Biomass fly ash and biomass bottom ash

Biomass fly ash and biomass bottom ash

2

Francisco Agrela1, Manuel Cabrera1, Marı´a Martı´n Morales2, Montserrat Zamorano2 and Mazen Alshaaer3 1 Construction Engineering Area, University of Cordoba, Leonardo Da Vinci Building, Rabanales Campus, Cordoba, Spain, 2Civil and Building Engineering Schools, University of Granada, Granada, Spain, 3Plasma Technology and Material Science Unit (PTMSU), Department of Physics, College of Science and Humanitarian Studies, Prince Sattam Bin Abdulaziz University, Al-Kharj, Saudi Arabia

2.1

Energy from biomass, and biomass ashes (BA)

Biomass is an ‘indirect solar fuel’ source and is used in countless applications as a renewable energy source. As this waste comes from animal and vegetable organic matter, its environmental and economic potential have made it a competitive alternative to traditional solid fuels. Consequently, biomass is a renewable energy with massive potential. Used every day by more and more people, it aims to cover the high demand for energy that society demands day after day presenting a solution to energy problems, although it also is inconvenient due to ash production when thermochemical processes are applied to produce energy. In this chapter we explain the main use of biomass to generate electricity, which produces another waste, biomass ashes, which could be applied in concrete manufacture.

2.1.1 Biomass to produce energy, a renewable alternative The combustion of fossil fuels in the conventional method of producing energy has been established for many years; however, during the past few decades, the shortage of fossil energy resources, the negative environmental impact derived of their use and their price variability have encouraged governments to look for alternative energy sources that diversify energy production improving their management efficiency, political independence, economic growth and environmental protection (Herna´ndez et al., 2018). Fossil fuels emit greenhouse gases (GHG), mainly carbon dioxide (CO2), into the atmosphere. In 2013, world CO2 emissions from the consumption of petroleum exceeded 11,830 million metric tonnes (EIA, 2015). In order to ensure safe energy supply, new energy models are being developed according to renewable, sustainable, efficient and cost-effective systems. New Trends in Eco-efficient and Recycled Concrete. DOI: https://doi.org/10.1016/B978-0-08-102480-5.00002-6 © 2019 Elsevier Ltd. All rights reserved.

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New Trends in Eco-efficient and Recycled Concrete

Renewable energy is obtained from never-ending natural sources, either because of the huge amount of energy that they contain, or because they are rapidly regenerated by natural means. In its different forms, renewable energies are derived directly from the sun, rain, biomass, wind, ocean tides and heat generated and stored in the Earth (Mohtasham, 2015). Among the renewable resources, biomass is defined as organic matter from different plants, agricultural, industrial and urban waste. It is characterised by its great energy potential that can be used to produce thermal energy, electricity and biofuel (for transport). This energy source shows important advantages, including its contribution to the economic and social development of the countries and regions where it is produced as well as the reduction of waste disposal and CO2 emissions, among other benefits (Herna´ndez et al., 2018). We could rightly think that the combustion of biomass does generate CO2, as well as other gases such as water vapour and carbon monoxide (CO), etc. However, biomass is considered as ‘neutral’ within the carbon cycle, it does not break the equilibrium of the atmospheric carbon concentration. Unlike solid fuels, the emission of CO2 during biomass combustion is considered neutral, since the CO2 emitted is part of the current atmosphere; it is the carbon that plants continuously absorb and emit (Fig. 2.1). It is possible to ensure that biomass is part of the solution in future energy planning. In this sense, the International Energy Agency (IEA) has predicted a

Figure 2.1 Biomass cycle.

Biomass fly ash and biomass bottom ash

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significant increase of biomass being used in coming years, estimating that by 2050 the potential energy production from biomass will be in the range of 100 to 300 EJ (2300 to 7100 Mtoe) per year, compared to the current 50 EJ (International Energy Agency (EIA), 2012). Particularly, European Union biomass is expected to contribute over 50% towards their renewable energy targets (International Energy Agency (EIA), 2012).

