Steel Waste Research Articles

Mechanical performance of geopolymer concrete with micro silica fume and waste steel lathe scraps

Environmental studies for solutions are among the most important agendas of the scientific world. Most of the new studies are taking into account environmental effects. However, it is interesting for the scientific world to find solutions for accumulated environmental problems, to reduce harmful production, and to turn wastes that cause environmental pollution into useful products. In addition to incorporating fly ash, a recognized environmentally friendly and sustainable material, geopolymer concrete, utilizes micro silica fume (micro silica) as a binding agent. Furthermore, waste lathe scraps are introduced to enhance and safeguard the concrete's mechanical properties. During the preparation phase, significant enhancements have been identified in the workability and setting time of concrete. A total of 16 test samples were prepared in this study. Micro silica of 0%, 5%, 10%, and 20%, and lathe scraps of 0%, 1%, 2%, and 3% were examined. Experimental findings revealed that incorporating 5% micro silica resulted in notable improvements in the compressive, flexural, and splitting tensile strengths, with the increases of 14.4%, 7.45%, and 6.18%, respectively. However, higher additions of 10% and 20% were found to decrease these strengths. In contrast, the inclusion of 1% lathe scraps led to considerable increases in the compressive, flexural, and splitting tensile strengths by 11.4%, 6.35%, and 8.23%, respectively. However, the addition of 2% and 3% lathe scraps resulted in the reduced capacity. The findings demonstrated that combining 5% micro silica with 1% lathe scraps provided the highest strength, with the improvements of 25.7%, 14.4%, and 12% in the compressive, flexural, and splitting tensile strengths, respectively. Considering the enhancements in the workability, setting time, and strengths observed in all the tests, the recommended optimal geopolymer mixture is 5% micro silica together with 1% lathe scraps.

Feasibility of utilising waste tyre steel fibres to develop sustainable engineered cementitious composites: Engineering properties, impact resistance and environmental assessment

This study investigates the feasibility of developing sustainable engineered cementitious composite (ECC) with polyvinyl alcohol fibre (PVAF) and recycled tyre steel fibre (RTSF) to reduce the total cost of construction materials and ease the environmental pressure for disposing of waste tyres, putting an emphasis on the engineering properties, impact resistance and environmental assessment. Experimental results reveal that partially replacing PVAF with RTSF can reduce the loss in fluidity and promote flexural performance, while the fibre bridging stress of ECC can reach up to 4.41 MPa because of the enhanced fibre bridging effect. The dynamic compressive properties of ECC present a typical strain rate effect regardless of the fibre content, and the replacement of 0.25 vol% RTSF would cause a further 5.03%–25.55% increase in total dissipation energy within the strain rates of 58.2–124.5 s−1, relative to ECC reinforced by 2.0 vol% PVAF. The synergetic effects of hybrid fibres on dynamic compressive properties of ECC can be explained by the bridging and pull-out effects of PVAF at lower strain rates, as well as the enhanced fibre-to-matrix bonding of hydrophilic RTSF at relatively higher strain rates. Upcycling of waste steel fibres in ECC holds promise as a cost-effective and eco-friendly solution for defence engineering constructions.

Integrated design of stepped solar still with internal glass reflector for environmental desalination

<p>Water, a scarce resource that is necessary for the survival of all life on Earth and also for economic, environmental sustainability, and social progress, is becoming increasingly difficult to come by. Recent research analysis revealed that reflector, absorber, and evaporator were the three distinct components of equipment employed in solar stills. These three essential pieces of equipment help the portable water solution's re-mineralization process run smoothly. These three components were used to assess the efficiency and quality of water. The 1x1 m3 specification was used to examine this experiment. Using absorbers, reflectors, and evaporators, this study developed three distinct conceptual stages. The initial phase includes cloth, stone, and waste steel used in the absorber process. Using a reflector, a secondary stage experiment compared using with and without glass. The stepped type absorber with mild steel, plain round, and square fin type trays are all dealt with in the territorial stage of the evaporator process. To assess the improved desalination system, an analysis between an enhanced stepped solar still and a plain stepped solar still is made. It increases the concentration of solar radiation on the water and simultaneously enhances the heat transfer and leads to a 30 % enhancement of efficiency.</p>

Microwave self-healing characteristics of bituminous mixtures with different steel slag aggregate and waste ferrite filler

