Ash Recycling : Cement, materials, waste and ashes: an effort to reduce CO2 emissions

Introduction

Environmental susceptibility is a growing concern that involves many actors in the building field. Special consideration must be given to the choice of materials when planning construction projects. Material availability, pollutant content, and potential for reuse or recycling are all factors connected with CO2 reduction. The latter topic has received particular attention in cement production, and so the recycling capability and the search for alternative binders are being widely investigated in order to potentially replace the Portland cement clinker.

Reuse and recycling: not really a new issue

The concept of a circular economy is widely used at the present time. The depletion of natural resources results in a number of fundamental issues related to the endless availability of materials. The misuse of precious landscapes, the mining of virgin natural areas and the lack of waste disposal sites increases people’s environmental susceptibility. Nonetheless, it is not just these factors that may trigger innovation and the capability to repair, fix and recycle. In the past, poverty switched on the human brain and materials were largely reused.

Building material evolution

The field of materials is broad. Evolution followed an important trend: from natural to artificial materials by heating them at a high temperature. In the building sector, the materials needed to firstly accomplish static requirements. Then, high-performance materials were developed. They were resistant to fire and impact, combined lightness and high mechanical behaviour, and implemented thermal insulation during times of energy crisis. The mixing of materials led to new composites, such as carbon fibres glued with polymers used for structural reinforcement applications. The evolution to smart materials took place rapidly. These materials are able to adapt to loading and environmental conditions. In spite of the continuous improvement of mechanical properties, the shortened service life of some structures suggested that durability should be reassessed. The materials also needed to be durable. Lately, research has moved towards eco-friendly materials and recycling. Today, it is almost forbidden to think about a new material if we cannot reuse or recycle it.

The cement-based world

The extraordinary development of Portland cement in the 19th century changed the construction field, although hydraulic cements capable of forming an amorphous gel mass in the presence of water to wrap stone aggregate and to harden date back to ancient Greece and Rome. By that time, volcanic ashes with their alumosilicate component was mixed with lime and water. Calcium carbonate, Al-Si-rich clays and ferrous compounds formed a ternary system and the basic chemistry of cements. Lately, several types of cements have been developed. The addition of mineral compounds with hydraulic or filling properties has been a main concern. They influenced the hydration rate and the mechanical and durability characteristics of the Portland cement-based binders.

Sustainability issue

Sustainability is a very broad field that is related to the materials, building system, availability, production, placing on site, maintenance, restoration, transportation and many other activities. Life-cycle evaluation and embodied energy calculations are also a concern. Moreover, costs are also an issue. We all live with money. Concrete has become a major construction material consisting of stone aggregates, cement, water and chemical additives. Among these ingredients, cement production emits a relatively large amount of CO2. Nowadays, cementitious demolition waste needs to be crushed and reused as aggregates to lower the landfill disposal. In addition, the Portland cement content may be limited by increasing the superplasticiser content. But above all, several mineral additives are studied and used to partially replace the Portland cement component.

Waste materials

This class of compounds is used to partially replace Portland cement [1]. Availability of the materials can vary depending on the type of industry, such as for steels and carbon. Every country has a different situation with respect to geological material availability and industry processes. Transportation routes need to be considered for sustainability. Cementitious recycled aggregates, foundry sand, cement kiln and marble dust, construction demolition, glass, plastics [2] and rubber are the common wastes used in concrete. They affect the properties of concrete in different ways. The partial reduction of compressive strength is due to the lower intrinsic strength of the wastes and the partial lack of hydration products to form the amorphous gel to bind the aggregates. On the other hand, the shape of some types, such as fibres, plates or foils, may increase the ductility of the compounds and implement the durability. It is also useful to evaluate the internal/external use of such combined materials and potential reuse at the end of the service life.