2.1.2 Sources of biomass for power generation Biomass is organic matter that comes from living organisms, and includes animalas well as vegetable-derived material. It is one of the most diverse and versatile renewable energy sources that can be used to provide heat, electricity and transport fuels. Generally, any definition of biomass must encompass three terms: organic, autochthonous and renewable. G

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It is based on organic matter available to humans. It is an autochthonous energy, so is non-dependent on other countries, at least during its obtaining phase. It is a renewable energy as it comes from the sun (Fig. 2.1).

Biomass for energy can include a wide range of materials, natural biomass, residual biomass and energy crops (Figs. 2.2 and 2.3): G

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Natural biomass. It is produced in natural ecosystems without human intervention to enhance or modify it. Natural biomass includes, fundamentally, waste produce during forest cleaning works and plantation remains, firewood and branches and coniferous and broad-leaved forests. Residual biomass. The intensive exploitation of natural biomass is incompatible with the protection of the environment. However, huge quantities of waste are available and they can be transformed into energy. This fact is considered a necessary tool to move towards a more sustainable circular economy, since it helps to avoid waste disposal and produces energy. In this way, residual biomass is defined as the biomass

Natural biomass Dry biomass Residual biomass Wet biomass

Biomass classification

Oil crops Energy crops

Alcoholic crops Woody crops Lignocellulosic crops Grass crops

Figure 2.2 Biomass classification.

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New Trends in Eco-efficient and Recycled Concrete

Natural biomass Waste from forest cleaning works

Dry biomass Rise straw

Wet biomass Livestock waste Residual biomass

Oil crops

Alcoholic crops

Lignocellulosic crops

Sunflower

Sugar beet Energy crops

Sorghum

Figure 2.3 Examples of biomass. derived from waste or by-products of agricultural, livestock and forestry activities, as well as the processes of agro-food industries and wood processing. It is usually classified into: Dry residual biomass. It comes from agricultural activities, forestry, food industries and wood, etc. This type of industry produces solid waste, such as sawdust, pomace, shavings and straw, etc., with a significant energy content. Wet residual biomass. It is biodegradable waste produced in wastewater treatment, industrial process and livestock waste. Energy crops. Energy can also be obtained from crops exploited with the sole objective of obtaining biomass and called energy crops. In fact, energy crops are available in many forms (forestry or agricultural crops) and they are characterised by their adaptation to poor lands, resistance to diseases, drought and robustness, the predictability of their disposition, which ensures the supply, as well as a spatial concentration that allows a mechanised management, not intensive in labour, vey little fertiliser input and is relatively cheap. Energy crops may be classified into: Oil crops. They are plants with seed or fruit used to extract oil that can have various applications both in food and in industrial processes. Oil crops can be also used for the production of energy directly as heating fuels or, after transformation processes, be applied as transport biofuels, such as biodiesel ester. Some of the more common oil crops are: oilseed rape, hemp, olive, sunflower, safflower, palm and coconut, among others. Alcoholic crops. They are used to produce bioethanol which can be can used directly as a fuel. Some examples are: starch and sugar crops (e.g., sugar beet and sugarcane) or Jerusalem artichoke. G

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Biomass fly ash and biomass bottom ash

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Lignocellulosic crops. They are named lignocellulosic species because their major components are lignin and cellulose. They are usually applied to produce heat and electricity through combustion processes; however, they can be used to produce biofuels as methanol and ethanol after conversion processes. They are classified as: Woody crops: short rotation coppice, poplar and eucalyptus, Grass crops: sorghum, kenaf, prickly pear, whole crop maize, reed canary grass or miscanthus.

2.1.3 Advantages and disadvantages of biomass energy The use of energy sources other the usual ones necessarily involves solving the problem of availability and profitability. One form of energy is not going to displace another if it is not, at least, equally accessible, manageable and economically affordable. Biomass, as an energy source, satisfies all these requirements, in addition to having further advantages: G

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Renewable energy source. It comes from abundant and natural resources as well as from waste, in consequence it is regenerated rapidly. Improves waste management. The elimination of agricultural and industrial waste, in many cases, supposes a problem, nevertheless its use to produce energy has important associated advantages, for example: the reduction of the contamination, fire risks and occupation of space in landfills; low production and transportation costs; reduction of CO2 emissions; generation of jobs; and contribution to rural development. Reduces agricultural environmental impacts. Using agricultural biomass as biomass energy instead of burning or disposing in landfill can not only reduce the risks of forest fires but also can reduce insect plagues. The creation of jobs in rural areas is encouraged. Cheap energy source. The cost of this type of energy could be up to 3 or 4 times cheaper. Multiple use. Nowadays, the performance and technology of systems to produce energy with biomass are very advanced so that they could be used in many applications due to different energy conversion technologies, for example, transport, heat and electricity production, etc. Local production. Consequently, the use of biomass reduces the need to import foreign fuels and the dependence on fossil fuels. Clean energy. Promoting biomass in energy production can reduce pollution emissions such as CO, HC and NO and better protect the environment.