To improve crack propagation issues of bituminous mixtures in an eco-friendly way, steel slag (SS) and waste ferrite were partially substituted for aggregate and filler in the bituminous mixtures to enhance the self-healing performance (SHP) of the mixture through microwave heating (MH). Firstly, the electromagnetic parameters of ferrite filler (FF) and SS were tested by the vector network analyzer. Secondly, the thermal conductivity (TC) of ferrite filler bituminous mixtures (FFBM) and steel slag bituminous mixtures (SSBM) was tested based on the transient plane source. Subsequently, the MH surface temperature was measured on FFBM and SSBM, and the MH efficiency and uniformity were analyzed. Finally, the SHP of two types of bituminous mixtures under different conditions was analyzed. The results revealed that both FF and SS have significant magnetic loss, and the absorption performance of FF is better at the same frequency and thickness. The TC of SSBM is higher than FFBM, but due to the porous characteristics of SS, it tends to stabilize. The MH efficiency of FFBM is lower due to its volume ratio, but it has better uniformity. The self-healing rate of SSBM is higher at 40 s MH time, but FFBM can obtain a higher self-healing rate by extending the heating time. This work contributes to understand the difference between different types of suction materials self-healing of bituminous mixtures under MH.

PERFORMANCE OF STEEL FIBER EXTRACTED FROM OLD WASTE TYRES ON MECHANICAL PROPERTIES OF CONCRETE FOR RIGID PAVEMENT CONSTRUCTION

The application of Waste Recycled Steel Fibers (WRSF) extracted from waste tyres in fiber-reinforced concrete production has great benefits in civil engineering. Thus, this creates a need to study the appropriate length and dosages of recycled steel fiber in fiber-reinforced concrete. In this experimental research, the effects of varying lengths (5cm and 10 cm) and dosages (1, 1.5, 2, 2.5, and 3%) of WRSF on various mechanical properties of fiber-reinforced concrete for rigid pavement construction were studied. The aggregates were taken from a stockpile, commercially available cement Dangote Ordinary Portland Cement (OPC), and potable water. Non-probable sampling techniques were adopted to collect extracted waste steel tyres. The quality test for sand (fine) and coarse aggregate satisfies the requirements specified in the ASTM. The concrete mix design was done in two categories; the first was a control mix concrete and the second was the experimental mix concrete with the addition of steel fibers with varying lengths (5cm and 10 cm) and dosages (1, 1.5, 2, 2.5, and 3%) of WRSF in concrete (percentages were determined by density for each fiber). The mix ratio of cement: sand: aggregate (1:2:3) with a constant water-to-cement ratio of 0.53 was used throughout this investigation. The outcome showed that the fiber fraction with 1.5% and 10cm of fiber had the maximum compressive strength which was 45.59 MPa, while for the fraction with 1% and 5cm of fiber, the maximum compressive strength was 43.85 MPa. The flexural strength had a maximum value of 5.88 MPa at 3% fiber content for 5cm fiber and 5.09 MPa at 3% for 10cm of fiber. The maximum tensile strength attained was 4.74 MPa and 3.50 MPa at 3% WRSF for 5cm and 10cm of fiber, respectively. The strain value had a maximum value at 3% for both 10cm and 5cm of fiber which were 0.53 and 0.6 respectively. The concrete strength which was obtained with the addition of steel fiber showed reasonable improvement in compressive, indirect tensile, and flexural strength. However, as the percentage of WRSF increased, the workability of concrete reduced.

Prediction of the compressive strength of lightweight concrete containing industrial and waste steel fibers using a multilayer synthetic neural network

