We are pleased to introduce the collection, 2022 Retrospective: Structural Materials. This collection showcases well-received spontaneous articles from the past couple of years, which have been specially handpicked by our Chief Editor, Prof. John L. Provis. The work presented here highlights the broad diversity of research performed across the section and aims to put a spotlight on the main areas of interest. All research presented here displays strong advances in theory, experiment, and methodology with applications to compelling problems. This collection aims to further support Frontiers’ strong community by recognizing highly deserving authors.

Paul Scherrer Institut (PSI)

Universiti Malaysia Pahang

University of Salerno

University of Hertfordshire

Frontiers in Materials

Structural Materials

Frontiers in Built Environment

2022 Retrospective: Structural Materials

Welcome to the Research Topic “2022 Retrospective: Structural Materials.” This curated Research Topic highlights esteemed spontaneous articles from the past few years, personally selected by our Chief Editor, Prof. John L. Provis. The work showcased here underscores the extensive spectrum of research conducted within the section and seeks to illuminate its primary areas of interest. All the studies presented here demonstrate significant advancements in theory, experimentation, and methodology, offering solutions to compelling problems. Therefore, the editorial team deems it crucial to give special attention to these matters.Three of the four articles in the present issue of Frontiers in Materials address different aspects of cement concrete, and the remaining articles focus on utilizing 3D digital image processing to identify voids and detect pavement distress associated with road conditions.The first paper by <a href="https://doi.org/10.3389/fmats.2021.801464">Noman et al.</a> addressed low-cost repair techniques for restoring the mechanical properties of fire-damaged concrete. In their study, the concrete samples underwent heating within the range of 400°C to 800°C using a propane gas furnace. The tests included ultrasonic pulse velocity, rebound hammer, and compressive strength measurements. The findings showed that a cement-based slurry injection (with a water curing repair technique) is the best low-cost repair technique for restoring residual strength and stiffness for concrete damaged by a fire that reached 700°C.In recent years, the three-dimensional ground-penetrating radar system has been an effective method of detecting road void disease. In line with the aim of this Research Topic, <a href="https://doi.org/10.3389/fmats.2022.850694">Huang et al.</a> highlighted the research on a three-dimensional ground-penetrating radar void signal recognition algorithm based on a digital image. The images were processed by binarization, corrosion, expansion, connected area inspection, fine length index inspection, and three-dimensional matching inspection. The authors concluded that the void identification algorithm can accurately identify the position of the void area between 2.2% and 17.3%. They also indicated that the volume index calculated by the algorithm has a specific engineering application value compared to other methods.Slag gypsum is beneficial to industrial backfill, making its use in mine backfill a good application prospect. <a href="https://doi.org/10.3389/fmats.2022.869875">Xiaobing et al.</a> presented their study on the effect of slag-gypsum binder as a substitute cement on the stability of backfill mining. In the investigation, slag-gypsum binder (SGB) was used as a cement replacement in the mine backfill. Workability, X-ray diffractometry, and scanning electron microscopy were employed in their study. The authors found that the SGB can effectively improve the strength of backfill and has a positive effect on early strength. They also noted that SGB and cement have very different hydration behaviors.The synergistic effect of combining supplementary cementitious materials as partial substitutes for clinker improves cement properties and reduces its clinker factor and, hence, its carbon footprint. Our final paper highlights the environmental and mechanical assessment of blended cement containing a high mineral admixture content. <a href="https://doi.org/10.3389/fmats.2022.880986">Rodrigues et al.</a> concluded that the formulated cements, particularly those in multi-addition cements, are potential alternatives for reducing greenhouse gas emissions while preserving the mechanical performance demanded by construction market practices.Overall, we hope that the comprehensive research showcased in this Research Topic enhances the understanding and interest of all researchers and practitioners dedicated to recent developments and advancements in structural materials and analysis.JP: writing&#x2013;review and editing. RP: writing&#x2013;original draft and writing&#x2013;review and editing. AN: writing&#x2013;review and editing. AK: writing&#x2013;review and editing.The author(s) declare that no financial support was received for the research, authorship, and/or publication of this article.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process or the final decision.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, or the reviewers. Any product that may be evaluated in this article, or any claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Editorial: 2022 Retrospective: structural materials