Despite the important advantages provided by biomass, it also has some disadvantages: G

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It requires a large space to be produced and stored before being converted into energy. It is not entirely clean. Although the levels of pollutants are far less than those produced by fossil fuels, the combustion of biomass produces some GHG and particle matter. High water footprint. A great deal of water is needed for some energy crops to be produced resulting in biomass having a high water footprint. High production costs, especially due to high transportation costs as well as necessary pre-treatment processes. For example, biomass is characterised by high humidity, so it is necessary to apply drying technologies which implies a previous energy consumption that increases production costs. Networks and distribution channels are not as developed as in the case of liquid and/or solid fuels.

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New Trends in Eco-efficient and Recycled Concrete

2.1.4 Technologies for converting biomass into useful energy Biomass conversion may be conducted on two broad pathways: chemical decomposition and biological digestion. The most-used conversion technologies for utilising biomass can be divided into three basic categories [summarised in Fig. 2.4 (Demirba¸s, 2001)]: G

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Direct combustion. This is the burning of biomass in the presence of oxygen. It is a proven technology widely used to convert biomass energy into heat and/or electricity with the help of a steam cycle (stoves, boilers and power plants). These processes are applied from a very small scale, for domestic heating, up to a scale in higher ranges to produce electricity. Biomass could be used as only fuel, but it is also possible to apply co-firing that has important advantages, particularly when electricity is an output. Biomass can be burnt mixed with a fossil fuel, such as coal; this process is named co-firing. In the case that the biomass is used for simultaneous production of heat and electricity the process is named a co-generation process or combined heat and power; in this case power plants produce heat used in district heating and ‘waste heat’ is recovered as a by-product for electricity production. Co-generation processes convert about 85% of the potential energy of biomass into useful energy. Thermochemical processes. These entail the application of heat and chemical processes into the production of energy products from biomass and can be subdivided into pyrolysis and gasification. Pyrolysis. Pyrolysis is a thermal degradation of a substance in the absence of oxygen. In this process, biomass undergoes partial combustion and is decomposed by heat, without producing the combustion reactions. The basic characteristics of this process are the following: the only oxygen present is the content of the waste to be treated, at working temperatures ranging between 300 and 800 C, and as a result of this process, liquid fuels (including tars, oils, methanol, acetone, etc.) and a solid residue rich in carbon (called biochar) are obtained. G

Direct combustion Pyrolysis Thermochemical processes Conversion processes biomass-energy

Gasification

Anaerobic digestion Biochemical processes Alcoholic fermentation

Figure 2.4 Conversion processes of biomass to energy.

Biomass fly ash and biomass bottom ash

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Gasification. Gasification is a thermochemical process in which biomass is transformed into a combustible gas known as syngas or synthesis gas. The operating temperature ranges between 700 and 1400 C, depending on the type of technology used and the conditions of the process, and a gasifying agent (water vapour, oxygen, air or a mixture of these) is used. The gas generated is more adaptable than the original solid biomass and it is characterised by its low or medium calorific value. It can be exploited in various ways through combustion processes to produce electricity and/or thermal energy or as synthesis gas, transforming it into products with higher added value. Biochemical processes. The biochemical processes of transformation of biomass into gaseous or liquid fuels, such as biogas or bioethanol, are supported by different types of microorganisms. The microorganisms, whether contained in the original biomass or added for the process, produce the degradation of the complex molecules of the biomass to more simple compounds characterised by their high-energy density. These procedures are usually applied in the case of natural or residual biomass with a high moisture content. The most-used biochemical technologies are anaerobic digestion and alcoholic fermentation: Anaerobic digestion. This is basically a fermentation process. The biomass is transformed through bacterial action without oxygen producing a gas compound of methane and CO2 that can be used to produce electricity through gas turbines or in heat and steam processes. Alcoholic fermentation. A variety of biofuels can be produced from waste resources including liquid fuels, such as ethanol or methanol, that can replace significant quantities of fossil fuels in many transport applications. Biofuels are produced through the chemical reactions transesterification and esterification. G