The use of waste and industrial steel fibers as part of the materials used in concrete can increase resistance and reduce cost and air pollution. It also saves energy. One of the important measures for technical inspections and assessment of the existing condition of structures, especially bridges, which is the most important communication factor, is to check the compressive strength. Considering that the calculation of compressive strength in the laboratory is done with the intervention of human power and is undoubtedly affected by human error, we decided to use it through.Predicting the mechanical properties of concrete reinforced with steel fibers based on artificial neural network models without the need to conduct any laboratory studies will save money in construction projects. Unlike classical methods in statistical theories, neural networks do not require any specific model or function along with limiting assumptions to linearize problems.For this purpose, this research was done with the aim of compensating this problem and with the aim of building a neural network with high accuracy that can make the desired predictions with the least error. In this research, this modeling was done using artificial neural networks (ANN) and Levenberg algorithm. The data used to train the neural network was collected from 45 different mixing schemes. Then the compressive strength of the sample is determined experimentally. The parameters considered for the ANN inputs are the values of steel fiber, water, water-cement ratio, cement and superlubricant. The objective data of this study included the compressive strength of each of these mixing designs at the ages of 7, 28 and 60 days. Then, to design the neural network, 75% of the data were considered as training data, 15% as target data and 15% as validation data. The compressive strength of concrete samples made from waste steel fibers increases. One of the reasons for this result is the placement and uniform distribution of fibers in the cement matrix, or in other words, the optimal amount of desired fibers in concrete. For experimental information and data, results can be seen with the help of neural network in data analysis. It was observed that the validation is correlated with a correlation coefficient above 99% and the constructed neural network has sufficient accuracy and validity.

Eco-friendly concrete produced with recycled materials

Using recycling materials to produce concrete become a major subject for research in the last decades. The work aims to examine the effects of the partial replacement of fine aggregate by recycled rubber. In addition to studying the influence of two different fiber types on mechanical properties of concrete with rubber replacement. Three types of rubbers that differ in particle size are used to find the effect of rubber particle size on compressive strength, splitting tensile strength, and flexure strength of the concrete mix. Two waste fiber types are used which are Rope Waste Fiber (RWF) and Steel Waste Fiber (SWF). The findings showed that minimizing the rubber particle size improved the mechanical properties. Also, using two types of waste fiber (rope and steel) together led to improve compressive strength.

Performance of SSC filled circular steel tube short columns under axial compression

Steel slag concrete (SSC) filled steel tube (SSC-FST), reusing the waste steel slag, is a new type of composite members, therefore shows advantages in sustainable use of resources. In order to investigate axial compressive performance of this new types of SSC-FST short columns with circular section, ten specimens with varied steel slag aggregate (SSA) replacement ratio and cross-sectional steel ratio (α) were tested. The failure patterns, load versus deformation relationship and mechanical indexes (axial capacity, elastic modulus and ductility) of the specimens were recorded and analyzed. The test results indicate that, generally, replacing natural aggregates with SSAs and improving α lead to better structural performance of circular SSC-FST short column specimens. Moreover, a refined finite element (FE) model considering the initial strain of core concrete before loading was established to simulate the performance of circular SSC-FST short columns under axial compression, and the FE model was validated through experimental results from this research and the literature. Based on the verified FE model, the mechanism of axially compressed circular SSC-FST short columns was further analyzed. Finally, simplified equations were proposed to predict the axial capacity of circular SSC-FST short columns, and generally the calculated results show good agreement with the numerical/experimental results.

Development of Iron-Silicate Composites by Waste Glass and Iron or Steel Powders.

There is growing interest in the opportunities regarding construction and demolition wastes, such as glass and metal powders, for developing a circular economy and their transformation into new materials. This management and recycling of construction and demolition waste offers environmental benefits and conservation of natural resources. In this paper, new magnetic composite materials were prepared by wet chemical synthesis methods using crushed glasses and iron and steel waste powders as raw materials. The prepared iron-silicate composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, infrared (IR), ultraviolet-visible, and electron paramagnetic resonance (EPR) spectroscopy, and magnetic measurements. The XRD data confirm the formation of varied crystalline phases of the iron ions. The presence of the Fe3O4 crystalline phase was detected in the composites containing the iron waste powders. The inspection of the SEM micrographs revealed slightly better homogeneity for the composite material containing larger amounts of iron waste and heterogeneous morphology with cracks and random crystallinity for the composite doped with steel waste. By doping with different contents of iron or steel waste powder, structural modifications in the silicate network and the formation of new bands in the IR spectra were evidenced. The UV-Vis spectra were characterized by the absorption peaks for both the tetrahedral and octahedral geometries of the Fe3+ ions and the octahedral coordination of the Fe2+ ions with oxygen anions. The EPR data show resonance lines with g ~2, 4.3, and 6.4, corresponding to the Fe3+ ions. Using hysteresis curves, the superparamagnetic properties of the iron-silicate composites were evidenced.

