colitis model mice analyzed by gut microbiota and proteomics.

Tuina can effectively alleviate ulcerative colitis-related symptoms, but the mechanism of action is unknown. The purpose of this research is to explore potential pathways for the treatment of tuina through gut microbiota and proteomics techniques. Thirty-two male BALB/c mice were divided into four groups, the control, model, mesalazine, and tuina groups. The ulcerative colitis model was established by freely drinking a 3% dextran sulphate sodium solution for 7 days. The mesalazine group and the tuina group, respectively, received 7 days of mesalazine and tuina treatment. Subsequently, their body weights, feces properties, colon length, histomorphological changes, gut microbiota, and colon proteomics were determined. Body weights, disease activity index score, colon histological scores, and microbiota diversity were restored in the tuina group. At the phylum level, Firmicutes was increased and Bacteroidota decreased. At the family level, Lachnospiraceae increased and Prevotellaceae decreased. At the genus level, the Lachnospiraceae_NK4A136_group was increased. Proteomics detected 370 differentially expressed proteins regulated by tuina, enriched to a total of 304 pathways, including biotin metabolism, Notch signaling pathway, linoleic acid metabolism, and autophagy. Tuina can effectively improve the symptoms of weight loss, fecal properties, and colon inflammation in ulcerative colitis mice and restore the gut microbiota diversity, adjusting the relative abundance of microbiota. The therapeutic effects of tuina may be achieved by modulating the signaling pathways of biotin metabolism, Notch signaling pathway, linoleic acid metabolism, and autophagy.

Efficient frequency conversion and the crossing-pump effect of stimulated Raman scattering in an aqueous sodium sulfate solution.

The cascaded stimulated Raman scattering (SRS) of an aqueous sodium sulfate solution was investigated as well as the generation of the crossing-pump effect. With the introduction of dual sample cells, the first-order Stokes of the O-H stretching vibrational mode was able to act as the pump light to excite the Stokes of the S-O stretching vibrational mode, and a new Raman peak was obtained at 4423 cm-1. The dual sample cell device not only lowered the SRS threshold, but also enhanced the four-wave mixing (FWM) process. Compared to the input laser of 7 ns/pulse, the first-order Stokes of O-H was compressed to a pulse width of 413 ps after passing through the dual sample cells. The SRS of aqueous sodium sulfate solution covered an ultrabroad wavelength ranging from 441 nm to 720 nm (a Raman shift ranging from -3859 cm-1 to 4923 cm-1). The cone-shaped launch ring of the FWM process was also recorded. This work provides a reference for the establishment of laser frequency conversion devices using an aqueous sodium sulfate solution as the Raman medium.

The Damage Performance of Uncarbonated Limestone Cement Pastes Partially Exposed to Na2SO4 Solution

Pore structure and composition of cement paste are the main two factors in controlling the sulfate attack on concrete, but the influence of carbonization on pore structure and composition is often ignored in sulfate attack. Therefore, will the damage performance of concrete partially exposed to sulfate solution be different avoiding the alterations of pore structure and composition due to carbonation? In this paper, the cement pastes were partially immersed in 5 wt. % sodium sulfate solution, with N2 as protective gas to avoid carbonation (20 ± 1°C, RH 65 ± 5%). Pore structures of cements were changed by introducing different contents of limestone powders (0 wt. %, 10 wt. %, 20 wt. %, and 30 wt. %) into cement pastes. The damage performance of the specimens was studied by 1H NMR, XRD and SEM. The results showed that the immersion zone of pure cement paste under N2 atmosphere remained intact while serious damage occurred in the evaporation zone. However, the damage of cement + limestone powders pastes appeared in the immersion zone rather than in the evaporation zone and cement pastes containing more limestone were more severely damaged. Compositional analysis suggested that the damage of the evaporation zone or the immersion zone was solely caused by chemical attack where substantial amount of gypsums and ettringites were filled in the pore volumes. Introduction of limestone powders led to the increase of the pore sizes and porosity of cement pastes, causing the damage occurred in the immersion zone not in the evaporation zone.

