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The type of steel furnace slag (SFS), including electric arc furnace (EAF) slag, basic oxygen furnace (BOF) slag, ladle metallurgy furnace (LMF) slag, and argon oxygen decarburization (AOD) slag, can significantly affect the composite properties when used as an aggregate or as a supplementary cementitious material in bound applications, such as concretes, mortars, alkali-activated materials, and stabilized soils. This review seeks to collate the findings from the literature to express the variability in material properties and to attempt to explain the source(s) of the variability. It was found that SFS composition and properties can be highly variable, including different compositions on the exterior and interior of a given SFS particle, which can affect bonding conditions and be one source of variability on composite properties. A suite of tests is proposed to better assess a given SFS stock for potential use in bound applications; at a minimum, the SFS should be evaluated for free CaO content, expansion potential, mineralogical composition, cementitious composite mechanical properties, and chemical composition with secondary tests, including cementitious composite durability properties, microstructural characterization, and free MgO content.

To reduce the environmental impact of concrete, recycled aggregates are of significant interest. Recycled concrete aggregate (RCA) presents a significant resource opportunity, although its performance as an aggregate in concrete is variable. This study presents a meta-analysis of the published literature to refine the understanding of how the moisture content of RCA, as well as other parameters, affects the compressive strength of concrete. Seven machine learning models were used to predict the compressive strength of concrete with RCA, including linear regression, support vector regression (SVR), and k-nearest neighbors (KNN) as single models, and decision tree, random forest, XGBoost, and LightGBM as ensemble models. The results of this study demonstrate that ensemble models, particularly the LightGBM model, exhibited superior prediction accuracy compared to single models. The LightGBM model yielded the highest prediction accuracy with R2 = 0.94, RMSE = 4.16 MPa, MAE = 3.03 MPa, and Delta RMSE = 1.4 MPa, making it the selected final model. The study, employing feature importance with LightGBM as the final model, identified age, water/cement ratio, and fine RCA aggregate content as key factors influencing compressive strength in concrete with RCA. In an interaction plot analysis using the final model, lowering the water–cement ratio consistently improved compressive strength, especially between 0.3 and 0.4, while increasing the fine RCA ratio decreased compressive strength, particularly in the range of 0.4 to 0.6. Additionally, it was found that maintaining moisture conditions of RCA typically between 0.0 and 0.8 was crucial for maximizing strength, whereas extreme moisture conditions, like fully saturated surface dry (SSD) state, negatively impacted strength.

High-density polyethylene (HDPE) is widely used for above-ground storage tanks (ASTs). However, there are currently no guidelines for the non-destructive testing (NDT) and evaluation (NDE) of HDPE ASTs. Moreover, the feasibility, limitations, and challenges of using NDT techniques for the field inspection of HDPE ASTs have not been well established. This study used both infrared thermography (IRT) and ultrasonic testing (UT) for the field inspection of HDPE ASTs. Highlighting the implementation challenges in the field, this study determined that: (1) ambient environmental parameters can affect IRT accuracy; (2) there is an ideal time during the day to perform IRT; (3) the heating source and infrared camera orientation can affect IRT accuracy; and (4) with proper measures taken, IRT is a promising method for flaw detection in HDPE ASTs. Additionally, UT can be used following IRT for detailed investigation to quantify the size and depth of defects. The manuscript concludes with a discussion of the limitations and best practices for the implementing of IRT and UT for HDPE AST inspections in the field.

High-density polyethylene (HDPE) above-ground storage tanks (AST) are used by highway agencies to store liquid deicing chemicals for the purpose of road maintenance in the winter. A sudden AST failure can cause significant economic and environmental impacts. While ASTs are routinely inspected to identify signs of aging and damage, current methods may not adequately capture all defects, particularly if they are subsurface or too small to be seen during visual inspection. Therefore, to improve the ability to identify potential durability issues with HDPE ASTs, additional non-destructive evaluation (NDE) techniques need to be considered and assessed for applicability. Specifically, this study investigates the efficiency of using infrared thermography (IRT) as a rapid method to simultaneously examine large areas of the tank exterior, which will be followed by closer inspections with conventional and phased array ultrasonic testing (UT) methods. Results show that IRT can help to detect defects that are shallow, specifically located within half of the tank’s wall thickness from the surface. UT has the ability to detect all defects at any depth. Moreover, phased array UT helps to identify stacked defects and characterize each defect more precisely than IRT.