Novel low-cost steel slag porous ceramic-based composite phase change material: An innovative strategy for comprehensive utilization of steel slag resources

This work creatively proposes a novel and low-cost steel slag porous ceramic (SSPC)-based NaNO3 salt composite phase change material (PCM) using industrial solid waste steel slag as the main raw material. In this study, the foaming-gel casting process prepares SSPC with high porosity (77–83%) and mechanical strength (0.36–1.13 MPa), which shows good adsorption of NaNO3 salts. The spontaneous infiltration method prepares and characterizes the NaNO3/SSPC composite PCM by performance properties. The results show that the NaNO3 salt is uniformly adsorbed in the pores of SSPC with good chemical compatibility, and the maximum filling rate reaches 86.26%. The porous media can effectively prevent the leakage of molten NaNO3 salt. The prepared composite PCMs have lower melting and freezing temperatures, lower subcooling by 5.47 °C, and higher thermal conductivity by 121.6% compared to pure NaNO3 salt. The latent heat retention of the composite after 50 thermal cycles is more than 96%, with good thermal reliability. Finally, a prospective process of using steel slag to prepare composite PCMs for collecting and storing waste heat from the steel industry is proposed. Reusing steel slag and storing waste heat from the steel industry are realized, both beneficial for sustainable development.

A promising catalyst for catalytic oxidation of chlorobenzene and slipped ammonia in SCR exhaust gas: Investigating the simultaneous removal mechanism

As the emission limits for NOx become further stringent, development of a synergistic control technology to prevent emissions of slipped ammonia (NH3) and chlorinated organics from steel production and waste incineration is both a prospect and a challenge for the environmental issues. In this work, the designed ruthenium-based catalyst (RuNb/ST) showed the excellent chlorobenzene (CB)/NH3 synergistic removal performance and N2/CO2 selectivity above 300 ℃. The activity of NH3 and CB were suppressed at relatively low temperature due to the competitive adsorption of CB and NH3. Whereas, there was a significant improvement in the N2/CO2 selectivity at relatively high temperature. The removal mechanism was studied by experiments, in which the accumulation of chlorine species, the role of NO2 and the oxidation of CB were considerable. In contrast to the carrier, the vast majority of the chloride ions adsorbed onto the active sites after dissociating from CB could be efficiently removed in form of Cl2 via the Deacon Reaction catalyzed by RuO2. CB was predominantly converted into CO2 and H2O with a fraction persisting as organics. Partial of organics were attacked by Cl ions to generate organic chlorine-containing by-products. The presence of NO2 molecule was found to promote the cleavage of the benzene ring within CB, and expedited the carbon migration towards the ultimate product of CO2, resulting in a notable reduction of byproducts.

Flexural Behavior of Reinforced Rubberized Reactive Powder Concrete Beams under Repeated Loads

Non-biodegradability of rubber tires contributes to pollution and fire hazards in the natural environment. In this study, the flexural behavior of the Rubberized Reactive Powder Concrete (RRPC) beams that contained various proportions and sizes of scrap tire rubber was investigated and compared to the flexural behavior of the regular RPC. Fresh properties, hardened properties, load-deflection relation, first crack load, ultimate load, and crack width are studied and analyzed. Mixes were made using micro steel fiber of the straight type, and they had an aspect ratio of 65. Thirteen beams were tested under two loading points (Repeated loading) with small-scale beams (1100 mm, 150 mm, 100 mm) size. The fine aggregate is replaced by 5, 10, and 15%, respectively, with crumb rubber. While replacement of silica fume was 10, 20, 30, and 50%, respectively, with very fine rubber. Also, chip rubber was added to the mixture as coarse aggregate with 5, 10, and 15%. Five tested beams were chosen as case studies to analyze and compare the results of the ABAQUS software with the experimental results. The results showed that the flexural behavior of RRPC beams that contains rubber was acceptable when compared with the flexural behavior of the RPC beam (depending on load-carrying capacity). The crack width was decreased by including waste rubber and steel fibers. There is a satisfactory agreement between the results of the numerical analysis and the results of the experimental testing. Slight ultimate load differences are targeted between the effects of the monotonic loading and the repeated loading.