Retention performance of alumina porous layer open-tubular column coated with γ–alumina nanoparticles in the highly volatile hydrocarbons separation

An alumina porous layer open-tubular (Al2O3 PLOT) column coated with γ–alumina nanoparticles (20 nm) for highly volatile hydrocarbons (C1 to C5) separation was described. Relative to the coating of bulk alumina, this column was easily coated with dynamic method under 0.4 or 0.6 MPa for 0.53 mm or 0.32 mm capillary, respectively. And the thickness of coating layer could be tuned by repeating the coating process after column was dried and activated at 300 °C for 3 h. The effect of deactivation agents on the physicochemical properties of nano γ–alumina was characterized by X–ray diffraction (XRD) measurement and nitrogen adsorption–desorption isotherms. The influences of deactivation agents, film thickness, conditioning and column dimensions on the inertness, polarity, selectivity and elution order of C1 to C5 separation were investigated in detail. The crystallite structure and size of nano alumina were not affected by the deactivation agents and remained constant during the column making processes, whereas specific surface area, pore volume and average half pore width altered significantly. The specific surface area decreased to 125.4 m2 g–1 or 174.0 m2 g–1 and the average half pore size distributions decreased to 1.6–8.4 nm or 2.4–14.3 nm when it was deactivated with potassium chloride or sodium sulfate solution, respectively. The deactivation agents and its concentrations impacted significantly on the retention performance of column. The column deactivated with sodium sulfate solution exhibited stronger polarity and lower selectivity than which deactivated with potassium chloride solution although both columns showed good inertness. The length, internal diameter and film thickness of the column had less influence on the selectivity and resolution for C1 to C5 hydrocarbons separation, whereas the conditioning temperature and time had an obvious influence. The column had distinguished polarity and selectivity which was different from either bulk or commercial alumina columns. Typically, the hydrocarbons were baseline separated with resolutions ranging from 1.65 to 15.33 within 9 min under programmed temperature below 100 °C, and the tailing factors ranging from 1.02 to 1.07.

TLC-densitometric identification of valproic acid

Valproic acid and its derivatives are drugs that have been used in many neurogenic diseases, but are mainly used in epileptic seizures. Due to the fact that valproic acid doesn’t have conjugated double bonds, it isn’t visible under UV light and its detection is difficult. Therefore, the aim of the study was to assess the suitability of individual visualizing reagents in the chemical qualitative and quantitative detection of valproic acid using the TLC technique. Solutions of various visualizing reagents were prepared, and then the reagents were applied on the previously prepared chromatographic plates with valproic acid by means of an immersion and spraying method in order to detect it. It was shown that all the visualizing reagents used, with the exception of helasol green, bromothymol blue, malachite green, bromocresol green in 2% sodium hydroxide solution and sulphate (VI) solutions of iron (II), nickel and cobalt, allowed for the qualitative detection of valproic acid. It is recommended to use a 1% solution of copper (II) sulphate (VI) for the detection of valproic acid for both qualitative and quantitative purposes, due to the largest areas of the chromatographic bands on most of the chromatographic plates used in the study. There is a potential possibility of examining the identity of valproic acid present in drugs on the basis of the RF value, the characteristic colour of the spot, and a comparative analysis of spectrodensitograms.

Deterioration of Mortar Bars Using Binary and Ternary Mixtures Immersed in Sodium Sulfate Solutions

In this study, the performance of several binary and ternary mixtures containing high-calcium fly ash and other pozzolans, such as Class F fly ash and silica fume, were investigated for their sulfate resistance using different sodium sulfate solutions. The mortar bars were placed in a similar sulfate solution as per modified ASTM C 1012/1012M (33,800 ppm SO42−) with a less severe sulfate solution (6000 ppm SO42−) has been tested to resemble actual field performance for a duration of 18 months. The phase composition of the mortar samples was investigated using X-ray diffraction and scanning electron microscope coupled with energy dispersive spectroscopy (SEM/EDS). Results show that the mortar bars placed in the moderate sulfate concentration experience less expansion and deterioration than the same bars placed in the higher sulfate concentration. Storage in sodium sulfate solutions resulted in the formation of ettringite and gypsum in both sulfate concentrations. Replacement of cement by high-calcium fly ash showed significantly higher amounts of ettringite formation, especially for the mortar bars stored in the higher sulfate concentration. SEM analysis revealed ettringite to be the primary cause of disruption and deterioration observed in the mortar bars.