Supplementary cementitious materials

These types of addition to partially replace Portland cement are thought to increase durability, optimise pumpability, reduce permeability and the degrading reactions, and improve the long-term mechanical properties of concrete. In addition, they help to lower CO2 production by reducing the Portland cement component in cementitious binders [3]. The most common material is the fly ash derived from the precipitation of exhaust gases of coal-fired power stations. Ground-granulated blast-furnace slags, metakaolin and calcinated clays [4] improve material performance and environmental protection. The combination of calcium carbonate-rich cement with 15 per cent silica fume yields a satisfactory compressive strength ranging from 30 to 40 Mpa at 28 days within cement-based blends. The durability parameters, such the chlorides and the freeze/thaw resistance, are positively affected [5]. Oil shale ashes in combination with calcium sulphoaluminate cements contribute to early strength [6]. Thus, many properties can be reached with mineral additions, but intensive research and standards are still required in this field.

Wood ashes

The wood ashes are produced by wood-burning plants. The ashes are spread over the land or transported to incineration plants. Nonetheless, their pozzolanic or filler activities may be beneficial for application within concrete [7]. Whether they are disposed of in landfills or reused depends on the country in question. The total organic carbon and the heavy metal content need to observe the limits imposed by legislation. In this regard, a wood biobased thermal power plant that supplied energy to a district network was investigated. The ashes of the boiler plant, the multicyclone and the electro-filter were analysed with respect to heavy metals, in particular water-soluble chromium (VI). The elemental content of the ashes complied with the limits imposed for the construction sector. Therefore, the wood waste boiler ashes were added in different percentages up to 30 per cent by mass to replace a Portland cement-based clinker. A lowered development of compressive strength for the cementitious blends added with wood ashes was observed. This is typical behaviour of some classic supplementary cementitious materials added to concrete. They exhibit lower hydration at an early stage. Nonetheless, starting from 28 up to 90 days, the strength of the wood ash blends reached the reference values of the Portland cement-based mix. A similar durability as for the reference blend is seen for the accelerated carbonation, chloride penetration and freeze/thaw resistance if the addition of the ashes does not exceed 5 per cent [8]. This is an interesting value considering the enormous amount of concrete produced worldwide. This would quickly allow most wood ashes to be reused. Some are already currently added within cementitious blends. When the ashes contain large amounts of heavy metals, such as chromium, chemical reduction procedures are currently under investigation.

Incineration plant ashes

A slightly different situation exists for municipal solid waste ashes, which generally exhibit a higher content of heavy metals. However, the content varies within each urban solid waste treatment plant. Use as supplementary cementitious or road base materials differs depending on the country’s legislation. Hg, Cd, Cu, Ni, Pb, Zn and Cr are the main elements detected within the slags and acid-washed ashes. In some countries the wastes are chemically prepared for disposal. In fact, it is not expedient to separate the most environmentally aggressive metals during the burning procedure and filtration phases. The source of metals is wide and present in urban waste such as housewares, food packaging, paper, textiles, fibreglass, plastic films, yard waste and much more [1]. On the other hand, the main elemental composition of the municipal solid waste ashes are Ca, Si, Fe, Al, K, Na and Mg. They look very interesting as a potential binder. The granulometry of the slag/ashes can vary, but the diameters are useful as an additional binder or aggregate material for concrete (Fig. 1).

Several treatments aim to lower the content of detrimental substances. Magnetic separation is used for magnetic materials and eddy currents for non-ferrous metals. The water, alkaline and acid-washing procedures contribute to the main treatments of the MSW ashes. The density separation technique and ageing are also used. The latter reduces the leaching of heavy metals. Additional techniques, such as wet grinding, phosphatation, carbonation, hydrothermal and thermal treatments aim to optimise and lower the content [1, 9]. The addition of the slag/ashes in binder cementitious systems is under investigation. In spite of some encouraging results, the deleterious effect of some chemical elements, such as aluminium, has been reported to cause expansion and strength loss [10]. Nevertheless, cement-based binders are known to immobilise detrimental substances such as heavy metals [9]. Therefore, it is important to keep the addition of slags/ashes to reasonably low levels in order to prevent degrading reactions and maintain satisfactory mechanical and durability performance.