Investigation of Fresh and Hardened Properties of Self-Compacting Concrete Using Wood Waste Ash and Steel Slag

Steel rolling mills are the main place where steel slag is produced. The steel slag accumulates at the bottom of the blast furnace slag. The current study sought to ascertain whether steel slag may replace some of the fine aggregate (sand) in self-compacting concrete. 15% of wood waste ash used as mineral admixture and varying amounts (15%, 30%, and 45%) of steel slag were used as sand replacement. Experimental tests were performed on freshly cured concrete that contained steel slag to determine its properties. Slump flow, U-box, L-box, V-funnel, splitting tensile strength, flexural strength, and elastic modulus are features of self-compacting concrete that comprises steel slag in a portion of river sand. Flexural Strength, and modulus of elasticity of Self-Compacting concrete incorporating steel slag in partial replacement of river sand were examined. The results of this study show that at a fixed water cement ratio of 0.45 and an SP dosage of 1.1% by weight of powder. Self-compacting concrete's compressive strength, splitting tensile strength, and flexural strength enhanced throughout the curing process.

Recycling of Materials from Renovation and Demolition of Building Structures in the Spirit of Sustainable Material Engineering

In the European Union, more than 20% of residential buildings were constructed prior to 1945 and are approaching the end of their projected lifespan. This necessitates renovations or demolitions of these structures. A study was conducted to analyze the quantity of waste generated during the dismantling of a representative building from that era. Construction waste is produced during the building process and its disposal can be costly. Sustainable material engineering offers a solution by promoting the reuse of waste materials. When designing new products, manufacturers should consider using waste materials and ensure they have an extended lifespan, efficient life cycle management, and sustainable consumption. A study has shown that researchers are currently exploring the potential use of waste steel, bricks, wood, and glass in composite materials. Direct reuse of waste in the construction industry has also been observed. However, users’ preferences show that while environmental concerns are important, good quality and price are still the primary factors that drive consumer demand. Therefore, materials made from waste materials must demonstrate excellent parameters to gain consumer acceptance. Education initiatives such as green building certifications and education programs for construction professionals and the general public can help promote sustainable practices. Despite progress made in sustainable development, there is still a need for further education to encourage the adoption of sustainable practices.

Application of waste recycle tire steel fibers as a construction material in concrete

Abstract A bulk volume of waste tires, an underrated global resource, is disposed of in landfills worldwide. Extracting recycled steel fibers from these tires is an evolving trend nowadays. The outcomes of concrete having steel fibers sourced from end-of-life tires are alike industrial steel fibers. Accordingly, the attention of researchers is focused nowadays on using alternative recycled steel fibers in place of industrial steel fibers in concrete. In the current study, the review focuses on waste recycle tire steel fiber (WRTSF)-reinforced concrete applications, considering different lengths and contents for finding the research gap in this research domain. The results and methods to recycle WRTSFs in existing studies are compiled and briefed. The adoption of appropriate composition and characteristics like length, diameter, content, etc., of WRTSFs can be made by using this study to improve the mechanical properties of respective composites. Therefore, the aim of this evaluation is to encourage the application of concrete reinforced with WRTSF for different construction purposes, including but not limited to pavements, tunnel linings, bridge decks, hydraulic structures, and slope stabilization. The study seeks to provide guidance on the appropriate composition and characteristics of WRTSFs to improve the mechanical properties of the composite material.

Comparative Environmental Analysis for Using Waste Polyethylene and Steel Slag in Semi-dense Asphalt Pavements

Abstract This study presents a life cycle assessment (LCA) evaluating the use of virgin materials, waste polyethylene (PE), and electric arc furnace steel slag (EAFSS) in semi-dense asphalt (SDA) surface courses, which is primarily used for low-noise pavements. Three types of SDA mixtures with virgin materials, waste PE, and EAFSS were prepared and water sensitivity tests were conducted to determine the mechanical performance. The LCA defined three scenarios using system expansion, namely (1) the reference scenario using virgin materials in SDA and disposal of waste PE and EAFSS by municipal solid waste incineration (MSWI) and landfill, respectively; (2) the test scenario using waste PE and straight run binder in SDA, with landfilling of EAFSS; (3) the test scenario using EAFSS in SDA, with the disposal of waste PE by MSWI. The data from the experiments, Swiss industries, official reports, and standard databases were used for the inventory analysis. The results show that compared to the reference, the scenarios using waste PE and EAFSS in SDA would reduce 15 % and 36 % of greenhouse gases emissions, respectively. The reason for the improved environmental performance when waste PE is used in asphalt can be attributed to the use of straight run binder to replace polymer-modified binder and the avoided MSWI. For the EAFSS scenario, the improved environmental performance is attributable to the avoided landfilling. The results also indicate that the recycling of waste PE and EAFSS in SDA would not have benefits in nonrenewable cumulative energy demand.