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2.1.5 Ash production from biomass combustion The process of energy production by combustion of biomass is considered to have important environmental advantages; however, it also has the disadvantage that it generates large amounts of ashes that affect the conversion process reducing the efficiency of combustion systems, causing extra cost for boiler cleaning and maintenance, and hinder further utilisation of biomass materials as combustion fuels (Frandsen, 2005; Wang et al., 2012; Werther et al., 2000). Ash production can become an environmental problem if they are not properly managed. Two types of waste are generated in the combustion of biomass (Picco, 2010): bottom and fly ashes. Biomass bottom ash (BBA) includes the coarse fraction and is formed by the total or partially burnt material. Bottom ashes are produced in the combustion chamber (James et al., 2013) and are composed of sand particles, mainly quartz, often mixed with mineral impurities contained in the biomass (Modolo et al., 2013); these impurities can be minerals, and they are usually responsible for slag production due to the melting point decreasing; they are also responsible of the presence of sintered ash particles in the bottom ashes. Biomass fly ashes (BFA) are the particles separated from the stream of gases outside the combustion chamber, so they are the finest fraction of the ashes. They are separated from the combustion gases by specially designed systems to avoid their emission

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New Trends in Eco-efficient and Recycled Concrete

into the atmosphere (Maschio et al., 2011). BFA have a mainly inorganic fraction and a minor organic fraction (unburned carbon). The quantity and quality of ashes produced during biomass combustion are strongly influenced by the characteristics of the biomass used (Masia´ et al., 2007) as well as the combustion technology and the combustion and operating conditions of the process (Rajamma et al., 2009). Thus, the combustion of wood generates fewer amounts of ashes to be managed because herbaceous biomass, agriculture wastes and bark have a higher ash content compared to wood (Van Loo and Koppejan, 2008; Masia´ et al., 2007). On the other hand, the composition of biomass varies not only according to vegetation type (Table 2.1) but also to soil conditions and atmospheric dust particles deposited during cultivation or storage, resulting in a very variable chemical composition of biomass ash (Michalik and Wilczy´nska-Michalik, 2012); for example the SiO2 content is relatively high in straw ash ( . 66 wt%) or beech bark (56 wt%), but very low in sunflower husk ash (,2.4 wt%) or corn bark (8.3 wt%); K2O content in ash varies from 31.4 wt% in sunflower husks to ,4 wt% in hand beech bark ash. P2O5 and CaO content is very high in corn barn ash ( . 36 wt%) or beech bark ash (17.7 wt%). Table 2.1 summarises some examples of chemical composition depending on the type of biomass. On the other hand, differences in operation temperatures influence the amount of organic species (several salts and heavy metals) that volatilise in the furnace, and consequently the relative composition of bottom and fly ashes (Rajamma et al., 2009). Finally, BBA represent the higher percentage of the total ashes produced in a grate furnace accounting for 60 90 wt% of the total ash generated (Obernberger and Supancic, 2009). However, in bubbling fluidised bed combustors (BFBC), the bottom bed ashes often represent the lower fraction varying between 5 and 17 wt% (Dahl et al., 2009; Latva-Somppi et al., 1998). The foreseeable increase of large-scale utilisation of biomass during the next years, according to the measures aimed at the implementation of renewable energy sources that will reduce the problems derived from fossil fuels, will result in large volumes of ash production. In this way, Directive 2009/28/CE to foster the use of energy from renewable sources, includes the goal of using 20% renewable energy by 2020; this goal will lead to the production of approximately 15.5 million tonnes of biomass ash per year in the EU-27 (Obernberger and Supancic, 2009; James et al., 2013).