The research study described in this paper investigated the potential to use steel furnace slag (SFS) as a stabilizing additive for clayey soils. Even though SFS has limited applications in civil engineering infrastructure due to the formation of deleterious expansion in the presence of water, the free CaO and free MgO contents allow for the SFS to be a potentially suitable candidate for clayey soil stabilization and improvement. In this investigation, a kaolinite clay was stabilized with 10% and 15% ladle metallurgy furnace (LMF) slag fines by weight. This experimental study also included testing of the SFS mixtures with the activator calcium chloride (CaCl2), which was hypothesized to accelerate the hydration of the dicalcium silicate phase in the SFS, but the results show that the addition of CaCl2 was not found to be effective. Relative to the unmodified clay, the unconfined compressive strength increased by 67% and 91% when 10% and 15% LMF slag were utilized, respectively. Likewise, the dynamic modulus increased by 212% and 221% by adding 10% and 15% LMF slag, respectively. Specifically, the LMF slag fines are posited to primarily contribute to a mechanical rather than chemical stabilization mechanism. Overall, these findings suggest the effective utilization of SFS as a soil stabilization admixture to overcome problems associated with dispersive soils, but further research is required.

Preliminary results are presented on the energy absorbing characteristics of a cementitious-polymer architecture bioinspired by the organic-inorganic composite structure of nacre. The proposed bioinspired architecture consists of an open cell, platelet-shaped 3D-printed thermoplastic lattice filled with high performance cementitious paste. The hypothesis is that, similar to nacre, the platelet arrangement and differences in mechanical properties of the thermoplastic lattice and cementitious platelets would result in increased energy absorption. Initial laboratory scale investigations were performed using notched beam samples subjected to static three-point bending. Stereo-digital image correlation was used to track global strain displacement field and Hillerborg’s method was used to estimate the total fracture energy. The results indicate that this “brick-and-mortar” hierarchy can increase the energy absorbing capacity of the composite by upwards of 2490% compared with the benchmark cementitious specimen. The load-deformation behaviour and total fracture energy of the bioinspired composite were found to be influenced by the platelet arrangement and size and the lattice thickness.

A ternary-blend concrete (65% cement, 25% slag, and 10% fly ash) containing fractionated reclaimed asphalt pavement (FRAP) as a partial replacement (0%, 20%, 35%, and 50%) for coarse aggregate was investigated through a comprehensive laboratory testing program. With increasing FRAP replacement, the concrete workability increased, unit weight decreased, and air content was mainly unaffected. The source of the measured strength and modulus reductions was linked to the interface between the FRAP particle and the paste. The incorporation of FRAP did not significantly impact the concrete free drying shrinkage but did reduce the restrained ring shrinkage strains. The freezing-and-thawing durability was acceptable for all tested FRAP contents. Fracture results indicated that FRAP addition did not statistically affect the initial or total fracture energy of the concrete. Mixtures containing up to 50% coarse FRAP may be used in concrete pavement and still produce acceptable fresh and hardened properties.