The synergistic effect of combining supplementary cementitious materials (SCMs) as partial substitutes for clinker improves cement properties and reduces its clinker factor and, hence, its carbon footprint. Limestone-calcined clay cement (LC3)—a family of clinker, calcined clay, and limestone filler mixes—is studied worldwide for its properties equivalent to those of Portland cement. Although slag and fly ash are no longer sufficient to keep up with current commercial blended cements, in the long run, these SCMs can support the development of optimized formulations for the future. By relating the environmental and the mechanical performances, the GHG emission intensity offers a broader assessment and selection perspective. In this article, 13 blended cements were evaluated: ternary, quaternary, and multi-admixture (i.e., OPC plus 4 SCMs) blends with clinker factor between 40 and 50%, composed of—in addition to calcined clay and limestone filler—blast furnace slag and fly ash. Compressive strength was measured at 3, 7, 28, 91, and 365 days. The greenhouse gas (GHG) emissions were estimated through life cycle assessment and related to the blends’ compressive strength unit. Quaternary and multi-addition cements consistently outperformed after 3 days of age, demonstrating the benefits of the synergistic effect between SCMs jointly on GHG emissions and compressive strength. Such an effect enables reducing not only the clinker factor and carbon footprint but also the GHG emission intensity, which relates both. This study showed that the formulated cements, particularly those composed of multi-additions (Series D), are potential alternatives for reducing the GHG emissions, whilst preserving mechanical performance demanded by construction market practices. From a multidisciplinary analysis standpoint, durability assessments are necessary to complement the reported findings, as low clinker contents can affect the pH of the concrete’s pore solution and carbonation which ultimately lead to deterioration.

The synergistic effect of combining supplementary cementitious materials (SCMs) as partial substitutes for clinker improves cement properties and reduces its clinker factor and, hence, its carbon footprint. Limestone-calcined clay cement (LC3)—a family of clinker, calcined clay, and limestone filler mixes—is studied worldwide for its properties equivalent to those of Portland cement. Although slag and fly ash are no longer sufficient to keep up with current commercial blended cements, in the long run, these SCMs can support the development of optimized formulations for the future. By relating the environmental and the mechanical performances, the GHG emission intensity offers a broader assessment and selection perspective. In this article, 13 blended cements were evaluated: ternary, quaternary, and multi-admixture (i.e., OPC plus 4 SCMs) blends with clinker factor between 40 and 50%, composed of—in addition to calcined clay and limestone filler—blast furnace slag and fly ash. Compressive strength was measured at 3, 7, 28, 91, and 365&nbsp;days. The greenhouse gas (GHG) emissions were estimated through life cycle assessment and related to the blends’ compressive strength unit. Quaternary and multi-addition cements consistently outperformed after 3&nbsp;days of age, demonstrating the benefits of the synergistic effect between SCMs jointly on GHG emissions and compressive strength. Such an effect enables reducing not only the clinker factor and carbon footprint but also the GHG emission intensity, which relates both. This study showed that the formulated cements, particularly those composed of multi-additions (Series D), are potential alternatives for reducing the GHG emissions, whilst preserving mechanical performance demanded by construction market practices. From a multidisciplinary analysis standpoint, durability assessments are necessary to complement the reported findings, as low clinker contents can affect the pH of the concrete’s pore solution and carbonation which ultimately lead to deterioration.