2.1.6 Environmental and health aspects of biomass ashes The environmental impacts of biomass ashes are related to their composition, which depends on the origin of the biomass. In the case of agricultural and forest biomass ashes’ composition was dominated by Si, Ca, K and P inorganic species; however, industrial waste ashes were high in Si and Ca and to a lesser extent in Al and Mg minerals (Vamvuka and Kakaras, 2011). In general terms, ashes from biomass could be classified, according to the European Community’s legislation, as nonhazardous industrial waste. Consequently, numerous possible applications of

Table 2.1 Composition (wt%) of ash depending on the type of biomass used (Xing et al., 2016) Biomass

Na2O

MgO

Al2O3

SiO2

P2O5

K2O

CaO

TiO2

MnO

Fe2O3

Mixed forestry pellets Miscanthus Wheat straw Chipped wood Pine Olive residue Peanut

2.310 2.189 0.270 1.481 1.391 2.170 0.234

4.275 1.654 1.021 2.751 4.787 2.413 2.539

4.476 0.442 0.232 2.782 0.413 0.584 3.743

26.446 40.794 46.639 19.322 2.496 5.192 23.692

1.847 3.721 2.207 2.786 9.742 3.523 2.996

6.422 10.961 11.746 5.678 11.131 33.034 10.874

21.244 8.311 10.149 26.269 39.153 8.360 5.129

0.684 0.473 0.471 0.656 0.486 0.477 0.832

2.080 0.074 0.129 4.062 0.120 0.000 0.210

2.786 0.397 0.239 2.485 0.370 0.458 1.871

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New Trends in Eco-efficient and Recycled Concrete

biomass ash have been widely studied, including, for example (Vassilev et al., 2013a), soil amendment and fertilisation, bulk utilisation or in construction materials. However, logistical problems and variation in the quality of the ash, the lack of legislation and regulations in many countries, among other factors, lead most countries to deposit the ash in landfills increasing the economic and environmental impact (Vamvuka and Kakaras, 2011). The management of biomass is becoming an increasing economic and environmental burden. The increasing amount of BBA and BFA all over the world, as a consequence of the use of biomass to produce energy, suggests necessary recycling to reduce landfill disposal costs as well as negative environmental impacts, but also as a consequence of the ‘zero-waste’ objective including in circular economy concept (Maschio et al., 2011). Consequently, it is necessary to study new potential uses for biomass ashes and the construction sector could be a good option.

2.2

Overview of biomass ash characteristics

2.2.1 Classification of biomass ashes Different typologies of biomass are usually applied in biomass thermal power plants (BTPP) for the production of electric energy. These typologies are mainly related to the vegetation of the region where they are located, and the nearby areas. Mixtures of different types of biomass are usually applied to produce electricity, such as biomass waste from mass pruning processes, from the agricultural industry as forestry waste, from urban areas as gardens or from inter-urban areas such as roads, etc. BTPP are usually close to the areas where huge quantities of biomass are obtained. There are mainly two technologies in BTPP to burn biomass: fluidised bed combustors (BFBCs) and circulating fluidised bed combustors (Modolo et al., 2013). Fig. 2.5 shows a thermal plant with a fluidised bed combustor (Hinojosa et al., 2014). These technologies present differences in the pattern of gas solid hydrodynamics in the reactor, the size of the bed particles, the heat and mass transfer rates in the reactor, and the temperature and flue gas composition profile along the reactor (Van Loo and Koppejan, 2008). These variables influence the characteristics of the ashes produced during biomass combustion. In every BTPP, two types of ash are obtained, BBA and BFA. BBA is the portion of non-combustible residue found in the furnace or incinerator, whereas BFA is the portion of ash that escapes through the chimney and is retained to prevent it from being released into the atmosphere (Cabrera et al., 2014). On the one hand, BBA are composed of sand particles purged from the original bed, inorganic components, as soil and sand, and the unburnt biomass fraction. BBA represent the higher percentage of the total ashes produced, around 55% 65% (Modolo et al., 2013). These ashes are obtained through discharges that are required to renew and to avoid agglomeration and defluidisation (Van Loo and Koppejan, 2008).

Biomass fly ash and biomass bottom ash

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Figure 2.5 Schematic of biomass thermal power plant-based on fluidised bed combustors (Hinojosa et al., 2014).