A Stretchable and Healable Gelatin Hydrogel Assisted by Hofmeister Effect for All‐in‐One Flexible Supercapacitor

The all‐in‐one flexible supercapacitor holds great promise for wearable electronics due to its stable electrochemical performance under continuous strain. However, it is a challenge to achieve good mechanical strength, self‐healing performance, and high energy density for their potential applications. For this purpose, a stretchable and healable all‐in‐one supercapacitor consisting of gelatin hydrogel electrolytes and polypyrrole electrodes is constructed. The gelatin hydrogel is obtained by the hydrolysis of leather waste shavings and then physically cross‐linked by simply soaking in a sodium sulfate solution. Benefiting from the Hofmeister series, the kosmotropic sodium sulfate ions can significantly enhance the hydrophobic interactions and chain bundling, thus greatly improving the mechanical strength of gelatin hydrogel. As a result, the as‐prepared all‐in‐one supercapacitor exhibits an area‐specific capacitance of 219.0 mF cm−2, a high energy density of 19.46 μWh cm−2, and long cycling life. In particular, the all‐in‐one supercapacitor can be deformed under varied stress–strain via decross‐linking and dissociation, simultaneously delivers reliable self‐healing capability. This work presents a simple and green technology to prepare an all‐in‐one supercapacitor, which motivates the development of biomass wastes toward high‐performance electronics.

Experimental Study for the Comparison between Core, Schmidt and Cubic Concrete Compressive Strength After Durability Test

Estimating the compressive strength of concrete exposed to different environmental conditions is an essential parameter in judging the quality of concrete and proving its ability to perform as planned. There are various destructive (DT) and non-destructive (NDT) methods for estimating compressive strength. Some NDT and DT procedures, such as Schmidt hammer and core testing, have been used for estimating concrete strength for a long time. These methods consider indirect and predicted tests to evaluate concrete strength in existing structures. Various factors influence these tests, so it is difficult to precisely assess the hardened concrete strength. Thus, this research aims to identify a coherent relationship that connects the results of these tests and correlates them with the results of cube compressive strength by assessing data from laboratory tests conducted under control and durability conditions using statistical techniques. To achieve this purpose, three grades of concrete were considered C250, C450, and C600 Kg/cm2. Six concrete cubes of size (15x15x15 cm) and four concrete columns of size (30x15x50 cm) were cast for each grade and tested to assess the cube, Schmidt hammer, and core compressive strength of concrete. Half of the columns and cubes were cured in drinking water (control condition) and the other half of the samples were subjected to durability cycles for 92 days (durability condition). In the durability condition, the samples were exposed to 10% sodium sulfate solution for a day, placed in a water tank for a day, heated to 100°C at a laboratory furnace for a day and then left at average laboratory temperature for a day to finish one cycle. The results were tabulated and graphed to notice any differences between DT, NDT and cube compressive strength tests. The results showed that the cube strength is higher than the core strength for all grades of concrete under control and durability conditions. The cube test indicates a lower compressive strength than the Schmidt hammer test method at grade C250 under control condition and greater compressive strength in the rest of the grades.

Removal of Sulphate Ions from Borehole Water Using Nanofiltration and Reverse Osmosis

Consumption of contaminated water poses health hazards to humanity and hence increases the need for treatment. An excessive level of sulphate in borehole water was detected in the drinking water supply of Garpéné village in Burkina Faso. This study aimed to evaluate the performance of nanofiltration and reverse osmosis for the removal of sulphate ions from borehole water. A combined nanofiltration and reverse osmosis pilot was used for the treatment of the raw borehole water and some prepared synthetic solutions. Different experimental conditions were used to evaluate the effects of the pressure, the nature of the solution to be filtered, and the ions accompanying the sulphate. The filtration tests were conducted at transmembrane pressures of 1, 2.5 and 4 bar with a nanofiltration NF270 membrane and at transmembrane pressures of 4, 6 and 8 bar with a reverse osmosis TW30 membrane. The membrane used were a thin-film composite (TFC) membrane from Filmtec. The pilot was fed with real water from the Garpéné borehole and synthetic solutions of calcium sulphate and sodium sulphate at three different concentrations (250, 500 and 1400 mg/L). The results demonstrated that the nanofiltration and reverse osmosis membranes achieved retention rates of sulphate ions ranging from 97.0% to 98.7% and 98.4% to 99.0%, respectively. The results also showed that the operating conditions had a significant effect on the retention of sulphate by reverse osmosis and nanofiltration membranes. With regard to the effect of the counter ion, it was observed that during nanofiltration, the retention rate of sulphate ions in the water to be treated was lower in the presence of calcium ions than that of sodium ions, whereas in reverse osmosis little influence of the counter ion was observed.