Environmental and Mechanical Evaluation of Blended Cements With High Mineral Admixture Content

The three-dimensional ground-penetrating radar system is an effective method to detect road void disease. Ground penetrating radar image interpretation has the characteristics of multi-solution, long interpretation period, and high professional requirements of processors. In recent years, researchers have put forward solutions for automatic interpretation of ground-penetrating radar images, including automatic detection algorithm for subgrade diseases based on support vector machines, etc., but there are still some shortcomings such as training models with a large amount of data or setting parameters. In this article, a three-dimensional ground-penetrating radar void signal recognition algorithm based on the digital image is proposed, and the algorithm uses digital images to characterize radar signals. With the help of digital image processing methods, the images are processed by binarization, corrosion, expansion, connected area inspection, fine length index inspection, and three-dimensional matching inspection, so as to identify and determine the void signals and extract the void area volume index. The algorithm has been verified by laboratory tests and engineering projects, and the results show that the void identification algorithm can accurately identify the void area position; the error level between the measured values and the measured values of length, width, buried depth, and area is between 2.2 and 17.3%, and the error is generally within the engineering acceptance range. The volume index calculated by the algorithm has a certain engineering application value; compared with the support vector machine, the regressive convolution neural network, and other recognition methods, it has the advantage of not needing a large amount of data to train or modify parameters.

The three-dimensional ground-penetrating radar system is an effective method to detect road void disease. Ground penetrating radar image interpretation has the characteristics of multi-solution, long interpretation period, and high professional requirements of processors. In recent years, researchers have put forward solutions for automatic interpretation of ground-penetrating radar images, including automatic detection algorithm for subgrade diseases based on support vector machines, etc., but there are still some shortcomings such as training models with a large amount of data or setting parameters. In this article, a three-dimensional ground-penetrating radar void signal recognition algorithm based on the digital image is proposed, and the algorithm uses digital images to characterize radar signals. With the help of digital image processing methods, the images are processed by binarization, corrosion, expansion, connected area inspection, fine length index inspection, and three-dimensional matching inspection, so as to identify and determine the void signals and extract the void area volume index. The algorithm has been verified by laboratory tests and engineering projects, and the results show that the void identification algorithm can accurately identify the void area position; the error level between the measured values and the measured values of length, width, buried depth, and area is between 2.2 and 17.3%, and the error is generally within the engineering acceptance range. The volume index calculated by the algorithm has a certain engineering application value; compared with the support vector machine, the regressive convolution neural network, and other recognition methods, it has the advantage of not needing a large amount of data to train or modify parameters.

Research on Void Signal Recognition Algorithm of 3D Ground-Penetrating Radar Based on the Digital Image

To effectively apply slag-gypsum binder (SGB) to most modern mines. An experimental study is conducted to investigate the feasibility and reliability of slag-gypsum binder (SGB) to replace cement in the mine backfill to effectively apply SGB in modern mines. The optimum parameters of aggregate and SGB fineness were investigated on a laboratory basis and the effect of concentration on the workability was analyzed. Also, industrial tests were carried out to study the cementitious properties of SGB and compared to cement. Finally, the hydration properties of SGB were analyzed using XRD and SEM. The results indicated that the main hydration products of SGB are C-S-H gel and ettringite, while Ca(OH)2 is consumed during the hydration process. In addition, the structure formed in the later stage of SGB is more compact. SGB is beneficial to the strength of the waste rock mixture. The sample prepared with the waste rock-rod milling sand-river sand mixture has a maximum strength of 7.54 MPa at 28 d. Reducing fineness is beneficial to form early strength. However, a specific surface area exceeding 510 m2/kg will harm the later strength. Surface area is less (400 m2/kg), the sample strength decreases when the dosage increases. Comparing the workability of SGB slurry and cement slurry, it is found that there is no essential difference between them under different mass concentration conditions. SGB slurry shows better fluidity and stability at a mass concentration of 80–82%. Industrial backfill experiments show the 3-, 7-, and 28-d strengths of SGB samples under 80% mass concentration conditions are 0.91 times, 1.43 times, and 1.2 times the cement samples. Therefore, using SGB instead of cement to backfill is a good application prospect.