In general, BBA present an appearance similar to a fine natural and dark sand, because the burnt process is stopped before the final consumption of the biomass, and it is obtained a mix of ash with unburned particles of biomass. On the other hand, BFA is obtained in industrial combustion and gasification installations at temperatures commonly between 800 and 1600 C. This type of ash is powdery and is extracted from filters through which the combustion vapours are released. BFA presents a high content in alkali constituents (Na, K) and less alumina content (Al2O3) than coal FA traditionally used as a pozzolanic additive in cement formulations (Rajamma et al., 2009; Rajamma, 2011). The composition of BFA depends mainly on its origin varieties from woody to herbaceous and other natural resources. Wang et al. (2008) compared the properties of BFA from co-firing with coal FA in concrete, and they concluded that BFA from co-firing biomass with coal within a certain blending ratio (25%) should be considered in concrete. Both BFA and BBA can be applied in the manufacture of mortar and concrete with some limits. Due to their properties in both kinds of ash, these ashes present some appropriate properties for the manufacture of cement-based materials, but the high content of sodium and potassium make their use difficult because they could cause shrinkage and deformation problems in the concrete matrix. However, BFA can be used in agriculture as a fertiliser, which is a lower application than their use in concrete. Cuenca et al. (2013) studied the possible use of BFA as a filler in selfcompacting concrete and Go´mez-Barea et al. (2009) studied the addition of BFA to

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New Trends in Eco-efficient and Recycled Concrete

manufacturing lightweight wall board and bricks with special properties. Ferna´ndez-Pereira et al. (2011) studied the possibility of applying 20% BFA by dry weight to manufacture clay bricks, obtaining appropriate results. BFA was applicable in all these cases depending on its properties and chemical composition. Several studies on the application of BBA and BFA in concrete and mortar manufacture have been carried out and concluded that they could be applied in the manufacture of cement-based materials, limiting the degree of substitution of aggregate/ cement application.

2.2.2 Chemical composition of BA In this section, we discuss the chemical properties of BFA and BBA. The chemical properties of BFA and BBA depend on the biomass origin that is burned for obtaining f electrical energy and on the technological process applied in the BTPP. Vassilev et al. (2013a) studied the chemical composition of BFA, using 86 samples of this ash with eight varieties of biomass origin. Table 2.2 summarises the oxide composition of these eight types of BFA. Table 2.5 shows the main content of the ashes. It would be favourable to obtain a high content of the sum of calcium oxide 1 silica oxide 1 aluminium oxide. These three components should exceed 60% to obtain appropriate properties to be applied in concrete and mortar manufacture. Higher quantities were obtained in CaO 1 SiO2 1 Al2O3 in ash from beech wood chips (SiO2 12.33, CaO 67.80, Al2O3 20.12), rice husks (SiO2 94.38, CaO 0.97, Al2O3 0.21) and switchgrass (SiO2 66.09, CaO 10.19, Al2O3 2.21) (Vassilev et al., 2013a). These three kinds of ash would be the best to use in the manufacture of concrete and mortar. BBA have been less studied and there are few works where a summary of the chemical properties of this kind of ash is included. Hinojosa et al. (2014) published a work entitled ‘Potential use of BBA as alternative construction material: Conflictive chemical parameters according to technical regulations’, in which the Table 2.2 Chemical properties of BFA samples (Vassilev et al., 2013a) Characteristics