Sulfate resistance of ultrahigh-performance concrete with recycled fine aggregate

The effect of sulfate attack (SA) on the strength of ultrahigh-performance concrete (UHPC) made with different replacements (0%, 50% and 100% by weight) of natural fine aggregate with recycled fine aggregate (RFA) was investigated. The UHPC samples were soaked in 10% sodium sulfate solution for 0–180 days (SA duration). The sulfate ion concentration, flexural strength and compressive strength of the UHPCs were investigated. The microstructure of the UHPCs before and after SA was analysed using a multi-technique approach (analysis of interfacial transition zones (ITZs), X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy and mercury intrusion porosimetry). The results showed that, with an increase in SA duration, the compressive strength of the UHPC made with RFA increased. The generated gypsum and ettringite refined the microstructure by converting harmful pores (>20 nm) into harmless pores (<20 nm) in the UHPC matrix. For the same SA duration, the sulfate ion concentration of UHPC increased with an increase in RFA content due to more ITZs, pores and microcracks introduced by the RFA. Strength development models for UHPC with different RFA contents under SA were developed.

Experimental investigation on the mechanical properties and pore structure deterioration of fiber-reinforced concrete in different freeze-thaw media

• The mechanical properties and pore structure deterioration of FRCs in five freeze-thaw media (i.e. WF, NF, SSF, SCF, HSF) were studied. • The influence of the SCF on the mechanical properties of FRCs is the greatest, followed by the HSF, SSF, WF and NF. • There is an optimal fiber dosage for the FRCs to resist freeze-thaw cycle and salt erosion. • The porosities and salt corrosion degree of the FRCs exposed on the freeze-thaw cycles significantly increased. More engineering infrastructures have been built in cold regions and even permafrost area with salt-rich media, such as seawater, saline soil or groundwater. In such environment, the concrete is not only destroyed by freeze-thaw processes, but also damaged by corrosion. Therefore, this study investigated the mechanical properties and pore structure deterioration of fiber-reinforced concrete (FRC) in five types freeze-thaw media, i.e., water (WF), natural air (NF), 5 % sodium sulfate solution (SSF), 5 % sodium chloride solution (SCF), a mixture of the 5 % sodium sulfate and 5 % sodium chloride solution (HSF). A total of 180 freeze-thaw cycles were carried out on steel fiber-reinforced concrete (SFRC), polypropylene fiber-reinforced concrete (PPFRC), and basalt fiber-reinforced concrete (BFRC). The compressive strength and elastic modulus of FRCs after freeze-thaw cycles were measured to observe the influence of different media and fibers on the mechanical properties. The deterioration of the pore structure of the FRCs was observed by SEM (scanning electron microscope), XRD (X-ray diffraction), and MIP (mercury intrusion porosimetry) tests. The results show that the influence of the SCF on the mechanical properties of concrete is the greatest, followed by HSF, SSF, WF and NF. Moreover, there is an optimal fiber dosage for the FRCs to resist freeze-thaw cycles and salt erosion, the optimal fiber dosages for the SFRC, PPFRC and BFRC were 2 %, 2 ‰ and 0.5 ‰, respectively. Additionally, compared to the concrete not exposed to freeze-thaw cycles in the NF, the porosities of the FRCs exposed to the freeze-thaw cycles significantly increased. Under the same freeze-thaw media, the porosity of the BFRC is larger than that of the PPFRC. Furthermore, for the FRCs in the SSF, the proportions of gel micro-pores (<10 nm) decreased with the number of freeze-thaw cycles, whereas the proportions of capillary pores (10–5000 nm) and macro-pores (>5000 nm) increased.

Calcium leaching from cement hydrates exposed to sodium sulfate solutions

Calcium leaching from cement hydrates to pore solution increases the porosity and reduces the bonding strength of cement hydrates, accelerating the degradation of concrete. Calcium leaching can be quantified by solid–liquid equilibrium curves, which have been studied in deionized water or ammonium nitrate. The research of solid–liquid equilibrium curve of calcium under sulfate attack is limited and its mechanism is poorly understood. Reported here provide insights into the dissolution process of calcium from cement hydrates exposed to the sodium sulfate solution. The experimental programme examines the effects of sulfate ion concentration and temperature. An external sulfate attack (ESA) model considering the influence of calcium leaching is established and validated. The results show that, compared to deionized water, sulfate ions impact strongly the leaching process. Qualitatively, the dissolution of calcium in cement hydrates is accelerated by increasing the concentration of sulfate ions and decreasing the environmental temperature. Quantitatively, the presence of sulfate ions modifies the equilibrium curve describing calcium leaching in deionized water. In addition, the prediction results of the ESA model considering the influence of calcium leaching are in good agreement with the experimental results.