Documents published in Scipedia

• Application of Nanomaterials for Enhancing the Performance of Recycled Aggregate Concrete-A Short Review

  X. Caihong, Z. Qingxin

  DBMC 2023.

  
  

  Abstract

  Incorporation of recycled concrete aggregates (RA) produced from construction and demolition wastes in concrete contributes to the sustainability from two perspectives: reducing environmental pollution and reserving natural resources by reducing the consumption of natural aggregates (NA). However, comparing to NA, RA has lower mechanical properties and higher water absorptivity due to the old mortar attached on RA surface, resulting in weakened interfacial transition zones (ITZs) in the concrete and the consequent reduced durability of concrete when RA was used to replace NA for producing recycled aggregates concrete (RAC). Therefore, extensive research work had been devoted to the enhancement of RAC performance and nanomaterials have demonstrated great potential in this regard. This paper reviews recent progress on the application of various nanomaterials for improving the microstructure as well as nano/mechanical properties of ITZs in RAC, and special attentions were given to the dispersion strategies for nanomaterials which determines the amount of nanomaterials required to achieve reliable improvement in RAC performance and thus the cost of using nanomaterials in RAC.

  Abstract
  Incorporation of recycled concrete aggregates (RA) produced from construction and demolition wastes in concrete contributes to the sustainability from two perspectives: reducing [...]

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• Modification of Recycled Cement with Different Additives

  L. Xu, J. Wang

  DBMC 2023.

  
  

  Abstract

  The unmodified recycled cement (RC) paste has unfavorable properties (like fast setting, high water demand and low strength) restricting application. This paper intends to rectify these shortcomings and improve RC with different additives. A comprehensive experimental program combining flowability, setting time, isothermal calorimetry and mechanical tests of modified recycled cement (M-RC) paste with different proportions were conducted. The results indicate that adding TEA and pectin could significantly improve the early flowability of RC and delay its setting, but it will have adverse effects on the mechanical properties of RC paste. Adding 0.5% TEA or 0.075% pectin can increase the fluidity by 240% and 293%. For the 0.5% TEA and 0.075% pectin groups, the 3-day compressive strengths were reduced by 44.3% and 49.5%, respectively.

  Abstract
  The unmodified recycled cement (RC) paste has unfavorable properties (like fast setting, high water demand and low strength) restricting application. This paper intends to [...]

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• Influence of Waste Fibers on the Total Porosity of Cementitious Composites

  K. Didulica, A. Pranjić, M. Štefančić, B. Mrduljaš, A. Baričević

  DBMC 2023.

  
  

  Abstract

  The amount of water is essential to explain the shrinkage process. It is determined by the water-cement ratio, but also by the moisture content. Water in hydrated cement paste occurs in various forms: chemically bound, interlayer, adsorbed, free water and water vapor. During cement hydration, the balance between various forms of water changes. For example, the amount of physically bound water diminishes, and the moisture content changes at similar environmental conditions. Moisture changes occur first in the larger pores and then in the smaller pores. The air pores are larger than the hydration pores (capillary pores, gel pores) and have a greater effect on strength and permeability properties, while the cement hydration pores have a greater effect on shrinkage. At early ages, when composite properties are not fully developed, higher shrinkage influences the development and propagation of cracks. These (micro)cracks represent weak points and affect durability. To reduce shrinkage and bridge the cracks, fibers are added to the cementitious composite. Depending on the type and geometry, the fibers can reduce shrinkage and bridge the cracks. To reduce the environmental impact of fiber production, the use of waste fibers in cementitious composites is proposed. The main objective of this study was to investigate the influence of alkali-resistant glass fibers from production waste on the development of the microstructure of cementitious composites, i.e., the influence on the total porosity. The pore content was determined on fresh cementitious composites after mixing, while the total porosity of the material was measured after 28 days using a mercury intrusion porosimeter. Results were complemented by X-ray computed microtomography (micro-CT). The influence of production waste fibers is presented as a function of length and fiber content. In addition to the reference mixture, results were also compared with mixtures containing factory-produced fibers.

  Abstract
  The amount of water is essential to explain the shrinkage process. It is determined by the water-cement ratio, but also by the moisture content. Water in hydrated cement paste [...]

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• Effect of Current Modes on Electrochemical Chloride Extraction Treatments for Concrete

  C. Song, C. Jiang, X. Gu, Q. Zhang

  DBMC 2023.

  
  

  Abstract

  The current mode plays an important role in the Electrochemical Chloride Extraction (ECE) treatment for concrete. In this study, the experiments were conducted to investigate how the ECE process would be affected by different current modes (continuous and intermittent current modes). The results showed that there were remaining amounts of chloride ions that were hard to be removed by both current modes. Additionally, the porosity under the two current modes showed the same upward trend compared to the initial value, and the chloride diffusion coefficient increased by 1 - 2 times. Also, the chloride-contaminated concrete subjected to the continuous current mode had a higher cumulative chloride extraction content than that subjected to the intermittent mode. The intermittent current mode could not increase the effectiveness of extracting chloride when the current density was not greater than 3 A/m2 and the on-off ratio was not less than 3.2.

