Recycling mine tailings as precursors for cementitious binders – Methods, challenges and future outlook

© 2021 Elsevier Ltd. All rights reserved. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1016/j.conbuildmat.2021.125333Increase in demand for mineral commodities such as coal, copper, iron, aluminium, gold, tungsten, zinc etc., has led to higher quantity of mineral waste produced such as solids, crushed rocks, overburden soil and tailings. The fine-grained mineral waste left after removal of valuable material from ore is called mine tailing and is one of the major wastes of the mining processes. Mineral wastes from mines, quarries and excavations are typically rich in SiO2, Al2O3, CaO and Fe2O3. This chemical composition makes them very attractive candidates to be used in the production of construction materials, as these oxides are also the main constituents of cement as well as of key alkali-activated binders. This contribution aims to provide a comprehensive overview of the nature of mine tailings, the current state-of-the-art in their utilisation in cementitious binders and the future potential. A rational summary of limitations associated with use of mine tailing in cementitious binder due to its low reactivity and potential solutions to overcome it is also provided. The study concludes with how the use of mine tailings in cementitious binder could benefit in achieving the global sustainability goals.Peer reviewe

THE EFFECT OF MINING WASTE ON THE DURABILITY INDICATORS OF CEMENT-BASED COMPOSITES

© 2021 The Authors.The need for infrastructure development is of major importance and the projected global infrastructure investment between 2013-2030 is estimated in the excess of £30 trillion to support the rapid growth of societies and economies worldwide (1). This trend puts civil infrastructure industry under immense stress to plan properly, construct fast and deliver resilient structures. Concrete is the dominant construction material and the key element in most infrastructure assets. However, concrete’s manufacture is extremely energy and resource intensive: >4 Billion tonnes of cement are produced annually, accounting to ~8% of global anthropogenic CO2 and resulting to an annual production of ~2 tonnes of concrete for every person on the planet. The production of concrete is a process associated with very high energy consumption. In Europe, the construction sector alone is responsible for the 36% of CO2 emissions and the 40% of all energy consumption. The utilisation of mining waste in cement-based composites is an area of growing interest worldwide, with mining and excavation waste increasing considerably the last decade. Our work focuses on the replacement of cement with mineral wastes and the initial findings suggest that even at 20% replacement, the mechanical properties are marginally affected. This contribution will discuss some preliminary data on the effect of mining waste on the durability indicators of cementitious composites (oxygen permeability, capillary sorption and ion diffusion). Keywords: Mining waste, Silicates, capillary water absorption,Peer reviewedFinal Published versio

Recycling of fly ash-slag Geopolymer binder in mortar mixes

© 2022 Imperial College London.Fly ash-slag based Geopolymer cement (GPC) has demonstrated mechanical properties and environmental advantages that make it one of predominant sustainable alternatives to Portland cement (PC). Despite the fact that numerous environmental analyses about geopolymers are being published, their environmental impact after the end of service-life has barely been explored. Given that construction-waste management is a major sustainability issue, the present study is investigating the potential of recycling fly ash-slag GPC as a fine aggregate in mortar mixes. The major physical properties of the fine recycled aggregates (FRA) were tested and compared to those of PC FRA and natural sand of similar fineness. The effect of incorporating FRA in low (25%) and high (50%) percentage in PC or GPC matrix mortars was investigated. The 28day compressive and flexural strength of mortars were tested. Also the 28day water absorption and flow of mixes incorporating GPC FRA were recorded. GPC FRA exhibited properties similar to those of PC FRA and poorer than those of natural sand. The results of compressive and flexural strength proved that FRA addition had a negligible effect in all cases. The influence of the high water absorption of GPC FRA, relatively to that of natural sand, was prominent on the workability of fresh mixes and possibly affected the water absorption of mortar prisms. The effect of GPC FRA proved to be similar to that of PC FRA on compressive strength, while none of the tested mortar properties appeared to be jeopardised by the incorporation of the GPC FRA in the mixPeer reviewedFinal Published versio

