Reuse of industrial waste for new applications towards sustainable manufacturing and zero waste

This research addresses the theme of circular economy, focusing on recycling waste materials produced by companies, mainly dealing with the construction sector. The project targets materials, both inorganic and organic, specifically production or processing waste, that do not find immediate recovery or reuse within the internal industrial production of the company. The insufficient knowledge about these waste materials (quantities produced, dimensions, chemical-mineralogical compositions) limits their potential applications also in other fields, causing significant economic loss and environmental impacts. The loss of materials, that could potentially be reused as secondary raw materials, produces an increase of landfilling, with evident environmental implications for society, economic burden on the producing company due to disposal costs and an increased consumption of raw materials, which could partially be replaced by recovered materials. In addition to industrial waste, construction and demolition waste (CDW), representing 30% of the waste stream in the EU, is one of the objects of this study. In fact, in the Marche region (Italy), CDW from the 2016 earthquake debris poses a challenge due to its accumulation and limited reuse but also offers potentials as a resource for new products. The project aims to investigate how to reintroduce different kinds of waste into the production cycle to promote circular economy, emphasizing waste valorization as secondary raw materials, as products for new building materials via a sustainable manufacturing. This will help to promote a transition from a lack of recycling, a mere down-cycling or even end-of-life, to an up-cycling process, where waste is not only recovered but also used for new high-grade products, which can increase the interest of companies in the recycling process due to an increased value of the new materials. Moreover, the replacement of Portland cement is a global goal, due to the fact that the production of ordinary Portland cement (OPC) is accompanied by a considerable CO2 emission (a ton of cement produces about a ton of CO2). Potential sustainable alternatives to OPC are geopolymers, which possess interesting physical-chemical-mechanical properties compared to traditional cements and are capable of significantly reducing CO2 emissions. The research focus is therefore based on the use of geopolymer binders to incorporate waste for new building materials, coupling different types of advantages. In view of these characteristics, the project is therefore in line with the Agenda 2030 goals for sustainable development and follows the EU directive on reduction of waste, CO2 emissions, and extraction of raw materials, as well as the Green Deal indications and requirements for EU countries and companies towards a transition to more sustainable industry. The research started with an investigation of the characteristics of the waste materials potentially usable for the incorporation in new products, both as aggregate and as precursors for the geopolymers (chapter 1). This part was carried out investigating and giving preference to those produced locally, when possible, in the view of creating a synergy among industries and a prompt to increase interest towards circular economy in the area, as part of the PhD project, financed by Regione Marche (I) for industrial innovation. The materials investigated include construction and demolition waste (CDW), coal fly ash, ceramic industrial wastes, quartz-composites, ornamental stone scraps, industrial sludges from cutting/polishing operations, used foundry sand. A group of fibrous materials, both synthetic and natural (i.e., wind-blades waste, surgical masks, and others) were also taken into consideration. In order to define the possible utilization of this wastes into geopolymers, all the materials were preliminarily characterized by chemical and mineralogical analyses: XRF (X-Ray Fluorescence), XRD (X-Ray Diffraction), SEM (Scanning Electron Microscope), and RAMAN (Chapter 2). This allowed to determine which of them could be used as precursor (e.g., fly ash in partial or total substitution of metakaolin) or as aggregate or fillers, and in which amounts (Chapter 3 and 4). Following different procedures for the sample preparation, as a function of the components used and the properties to obtain, all the geopolymer samples produced were characterized through mechanical, physical, and durability tests (Chapter 4). Regarding the mechanical analyses, compressive and flexural strengths were evaluated after 7 and 28 days curing, as from the standard norms for cement (EN 196-1). In particular, the geopolymers samples obtained using quartz- composites aggregates (Chapter 4) allowed to obtain very good results in terms of mechanical/physical properties (including density, open porosity and water absorption). In fact, these samples, produced with the aim of achieving properties similar to those of OPC products, exhibited high compressive strength values, qualifying them even as equivalent to structural concretes (mechanical strength greater than 20 MPa). This evaluation was carried