  Abstract
  The current mode plays an important role in the Electrochemical Chloride Extraction (ECE) treatment for concrete. In this study, the experiments were conducted to investigate [...]

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• Comparison of Non-calcined and Calcined Red Muds from Different Sources as Potential Supplementary Cementitious Material

  S. Chen, A. Ghani Razaqpur

  DBMC 2023.

  
  

  Abstract

  Red mud (RM) is a hazardous by-product of alumina refining processes. Due to its high alkalinity, large specific surface area, and complex and variable composition, RM is hard to treat or utilize on a large scale. To date, more than 4 billion tons of RM have been stockpiled globally and is still growing by more than 120 million tons annually. Many investigations have focused on the largescale utilization of RM as a construction material, either in its virgin form or after heat treatment. However, the huge differences among the chemical/mineralogical compositions of RMs due to differences among bauxite ores from different sources and/or different refining processes, it is difficult to prescribe a unique process for activating RM cementitious/pozzolanic properties. The present study aims to identify the key chemical and process-dependent factors that influence the cementitious/pozzolanic properties of RM. Two types of RM from different sources are investigated to determine the effects of the chemical/mineralogical composition, the alumina refining process, and calcination on the mineralogical phases and compressive strength of mortar cubes made with ordinary Portland cement (OPC)- RM blended cement. Mortar made with 15 wt.% OPC replaced by one type of virgin RM produced by the Bayer’s process was found to have better strength than a control mortar made with 15% OPC replaced by sand, which indicates that this RM had cementitious/pozzolanic property without requiring heat treatment. On the other hand, the RM produced by the bauxite calcination method needed heat treatment to improve its pozzolanicity, but, despite the improvement, mortar made with its optimally treated form had lower 91-day compressive strength than the companion control mortar made with extra sand as RM replacement. The compressive strength of mortars made with calcined RM was not only affected by the phase changes of the virgin RM properties brought about by calcination but also by changes to its physical.

  Abstract
  Red mud (RM) is a hazardous by-product of alumina refining processes. Due to its high alkalinity, large specific surface area, and complex and variable composition, RM is [...]

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• Rebar-matrix Interface in Extrusion Molded Cementitious Filaments

  R. Yang, Q. Zeng, Z. Wang

  DBMC 2023.

  
  

  Abstract

  Extrusion molding enables automatic construction through the pattern of material addition. The unique molding would result in several features that are different from those in conventional framework molding, such as weak interlayer bonding and a laminar structure with high anisotropy. To strengthen the interlayer bonding, reinforcement is often implanted to penetrate through the extrusion molded filaments (EMF). However, different from the pre-implantation of reinforcement before concrete casting, the post-implantation of reinforcement in EMF may trigger the problems concerning the interface bonding between reinforcement and material matrix. In this work, saddle stitches, a type of Ushape rebar or fiber, are taken as an example to demonstrate the effect of the post-implantation of reinforcement on the interfacial structure between the rebar and EMF matrix. X-ray computed tomography (XCT) is employed to non-destructively probe the post-implanted saddle stitches as well as the surrounded EMF materials. Regions of interest (ROI) are set on the rebar-matrix interfaces to elaborately analyze the material or defect distributions around the saddle stitches. Results demonstrate that insufficient material filling is observed at the outboard of the U-shape rebar, while material pressing is shown at the inboard. Large defects unevenly form in the rebar-matrix interface. Mechanisms of the observations may be ascribed to the low flowability of EMF materials that can hardly migrate to the rebar-matrix interfacial gaps spontaneously. The findings would deepen the understandings in rebar-matrix interfacial structure of EMF with post-implantation of reinforcement.

  Abstract
  Extrusion molding enables automatic construction through the pattern of material addition. The unique molding would result in several features that are different from those [...]

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• Effectiveness of Paraffin-based Curing Compound for Mitigation of the Plastic Shrinkage and Related Cracking of the 3D-printed Concrete Elements

  S. Markin, V. Mechtcherine

  DBMC 2023.

  
  

  Abstract

  3D-printed concrete elements are highly vulnerable to early-age shrinkage and cracking compared to conventionally cast concrete elements. Material compositions of printable concretes and complete renunciation of shuttering are responsible for accelerated capillary pressure build-up, leading to volumetric constructions of the 3D-printed layers at a very early age after extrusion and enhanced shrinkage at a later age. Shrinkage-induced cracks can severely impair the durability and appearance of 3D-printed concrete structures. The study at hand analyses the efficacy of the paraffin-based curing agent for reducing the shrinkage and cracking of 3D-printed elements and structures. Development of the temperature and capillary pressure, as well as shrinkage strains, were reported for the cured and uncured specimens. The study results show that applying paraffin-based curing agents can considerably reduce shrinkage-indued deformations of the 3D-printed elements produced even under extreme environmental conditions.

  Abstract
  3D-printed concrete elements are highly vulnerable to early-age shrinkage and cracking compared to conventionally cast concrete elements. Material compositions of printable [...]