Effect of recycled geopolymer concrete aggregate on strength development and consistence of Portland cement concretes

Numerous studies have shown that production of geopolymer cement concretes can have lower carbon emissions compared to Portland cement concretes. However, for a full lifecycle assessment of environmental impacts, scenarios for the end of structures’ design life of must be considered, including reuse options. The work presented here is part of a wider study investigating the recyclability of fly ash-slag geopolymer cement (GC) concrete as an aggregate in Portland cement (PC) concretes. Three types of GC concretes with varying Na2O % per mass of precursor and SiO2/Na2O molar ratio were produced in the laboratory. All other mix design parameters were kept constant. The concretes were investigated thoroughly through physical and mechanical testing and chemical characterization at various ages and then crushed mechanically to form recycled geopolymer concrete aggregates (RGCA). Two series of PC concretes with 20% aggregate replacement by RGCA were produced – one of S1 consistence class and one of S3 consistence class (design slumps of 10-40mm and 100-150mm). The effect of RGCA on PC concrete fresh properties was investigated. The compressive strength development was assessed by testing at 7, 28 and 90 days. All results were evaluated against concretes with recycled Portland cement concrete aggregates (RCA) and natural limestone aggregates. These results were paired with calorimetric studies of pastes produced with recycled concrete aggregate leachate. Although mix designs were adapted according to water absorption requirements, the consistence of concretes appeared to be largely dependent on the type of aggregate. The results showed that strength trends remained unaltered between the two concrete series and were mostly influenced by the aggregate type. Mixes with RGCA presented overall higher strengths than the RCA and limestone aggregate concretes. Tests at 90 days showed a continuous increase of compressive strength, while the trends between the concretes remained unaltered. Overall, this study has shown that RGCA affect new concretes in a different way to RCA. However, none of the factors investigated here should prevent the use of RGCA in new concretes

Chemical aspects related to using recycled geopolymers as aggregates

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Despite extensive research into sustainability of geopolymers, end-of-life aspects have been largely overlooked. A recycling scenario is examined in this study. This requires an investigation of alkali leaching potential from a geopolymeric matrix. To study the feasibility of geopolymer cement (GPC) recycling, the migration of alkalis was evaluated for the first time on a microstructural level through energy dispersive X-ray (EDX) scanning electron microscopy (SEM) elemental mapping and leaching tests. Macroscale impacts were assessed through an investigation of Portland cement (PC) mortar properties affected by alkali concentration. Leaching tests indicated that alkalis immediately become available in aqueous environments, but the majority remain chemically or physically bound in the matrix. This type of leaching accelerates the initial setting of PC paste. Elemental mapping and EDX/SEM analysis showed a complex paste-aggregate interfacial transition zone. Exchange of calcium and sodium, revealed by the maps, resulted in the migration of sodium into the PC paste and the formation of additional calcium-silicon-based phases in the geopolymeric matrix. Strength values of mortars with 25% and 50% recycled aggregates (RA) showed negligible differences compared with the reference sample. Screening tests indicated a low potential for GPC RA inducing alkali-silica reaction. Transport of GPC RA alkalis and the underlying mechanisms were observed. This transport phenomenon was found to have minor effects on the properties of the PC mortar, indicating that recycling of geopolymers is a viable reuse practice.Peer reviewedFinal Published versio

Proceedings of Abstracts, School of Physics, Engineering and Computer Science Research Conference 2022

© 2022 The Author(s). This is an open-access work distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. For further details please see https://creativecommons.org/licenses/by/4.0/. Plenary by Prof. Timothy Foat, ‘Indoor dispersion at Dstl and its recent application to COVID-19 transmission’ is © Crown copyright (2022), Dstl. This material is licensed under the terms of the Open Government Licence except where otherwise stated. To view this licence, visit http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email: [email protected] present proceedings record the abstracts submitted and accepted for presentation at SPECS 2022, the second edition of the School of Physics, Engineering and Computer Science Research Conference that took place online, the 12th April 2022