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The deposition of textile waste into landfill has reached an unsustainable level and raises serious environmental issues across the world. Transforming textile waste into fiber reinforcement in cementitious composites offers a sustainable resolution toward a circular textile economy. This article presents a comprehensive review of environmental concerns, recycling routes for textile waste, together with an in-depth review of the engineering properties of concrete incorporating recycled textiles. In general, the incorporation of these recycled fibers from textile waste enhances strain capacity, crack control, durability, and energy absorption of concrete via dual effects: bridging action (direct mechanism) and refinement of pore distribution (indirect effect). An improvement in compressive strength can be achieved by the utilization of a small dosage of recycled fibers or recycled fiber fabrics in concrete (strength < 40 MPa). Finally, the cost and environmental benefits for eco-efficient building application are also evaluated to draw the attention of researchers toward these potentially recyclable waste materials. [ABSTRACT FROM AUTHOR]

Geopolymer aggregate (GPA) is a novel coarse aggregate synthesized from low-calcium fly ash with a highly alkaline activator. It is also classified as a lightweight aggregate, having a density of 1709 kg/m³ (106.7 lb/ft3). This paper reports the findings of the detailed investigation of mechanical properties of GPA concrete, which was observed up to a period of 1 year. The characteristics of GPA concrete were benchmarked against conventional basalt aggregate concrete. Compressive, flexural, and splitting tensile strengths, elastic modulus, and Poisson's ratio of GPA concrete ranged from 42.1 to 50.81 MPa (6.1 to 7.37 ksi), 4.75 to 5.27 MPa (0.69 to 0.76 ksi), 3.02 to 3.66 MPa (0.44 to 0.53 ksi), 20 to 20.5 GPa (2900 to 2973 ksi), and 0.13 to 0.11, respectively within a 90- to 365-day period. The correlations between existing concrete standards and major mechanical properties of GPA concrete are discussed. Relationships are developed between compressive strength and mechanical properties including flexural strength, splitting tensile strength, and elastic modulus using statistical regression analysis. The suitability of using the existing relationships in Australian standards and American Concrete Institute codes for GPA concrete are critically examined. In addition, the microstructure of GPA concrete was examined using scanning electron microscopy (SEM) imaging and microhardness testing. The thickness of the interfacial transition zone (ITZ) is estimated to be 55, 50, and 45 µm (21.65 x 10-4, 19.68 x 10-4, and 17.72 x 10-4 in.) at 28, 90, and 365 days, respectively. Overall, the observations of this study verify the potential of using GPA concrete in various structural applications, making it a viable and sustainable alternative to conventional aggregate concrete. [ABSTRACT FROM AUTHOR]

Production of alkali-activated binders is a developing research field that utilizes industrial/agricultural by-products and solid waste for the development of sustainable concrete. This paper comprehensively reviews the literature relating to rice husk ash (RHA)–based alkali-activated binders incorporating fly ash (low/high calcium) and blast furnace slag. The literature demonstrates that the properties of raw material significantly influence the formation of the alkali-activated gel matrix. Every precursor (low/high calcium fly ash and slag) that is used to develop alkali-activated binders with RHA have their own reaction mechanism dependant on their specific chemical composition. Hence the incorporation of RHA influences each binder in a unique way depending upon the alkali activation process and reaction mechanisms. The incorporation of RHA, in the range 5%–15%, with blended slag alkali activated binders, yields better compressive strength, when compared with RHA blended with fly ash (low/high calcium). The review presented in this paper is very useful to understand the behavior of alkali-activated binders incorporating RHA and in advancing the research into the successful application of RHA as a binder for alkali-activated materials. [ABSTRACT FROM AUTHOR]