Digital holographic microscopy (DHM) is a surface topography measurement technique with reported sub-nanometer vertical resolution. Although it has been made commercially available recently, few studies have evaluated the uncertainty or noise in the phase measurement by the DHM. As current research is using the DHM to monitor surface topography changes of dissolving materials under flowing water conditions, it is necessary to evaluate the effect of water and flow rate on the uncertainty in the measurement. Uncertainty in this study was concerned with the temporal standard deviation per pixel of the reconstructed phase. Considering the effects of solution flow rate, magnification, objective lens type (air or immersion), and experimental configuration, measurements under static conditions in air and in water with an immersion lens yielded the smallest amount of uncertainty (mean of ≤ 0.5 nm up to 40× magnification). Increasing the water flow rate resulted in an increase in mean uncertainty to ≤ 0.6 nm up to 40× with an immersion lens. Observations of a sample through a glass window at 20× magnification in flowing water also yielded increasing uncertainty, with mean values of ≤ 0.5 nm, ≤ 0.8 nm, and ≤ 1.1 nm for flow rates of 0 mL min−1, 15 mL min−1, and 33 mL min−1. Different hologram acquisition rates (12.5 s−1 and 25 s−1) did not significantly impact the uncertainty in the phase. Collecting holograms in single-wavelength versus dual-wavelength modes did impact the uncertainty, with the mean uncertainty at 10× magnification for the same wavelength being ≤ 0.5 nm from the single-wavelength mode compared to ≤ 1.5 nm from the dual-wavelength mode. When the quantified uncertainty was applied to simulated dissolution data, lower limits of measured dissolution rates were found below which the measured data may not be distinguishable from the uncertainty in the measurement. The limiting surface-normal dissolution velocity is −10−11.7 m s−1 for experiments with an immersion lens in flowing water conditions and −10−11.7 m s−1, −10−11.4 m s−1, and −10−11.0 m s−1 for static (0 mL min−1), slow (≤ 15 mL min−1), and fast (≤ 109 mL min−1) flowing water conditions in experiments with a glass window, respectively. The data presented by this study will allow for better experimental design and methodology for future dissolution or precipitation studies using DHM and will provide confidence in the data produced in postprocessing.

Waste tire rubber is regarded as a potential source for Zn recovery and recycling. In this study, the occurrence of modes of Zn was first characterized by an electron probe microanalyzer (EPMA), and the result indicated both ZnO and ZnS were present in the tire rubber. The Zn leaching recovery was optimized by response surface methodology, and temperature was identified as the most significant variable. The highest recovery of over 98% was obtained at 90 °C for 400 min when using 2.0 mol/L HNO 3 as the lixiviant. After that, the Zn-containing leach liquor was subjected to solvent extraction for further separation and purification using bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) and 2-ethylhexylphosphonic mono-2-ethylhexyl (PC88A) as extractants. Various parameters, such as equilibrium pH, extractant concentration, and organic-to-aqueous (O/A) ratio, were investigated to maximize the Zn extraction while minimizing the contamination of impurities. The result indicated that 0.1 mol/L Cyanex 272 exhibited a higher separation factor for Zn over major impurities compared to PC88A under the same conditions. To produce the high-purity ZnO, the Zn-loaded organic phase was subjected to stripping tests, and over 92% of Zn was stripped out with trace amounts of impurities. Finally, the pH value of the stripped solution was increased to precipitate Zn, and a final ZnO product with a purity of over 99% was obtained. This study provided a reference for waste tire rubber management and utilization. Graphical Abstract

This laboratory-scale study presents the development and validation of a hydraulic fracturing technique to directly measure the tensile strength of cemented paste backfill (CPB), providing an alternative to traditional strength testing methods. Fracture initiation pressure (FIP) was used as the primary measure of CPB strength. Experimental results were compared with traditional benchmark measures such as uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), and critical Mode-I fracture toughness (K Ic ). Regression analysis of experimental results revealed a strong linear relationship between FIP and these benchmark strength measures, indicating that FIP can be used as a reliable predictor of CPB strength. However, traditional linear elastic failure models did not adequately explain the observed FIP values, as they significantly over-predicted the CPB tensile strength. To address this, the Point Stress (PS) model was applied, which provided a more accurate prediction of tensile strength, especially in cases involving small boreholes. The PS model explained observed effects of borehole size on the material’s response to hydraulic pressurization. This study confirms that hydraulic fracturing, interpreted through the PS model, is an effective method for determining CPB strength and provides a practical alternative measure to conventional testing methods.