The quantity and diversity of municipal solid waste is high. Therefore, a wide range of contents is observed for the main heavy metals. Over a period of ten years and looking only at an incineration plant, elements variation of up to 200 times can be observed for Hg, Cu, Cr and other elements. A similar variation is seen worldwide [1]. It is neither wise nor possible to control the waste input into a plant in order to obtain the appropriate composition of the waste ashes and thus choose the specific selection of heavy metals. Part of the heavy metals remains confined to the slags/ashes and is recovered by separation: Fe, Cu, Fe-Cr-Ni and Al. The material subsequently goes to landfill disposal according to the country’s acceptable limits. The remaining solid waste fraction gained from the wastewater goes into the hydroxide muds and Zn is extracted.

Incineration plants gather the municipal solid wastes and a concentration of harmful substances comes from various sources. Regardless of the material origin, the poisoning effect is mainly due to the concentration of chemical elements in the material or the leachate. Thus, it may be possible to take a reasonable amount of slags/ashes and dilute them into a cementitious compound, much like in quantities no higher than 10 per cent by mass of cement. The waste slags/ashes exhibit pozzolanic activity and contribute to the binding of the cement-stone aggregate systems. At the same time, appropriate mechanical and durability properties can be achieved. In addition, the harmful elements, in particular heavy metals, are immobilised within the cement-based matrix. The cementitious compounds are sometimes used to immobilise radioactive waste. If some elements, such as Cr or others, are difficult to trap, appropriate cleaning techniques may be applied to the slags/ashes prior to mixing within concrete. It is worth investigating this in a systematic manner.

Concluding remarks

Portland cement clinker can be partially replaced with special waste, mineral materials and ashes to reduce CO2 production, especially if the substances exhibit pozzolanic binder and filler capacities. Harmful elements such as heavy metals are still an issue. Nonetheless, appropriate immobilisation within cementitious systems, reduction of the concentration and dilution, and further development of the slag/ash cleaning techniques will allow them to be reused almost completely.

References

[1] Siddique R. and P. Cachim, Waste and supplementary cementitious materials in concrete, Woodhead Publishing Series in Civil and Structural Engineering, 2018.

[2] Guerini L. and C. Paglia, Mechanical, physical and durability properties of cement mortars added with PP/PE-based food packaging waste material, International Conference on Concrete Engineering and Technology, Zurich, Switzerland, 14–15 January 2022.

[3] Neville A. M., Properties of concrete, Pearson Education, 1999.

[4] Scrivener K. and A. Favier, Calcined clays for sustainable concrete, Proceedings of the 1st International Conference on Calcined Clays for Sustainable Concrete, RILEM Bookseries, Springer, 2015.

[5] Paglia C., E. Giner Cordero and A. Jornet, The Portland cement limestone-silica fume system as a sustainable cementitious material, fib International Congress – Concrete Innovation for Sustainability, Oslo, 12–16 June 2022.

[6] Paglia C., F. J. Wombacher and H. K. Boehni, Hydration, strength and microstructural development of high early-strength C4A3S activated burnt oil shale-based cement system, ACI Materials Journal, 98 (5), pp. 379-385, 2001.

[7] Tamanna K., S. N. Raman, M. Jamil and R. Hamid, Utilization of wood waste ash in construction technology: A review, Construction and Building Materials 237, 2020.

[8] Giner Cordero E. and C. Paglia, Wood waste ash re-use for cementitious blends, 8th International Conference on Architecture, Materials and Construction, Lisbon, 2022 (to be submitted).

[9] Joseph A. M., R. Snellings, P. Van den Heede, S. Matthys and Nele De Belie, The use of municipal solid waste incineration ash in various building materials: A Belgian point of view, Materials 11, 141, 2018.

[10] Czop M., B. Łaźniewska-Piekarczyk, and M. Kajda-Szcześniak, Analysis of the possibility of using slags from the thermal treatment of municipal waste as potential component of cement – case study, Materials, 14, 6491, 2021.