Beech wood chips

Corn cobs

Plum pits

Rice husks

Switchgrass

Sunflower shells

SiO2 CaO Al2O3 Fe2O3 K2O MgO P2O5 SO3

12.33 67.80 0.12 1.09 2.59 11.43 2.29 0.80

27.65 13.19 2.49 1.55 35.49 2.05 2.49 7.14

3.59 14.65 0.11 0.68 44.88 11.62 20.12 2.47

94.38 0.97 0.21 0.22 2.29 0.19 0.54 0.92

66.09 10.19 2.21 1.36 9.62 4.70 3.91 0.83

23.46 15.18 8.67 7.27 28.29 7.27 7.07 4.03

Biomass fly ash and biomass bottom ash

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most limiting chemical factors which determine its possible real applications were analysed. In this case, BBA from olive waste was studied in the Andalusian region, Spain, since in this area large amounts of this type of biomass are used in BTPP. Olive waste used in the production of this BBA were: olive cake, olive tree and, in some cases, pine or poplar waste. These were added in the mix of biomass to be burned. In the work of Hinojosa et al. (2014), three BTPP were studied with different calcination systems and the results obtained are summarised in Table 2.3. Sklivaniti et al. (2017) also studied BBA from olive tree trimmings. An important quality of this BBA is its organic matter content. In this work, between 2.96% and 19.97% was obtained, and medium value was 7.42%, obtaining 4.1% in one biomass thermal plant (Hinojosa et al., 2014). In general, BBA properties are very variable, as shown in Table 2.3. Therefore, a specific study must be carried out depending on each type of calcined biomass, as well as the technology of burning used. BBA presents high contents in CaO (17% 30%) and in some cases of SiO2 (up to 72%), which could allow for its application in the manufacture of concrete and mortar. Otherwise, it is recommended to apply some type of processing to reduce the organic matter content and to use partial substitutions due to the excess content of K2O and MgO.

2.2.3 Physical and microstructural properties of BA This section focuses on the physical and microstructural properties of biomass ash oriented to the manufacture of construction materials such as cement, concrete or binders. In particular, we will make a brief summary of the properties of the particle Table 2.3 Chemical properties of BBA (Hinojosa et al., 2014, Sklivaniti et al., 2017, Modolo et al., 2013) Characteristics

SiO2 (%)

CaO (%)

Al2O3 (%)

Fe2O3 (%)

K2O (%)

MgO (%)

LOI (%)

BBA from burnt Olive waste (Hinojosa et al., 2014) BBA from burnt Olive plant trimmings (Sklivaniti et al., 2017) BBA from forestry Biomass (Modolo et al., 2015)

51.5

20.02

1.44

2.42

16.2

4.5

13.26

6.84

31.41

2.73

1.39

12.31

2.45

41.49

72.20

17.16

2.32

0.78

0.75

1.97

2.18

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New Trends in Eco-efficient and Recycled Concrete

size distribution, diffraction X-ray, water absorption capacity, density and thermogravimetric analysis. Particle size distribution presents clear differences between BFA and BBA. Usually BFA has a maximum nominal size around 200 µm and presents a distribution similar to coal FA, as these ashes are also obtained in filters. Fig. 2.6 shows the particle size distribution of three types of BFA from the calcination of forest residues (Berra et al., 2015). Particle size distribution of BBA are used to present more differences than in the case of BFA. Fig. 2.7 summarises the size distribution of different BBA. In these cases, maximum nominal sizes between 2 and 9 mm were obtained (Modolo et al., 2015; Cabrera et al., 2014), 4 mm being the medium maximum nominal size in this type of BA. With respect to X-ray powder diffraction (XRD), we can find different patterns between BFA and BBA. Fig. 2.8 includes several XRD patterns obtained by Berra et al. (2015) and Vassilev et al. (2013b). The XRD patterns of BFA did not reveal the presence of highly water-soluble crystalline phases, such as halite (NaCl) or sylvite (KCl). In some samples, it is possible to be identified as a crystalline sulphate phase (BFA-7) (Berra et al., 2015). In general, with XRD patterns it is possible to observe newly formed amorphous (non-glass) inorganic material. BFA present particles composed of non-fused phases produced from different biomass varieties. BFA are mainly amorphous waste because of their components from original biomass lose crystallisation in water at 300 1100 C, resulting in amorphisation (Vassilev et al., 2013b; Richaud et al., 2004). XRD patterns in BBA present differences with those based on BFA. Fig. 2.9 summarises the XRD patterns of BBA from olive plant trimmings (Sklivaniti et al., 2017), olive cakes, olive trees and pines (Cabrera et al., 2014). Calcite is the main constituent of the ash mass. The X-ray diffraction patterns of BBA reveals that

Figure 2.6 Particle size distribution of three BFA from the combustion of forest biomass (Berra et al., 2015). BFA, Biomass fly ash.