out also taking into consideration different types of quartz-composites (type A and B) which allowed to determine the contribution of the resin amounts towards the mechanical properties, with quartz component varying from 92-94 wt. %, to about 70 - 80 wt. %. Geopolymer samples were produced by adding 60, 70 and 80 wt. % of quartz-composites on the total, with best results in terms of mechanical resistance obtained for the sample containing 70 wt. % of type A composites (with compressive strength 62.5 MPa). CDW (Chapter 4) revealed to be a difficult material to deal with, given its wide compositional heterogeneity, but also its chemical composition, rich in Ca, which affected the preparation and performances of the geopolymers produced. The work, carried out as an intra-laboratory collaboration has been reported briefly here but it will be presented in more details in a companion UNICAM PhD thesis (F. Volpintesta). Special attention has been given also on testing the possibility to recycle industrial sludges, in the form of slurry or dust, coming from cutting and polishing processes of different materials, like natural stones, quartz-composites, gres (par. 3.1.7). These wastes represent a huge problem for companies and are landfilled at high economic costs. Therefore, there is a high interest in finding a way to avoid landfilling by also find a new application for them. The study carried out allowed to demonstrate that these sludges can be successfully incorporated into a geopolymeric matrix, in spite of their heterogeneity. Tests have been carried out also to verify their field of applications: thanks to the possibility to treat them with a foaming agent, it was possible to produce bubbles in the samples, producing a sponge-like material. These results suggests that they are suitable for the preparation of low-density materials reaching, in fact, density values of 0.98 to 1.07 g/cm3 at 28 days, well below the limit for lightweight mortar products fixed at 1.5 g/cm3 . These successful tests suggest an interesting application as a new light-weight insulating materials. Some tests were conducted to incorporate different natural and artificial fibrous waste material into the geopolymers with the aim of improving their mechanical properties and to recycling unwanted waste. They include resin-based waste from decommissioning of wind-blades, surgical masks, natural fibers like silk, in the form of discharged silkworm cocoons. The samples incorporating fibrous waste showed in general higher flexural strength values compared to the same samples without fibers, and they are very promising for the production of panels (thanks to the good flexural strength) and/or insulating materials. Since much of this thesis work has focused on the recovery and reuse of quartz-composite materials, various durability tests were also conducted on these types of samples. The "efflorescence" test was used to assess whether the geopolymerization reaction was complete and to determine if the sample would develop salt efflorescence when exposed to unfavorable environmental conditions. The results show that after 60 days of partial immersion in water, there was no salt efflorescence from the sample, demonstrating the samples good resistance. Another type of durability test conducted on these samples was the acid (HCl 18.5%) and salt (Na2CO3 5%) attack test. In this case as well, the analysis showed good results. After 60 days of complete immersion of the samples in acidic and alkaline solutions, the samples did not lose their integrity and maintained their mechanical characteristics. Another test conducted was the freeze-thaw resistance test, mandatory for all applications intended for external use. Fifty freeze-thaw cycles were performed, after which the mechanical strengths of the samples were measured and compared with a reference sample. The results demonstrated good resistance, as the samples subjected to this test showed similar mechanical strength values compared to the laboratory reference samples. The results obtained demonstrate that geopolymers can replace OPC products for a variety of applications, thanks to the good mechanical/physical properties obtained (Chapter 5). Chemical resistance and durability to different environmental conditions are certainly of interest for possible applications for extreme uses (e.g., for materials in contacts with highly acidic substances, or for the confinement and stabilization of fluids rich in polluting chemicals). Moreover, it was demonstrated with various examples, that it is possible to recycle and use many types of waste materials, mainly as aggregate or fillers, alone or even mixing different types, for the production of geopolymeric materials, obtaining products with interesting physic-mechanical properties. This addresses the need to find alternative allocations to landfilled sludges, and possible up-grade solutions. Finally, the testing of various fibrous materials in geopolymeric binders not only revealed to allow obtain good mechanical results, and new research directions, but also opens the way to new industrial high-grade applications which can attract industrial attention and further opening possibilities to reduce landfilling of waste.