Quarry aggregate reserves are depleting rapidly within Australia and the rest of the world due to an increasing demand for aggregates driven by expansion in construction. The annual production of premix concrete in Australia is approximately 30 million cubic meters, while 3–5% of concrete delivered to site remains unused and is disposed of in landfill or crushing plants. The production of coarse aggregates using this waste concrete is potentially a sustainable approach to reduce environmental and economic impact. A testing program has been conducted to investigate mechanical performance and permeation characteristics of concrete produced using a novel manufactured coarse aggregate recycled directly from fresh premix concrete. The recycled coarse aggregate (RCA) concrete satisfied the specified 28-day design strength of 25 MPa and 40 MPa at 28 days and a mean compressive strength of 60 MPa at 90 days. Aggregate grading was observed to determine strength development, while low water absorption, low drying shrinkage, and higher packing density indicate that the RCA concrete is a high-quality material with a dense pore structure. The rough fracture surface of the aggregate increased the bond between C-S-H gel matrix and RCA at the interfacial transition zone. Furthermore, a good correlation was observed between compressive strength and all other mechanical properties displayed by the quarried aggregate concrete. The application of design equations as stated in Australian standards were observed to provide a conservative design for RCA concrete structures based on the mechanical properties. [ABSTRACT FROM AUTHOR]

• Hydrophilic fabric fibres refine the pore structure of cementitious matrices. • Hybrid hydrophobic and hydrophilic fibres counterbalance the pore-refining effects. • Fabric waste fibres suppress the crack propagation and mitigate the quantity of cracks. • Hybrid fabric fibres significantly reduces the shrinkage of cementitious composites. • Blended fabric waste fibres can improve strength properties of cement-based materials. In this study, the strength properties, shrinkage and microstructures of mortar incorporating blended fabric fibres were characterised via X-ray micro CT and nanoindentation. Three different hybrid recycled fabric fibres, namely Kevlar/Nomex, Kevlar/Nylon and Nomex/Nylon fibres were investigated at three different blend ratios (2:1, 1:1, and 1:2). The fibre content was maintained at 0.3 % for all mixes. The findings indicated that the optimum blend ratio for hybrid fabric fibres is 1:1. At this optimum fibre blend ratio, an enhancement by 2.7 %, 4.8 % and 5.9 % in compressive strength, together with 9.8 %, 12 % and 13.4 % in flexural strength of mortar are recorded, corresponding to the inclusion of hybrid Nomex/Nylon, Kevlar/Nomex and Kevlar/Nylon fibres respectively. Kevlar/Nomex fibres display no effect in drying shrinkage mitigation, irrespective of blend ratios. In contrast, Kevlar/Nylon and Nomex/Nylon fibres result in reducing 180-day shrinkage rates by up to 13.9 % and 11 % respectively. The hybrid fabric fibres were found to refine the pore network, especially the highly hydrophilic Kevlar/Nomex fibres. Also, an increase in curing days from 7 to 90 days densifies the microstructure near the fibre-matrix interface due to the growth of hydration products (LD/HD C S H and CH). These research findings can open the pathway for utilising textile waste in landfill as reinforcing members for cementitious matrices toward sustainable building and construction. [ABSTRACT FROM AUTHOR]

• HVFA concrete has low early age strengths above 60% cement replacements. • HVFA concrete has a higher sulphate/acid/chloride resistance and a lower shrinkage. • Higher the fineness of fly ash and additives, greater the reactivity and hydration. • Nano silica significantly enhances the properties of HVFA concrete at all ages. • Material combinations could be optimized to develop effective HVFA concretes. Fly ash is commonly used as a partial cement replacement material, but this is limited to replacement levels of 30% or less, with significant quantities of fly ash still not utilized globally. There has been significant recent research into High Volume Fly Ash (HVFA) concrete to enable the utilization of fly ash and to reduce CO 2 emission by reducing cement demand. This comprehensive review summarizes up to date literature on HVFA concrete with more than 50% of cement replacement using ASTM Class F low calcium fly ash. Firstly, the available HVFA literature in which only fly has been used to replace cement, is categorized based on the replacement level and the mechanical and durability property results are summarized. Secondly, the remaining literature is categorized based on the different material additions to modify the HVFA concrete and the results are compared. The summarized results are discussed to elucidate the mechanisms underlying the reported results. The effect of each material addition on the HVFA concrete properties are also discussed to identify potentially more suitable additives for future development. Overall, this paper will provide an understanding of the current state of HVFA concrete research and the gaps in research for the development HVFA concrete containing higher replacement levels and achieving the required performance. Hence, summarised knowledge would significantly be beneficial to design prospective research towards a sustainable cement-free concrete using industrial waste. [ABSTRACT FROM AUTHOR]