Degradation of Reactive Black 5 Dye (RB5) using Iron Electrode (Fe.S) Derived from Iron Sludge Steel Waste via Electrochemical Method

A study has been performed on the degradation of Reactive Black 5 (RB5) as pollutant using iron electrode (Fe.S) derived from iron sludge steel waste via electrochemical method. The Fe.S and titanium graphite electrodes were applied as anode and cathode respectively during the studied. The studied operating parameters were the effect of current density, initial pH, initial concentration, and the reusability of the Fe.S electrode. The experiment was carried out for 120 minutes of electrolysis time. From results revealed that Fe.S could be used three times with complete degradation of RB5 from initial concentration of 50 ppm. A complete RB5 colour removal can be achieved in any pH range (3,6, and 9). The current density at 4 mA/cm2 showed a faster RB5 degradation compared to at 2, 6,8, and 10 mA/cm2.

Wear-Resistant Composites Produced from Tool Steel Waste for Contact Joints of High-Speed Printing Machines

Wear-Resistant Composites Produced from Tool Steel Waste for Contact Joints of High-Speed Printing Machines

PERFORMANCE EVALUATION OF SPECIAL CONCRETE WITH STEEL SLAG AND WASTE GLASS

In this work, tests on concrete built from waste glass and steel slag are presented. Both lightweight aggregate concrete and concrete with limestone aggregate underwent reference testing. Slump, density, elastic modulus, compressive strength, and flexural strength were used to analyse the material characteristics of various kinds of concrete. To examine the impact of aggregate type on the fire performance, fire tests on plain concrete columns with a diameter of 250 mm and a height of 800 mm were carried out. The study's findings show that it is possible to use steel slag and/or waste glass in place of all coarse aggregate or a portion of fine aggregate. Concrete made from steel slag may be made more workable and less dense by adding waste glass. The compressive strength, flexural strength, and elastic modulus of the steel slag concrete were equivalent to or even greater than those of the control limestone aggregate concrete. Only a slight effect on the mechanical qualities of concrete was noticed when coarse aggregate was substituted by up to 17.5% waste glass. Steel slag and waste glass have shown the capacity to increase the fire resistance of concrete due to their superior thermal and/or mechanical qualities.

Experimental investigation and analytical prediction of flexural behaviour of reinforced concrete beams with steel fibres extracted from waste tyres

In recent years, studies on the use of car tyre wastes in concrete have gained momentum. Especially, the effect of recycled waste steel wires (RWSWs) from tyres to be mixed into concrete for using in newly designed reinforced concrete buildings on the performance of construction elements is a fairly new research area. In this study, the bending behaviour of 12 reinforced concrete beams was investigated having 1/3 geometric scale, 100 × 150 × 1000 mm in size, and produced with RWSWs additive in different volumetric ratios (1%, 2%, and 3%) under vertical loads. Another main parameter selected in the study was the amount of varying tension reinforcements (2ϕ12, 2ϕ10, and 2ϕ8). The load-carrying, stiffness, ductility, and energy dissipation capacities of the RWSW reinforced bending beams were compared with the primary aim of this study which was to examine and present the contribution of RWSWs on the improvement of the bending performance of the reinforced concrete beams. The results revealed that the mechanical properties of the hybrid beams with RWSWs vary depending on dosages but are comparable with those of the beams-only with the same fibre dosage. A positive effect was obtained for the hybrid beams containing 2–3% RWSWs. Besides, RWSWs were found to be highly well mobilised at larger crack widths, and the post-cracking strength of RWSW mixes was significantly higher. Considering both mechanical properties of the beams and fresh properties such as the workability, 2% of RWSWs is recommended to be utilised in the reinforced concrete beams. On the other hand, the results were compared with the predictions of the methods given in the literature and standards. Moreover, an equation was derived to better predict the capacity of the hybrid beams using RWSWs.