To effectively apply slag-gypsum binder (SGB) to most modern mines. An experimental study is conducted to investigate the feasibility and reliability of slag-gypsum binder (SGB) to replace cement in the mine backfill to effectively apply SGB in modern mines. The optimum parameters of aggregate and SGB fineness were investigated on a laboratory basis and the effect of concentration on the workability was analyzed. Also, industrial tests were carried out to study the cementitious properties of SGB and compared to cement. Finally, the hydration properties of SGB were analyzed using XRD and SEM. The results indicated that the main hydration products of SGB are C-S-H gel and ettringite, while Ca(OH)2 is consumed during the hydration process. In addition, the structure formed in the later stage of SGB is more compact. SGB is beneficial to the strength of the waste rock mixture. The sample prepared with the waste rock-rod milling sand-river sand mixture has a maximum strength of 7.54&nbsp;MPa at 28&nbsp;d. Reducing fineness is beneficial to form early strength. However, a specific surface area exceeding 510 m2/kg will harm the later strength. Surface area is less (400 m2/kg), the sample strength decreases when the dosage increases. Comparing the workability of SGB slurry and cement slurry, it is found that there is no essential difference between them under different mass concentration conditions. SGB slurry shows better fluidity and stability at a mass concentration of 80–82%. Industrial backfill experiments show the 3-, 7-, and 28-d strengths of SGB samples under 80% mass concentration conditions are 0.91 times, 1.43 times, and 1.2 times the cement samples. Therefore, using SGB instead of cement to backfill is a good application prospect.

Effect of Slag Gypsum Binder as a Substitute to Cement on the Stability of Backfill Mining

Researchers have conducted numerous investigations on various repair techniques to restore the mechanical properties of fire-damaged concrete. However, most of the techniques are either expensive or require high technical skills. As a continuous work, this study involved an experimental investigation of the effects of various low-cost repair techniques to restore the mechanical properties of fire-damaged concrete. Sixty cylindrical concrete samples were subjected to heat ranging from 400°C to 800°C in a propane gas furnace. The samples were categorized into six groups, including the following: undamaged, fire-damaged, and fire-damaged and repaired using water curing; fire-damaged and repaired using cement-based slurry injection; re-curing fire-damaged and repaired with steel wire mesh along with epoxy resin wrapping; and fire-damaged and repaired using epoxy injection. The samples were tested for non-destructive and destructive tests, including ultrasonic pulse velocity, rebound hammer, and compressive strength. The results showed that the use of cement-based slurry injection along with the water-curing repair technique could regain almost 90% of its ultimate strength and secant stiffness, making it the most effective low-cost repair technique for fire-damaged concrete exposed up to 700°C.

Researchers have conducted numerous investigations on various repair techniques to restore the mechanical properties of fire-damaged concrete. However, most of the techniques are either expensive or require high technical skills. As a continuous work, this study involved an experimental investigation of the effects of various low-cost repair techniques to restore the mechanical properties of fire-damaged concrete. Sixty cylindrical concrete samples were subjected to heat ranging from 400&#xb0;C to 800&#xb0;C in a propane gas furnace. The samples were categorized into six groups, including the following: undamaged, fire-damaged, and fire-damaged and repaired using water curing; fire-damaged and repaired using cement-based slurry injection; re-curing fire-damaged and repaired with steel wire mesh along with epoxy resin wrapping; and fire-damaged and repaired using epoxy injection. The samples were tested for non-destructive and destructive tests, including ultrasonic pulse velocity, rebound hammer, and compressive strength. The results showed that the use of cement-based slurry injection along with the water-curing repair technique could regain almost 90% of its ultimate strength and secant stiffness, making it the most effective low-cost repair technique for fire-damaged concrete exposed up to 700&#xb0;C.