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• Effect of Delayed Addition of TEA on the Performance of Cement with Different Sulfate Carriers

  Z. Lu, X. Peng, Z. Sun, D. Stephan

  DBMC 2023.

  
  

  Abstract

  Concrete 3D printing is one promising technique used for future construction. However, a bottleneck limiting this technology's rapid development and practical application is the conflicting requirements for concrete properties before and after printing. Hence, the application of the accelerator and a deep understating of its working mechanism is becoming more and more important. As one commonly used accelerator, triethanolamine (TEA) has the advantage of low dosage and high efficiency to reach a fast setting. However, it is still unclear how the delayed addition of TEA affects the performance of cementitious materials. Besides, it is found that the different types and combinations of sulfate carriers can significantly affect the performance of TEA on cement hydration. However, the combined effect of different sulfate carriers and different addition times of TEA on the hydration process of cement paste is also unclear. The effect of different TEA addition times on the hydration and setting performance of cement pastes containing different types and combinations of sulfate carriers were thoroughly investigated in this study. It is found that, depending on the types of sulfate carriers inside, the different addition times of TEA can significantly affect the hydration and setting performance of cement paste. Specifically, to artificial cement (ArC) prepared with hemihydrate (HH), the different TEA addition times do not affect the setting performance of cement paste. To ArC with the two sulfate carriers of anhydrite (AH) and HH, a flash setting was observed when the TEA was added simultaneously. However, a delayed addition of TEA for just 5 min can make the setting performance back to normal. To ArC with only AH, a longer addition time of 15 min is needed to eliminate the flash setting behavior. This phenomenon could be originated from the competed interaction of sulfate ions released from sulfate carriers and TEA with the aluminate phase contained in the clinker.

  Abstract
  Concrete 3D printing is one promising technique used for future construction. However, a bottleneck limiting this technology's rapid development and practical application [...]

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• Adapting Construction and Demolition Waste to Circular Building Materials and Evaluation of Life Cycle Environmental Impacts

  A. Kul, E. Ozcelikci, B. Furkan Ozel, G. Yildirim, A. Ashour

  DBMC 2023.

  
  

  Abstract

  Upcycling of construction and demolition wastes (CDWs) into value-added building materials that can be used for rapid construction and housing in developing countries and disasterprone areas is important and offers a fast, cost-effective, and eco-friendly solution. The objective of this study is to investigate the feasibility of using different-size recycled concrete aggregates (RCAs) in geopolymers made entirely from CDW-based masonry components including hollow brick (HB), red clay brick (RCB), and roof tile (RT) as precursors. To improve the strength development, thermal curing was applied to the developed geopolymer mortars. Compressive strength test was used for mechanical characterization. To assess the environmental impact of geopolymer mortars, a cradle-to-gate life cycle assessment (LCA) was carried out. Findings revealed that RT-based geopolymer mortars were with the highest compressive strength level reaching 66.2 MPa based on different mixture and curing parameters. Compressive strength increased with the reduction in the maximum RCA size. According to LCA analysis, CDW-based geopolymer mortars resulted in similar strength but significant reduction in CO2 emissions (up to 60%) and are with comparable energy consumption to the Portland cement-based mortars.

  Abstract
  Upcycling of construction and demolition wastes (CDWs) into value-added building materials that can be used for rapid construction and housing in developing countries and [...]

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• A Conceptual Model for Assessing Circularity Potential of Building Materials at the Product Manufacturing Stage

  N. Tharaka Dharmasiri Pathberiyage, E. Delzendeh, F. Cheung, M. Mateo Garcia

  DBMC 2023.

  
  

  Abstract

  The construction industry presently accounts for 30% of natural resource extraction and 25% of solid waste generation. The prevailing economy is “Linear” which is summarised as take-makedispose. On the contrary, the “circular economy” model is a systematic model to restore, regenerate and expand the lifecycle of materials. Most of the existing circularity assessment methods are focused on the end-of-life wastage of building materials while neglecting resource consumption and wastage at the product manufacturing stage. Further, these methods only consider direct material flows for assessing the circularity potential of building materials and overlook the indirect material flows associated with product manufacturing. There is a need to develop metrics to assess the circularity performance of building materials more holistically. Therefore, this study proposes a conceptual model to assess the circularity potential of building materials by analysing both direct and indirect material flow processes of the product manufacturing stage including raw material extraction, transportation, and manufacturing. The method used to design the conceptual model includes a comprehensive literature review in two stages. First, the existing circular assessment methods are reviewed to identify the methods used for assessing the circularity potential of building materials. Secondly, the circularity options are explored to develop the circularity metrics. According to the findings of this study, in the absence of a comprehensive method to assess the circularity potential of building materials, the life cycle assessment and material flow analysis are the most prominent circularity assessment methods used. Furthermore, circularity options such as industrial waste (by-products), biodegradability, biofuels, renewable energy, reusability, recoverability, recyclability and product life span are identified as the circularity metrics for building materials at the product manufacturing stage.

  Abstract
  The construction industry presently accounts for 30% of natural resource extraction and 25% of solid waste generation. The prevailing economy is “Linear” which [...]

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