• AAS–RA concrete attained 90% compressive strength produced by AAS–RA concrete. • AAS–RA concrete showed lower chloride permeability and sorptivity in long term. • Extending water curing upto 28 days reduces drying shrinkage of AAS-RA concrete. • Porous external surface is attributed to initial water absorption in AAS-RA concrete. • AAS-RA concrete displayed 52% carbon emission reduction than PC concrete. Adaption of reclaimed resources within the construction industry, in order to move towards environmental sustainability and a carbon neutral society is essential. To address this issue this study focused on the investigation of the long term performance, carbon emissions and coast savings of Alkali-activated slag (AAS) concrete incorporating recycled coarse aggregate (AAS-RA) up to one year of age. The performance and sustainability aspect of AAS-RA concrete was then compared with AAS concrete incorporated with natural quarry aggregate (AAS-NA) and PC concrete, respectively. Both AAS concretes achieved similar compressive strength of approx. 40 MPa and tensile strength of approx. 3.3 MPa after one year. Hence, full replacement of quarried coarse aggregate using recycled aggregate in AAS concrete did not display any evidence of an adverse impact to the strength characteristics. However, the 7-day and 28-day water cured AAS concretes demonstrated 32% and 16% higher drying shrinkage at one year in excess of the maximum permissible limit specified in AS3600. Both AAS concretes displayed high water absorption but low chloride permeability and sorptivity. A highly porous external surface layer interconnected with numerous capillaries and microcracks is hypothesised to be the reason for the high water absorption. Gel formation densified the microstructure and filled the capillaries in the bulk matrix, which in turn resulted in the lower permeability and secondary sorptivity. The AAS-NA and AAS-RA concretes displayed 43.5% and 52% carbon emission reduction compared to an equivalent strength of PC concrete having similar binder content. [ABSTRACT FROM AUTHOR]

• Two HVFA concrete mixes utilizing 65% and 80% cement replacement, were developed. • Nano-silica & Hydrated lime increased early age hydration involving C 3 A and C 4 AF phases. • Formation of Monosulfoaluminate contribute to early age compressive strength gain. • HVFA concrete displayed 51–60% carbon savings and a reduced Global Warming Impact. • HVFA concrete shows a 10% cost reduction compared with Portland Cement concrete. This study investigates strength development, reactivity and environmental/economic benefits of blended High Volume Fly Ash (HVFA) concrete mixes utilizing 65% and 80% cement replacement utilizing a combination of fly ash and hydrated lime, with and without nano-silica. The carbon and non-carbon emissions are considered as environmental impacts while life cycle costs from cradle-to-gate, which is from material extraction to production, are considered for comparison of the economic benefits. The compressive strength of the HVFA mixes increased with the addition of nano-silica. The HVFA–65 and HVFA–80, without nano-silica, achieved 25.0 MPa and 14.5 MPa at 7 days, respectively, and 42.7 MPa and 29.5 MPa at 28 days. With the addition of nano-silica the HVFA–65 ns and HVFA–80 ns concrete had compressive strengths of 37.5 MPa and 28.8 MPa at 7 days and increased to 47.1 MPa and 40.1 MPa at 28 days. Incorporating 3% nano-silica into HVFA concrete increased the early age hydration reaction. This is attributed to the reaction of the C 3 A and C 4 AF phases and the formation of monosulfoaluminate, which contributed to the early age strength gain. The majority of Ca2+ ions were consumed during the initial hydration, with few Ca2+ ions remaining for the subsequent hydration reaction with the C 3 S phase. The HVFA concrete mixes displayed between 51 and 60 % carbon savings and a reduced Global Warming Impact. The non-Greenhouse Gas emissions, i.e. SO 2 and NO x , reflects minor savings in the Acidification Impact (AI) and Photochemical Oxidant Formation Impact (POFI) environmental impact indicators. Further, HVFA concrete incorporated with hydrated lime shows a 10% cost reduction compared with Portland Cement concrete. [ABSTRACT FROM AUTHOR]