Biomass fly ash and biomass bottom ash

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Figure 2.7 Particle size distribution of BBA from BTPP (BBA1-BBA12 Modolo et al., 2014; BBA13 and BBA14 Cabrera et al., 2014). BBA, Biomass bottom ash; BTPP, biomass thermal power plants.

except for calcite (CaCO3), which accounted for about 70 wt% of the BBA mass, another three major minerals were identified, fairchildite (K2CaCO3), quartz (SiO2), grossular (Ca3Al2Si3O12). In general, BBA has an amorphous structure during combustion at high temperatures. Mineralogical characterisation reveals that the composition of BBA presents some crystalline phases within an amorphous matrix. Oxides, such as quartz (SiO2), are typically produced at high temperatures during the combustion process. Carbonates (calcite) were also detected because biomass fuel contains a high content of wood waste. Other properties that can be considered in this category are the density and capacity of water absorption or friability, mainly in the case of BBA. This kind of BA presents a particle size distribution similar to natural sand; however, its friability coefficient has a higher value, with a medium value of 30.28% (Cabrera et al., 2014), while standard natural sand usually presents values of approximately 15% (Ledesma et al., 2015). This means that BBA has less resistance to wear than conventional or standard natural sand, and it is necessary to take this value into account in order to apply partial substitutions of natural sand by BBA in mortar and concrete manufacture. In terms of water absorption capacity, BBA has higher values, presenting more than 18% in BBA from burned olive waste. At the same time, its density has a lower value in comparison with natural aggregates, presenting values of 1.6 2 g/cm3, very much lower than natural sand with around 2.6 g/cm3 (Cabrera et al., 2014).

2.2.4 Leaching characteristics of BA Not much research has been done on the environmental impact by leaching of BA. Van Loo and Koppejan (2008) showed that the content of the water-soluble fraction

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New Trends in Eco-efficient and Recycled Concrete

Figure 2.8 XRD patterns of BFA (BFA1 BFA5 Berra et al., 2015). BFA, Biomass fly ash.

Vassilev et al., 2013b; BFA6 and BFA7

in BA (61%) is higher than those ashes obtained from coal thermal process (7.2%). The pH values of the different BFA leachates vary in the range of 4.5 13.4 (Vassilev et al., 2013b). The pH values of woody BFA are normally higher than straw and cereal BA due to the higher Ca and lower S and Cl concentrations in the former ashes (Van Loo and Koppejan, 2008). Since BFA can present high values of K, Cl, Ca and Mg, and it could contain certain quantities of other hazardous components, environmental risks and health concerns related to BFA with leaching process could be produced where hazardous elements, such as K, Cd, Cl, Cr, Mo and others, can induce alkalinity and leaching in the application of these ashes in civil construction. It is necessary to take into account the reduction of leaching values when some waste is applied in a cementbased material (Galvı´n et al., 2013).

Biomass fly ash and biomass bottom ash

39

Figure 2.9 X-ray diffractograms of BBA: (A) BBA from olive plant trimmings burnt (Sklivaniti et al., 2017); and (B) BBA from olive cake, tree and pine burnt (Cabrera et al., 2014). BBA, Biomass bottom ash.

In the work of Cabrera et al. (2016), 30 samples of BBA were analysed and it was observed that 37% of tested BBAs were classified as inert, 13% as nonhazardous and 50% as hazardous, confirming that they are unsuitable materials for the application as an isolated aggregate in civil engineering. The heavy metals released in higher levels were, in order of relevance, As, Hg, Cr, Ni, Cu, Se and Mo. In this work, a tank test by EA NEN 7375 (2004) for analysing the behaviour of mortar mixtures with BBA after their hardening was applied. All the specimens were classified as inert and it was demonstrated that secondary materials such as BBA can be reused in cement-based materials from an environmental point of view

40

New Trends in Eco-efficient and Recycled Concrete

as long as there is adequate management of these materials applied by engineers, constructors or plant managers.

2.2.5 Processing and improvement techniques of BA Several processing systems can help to modify the properties of BA to improve their properties. Specifically, there are three research works in which different processes were carried out. Firstly, in the work of Rosales et al. (2017) several BBA processes were applied to produce recycled mortar. The presence of light particles and organic matter confer to the BBA certain physical and chemical characteristics which significantly reduces the possibility of reuse. Three processes were applied on BBA, crushing, burning and eliminating lightweight particles. In this work, physical and chemical properties of unprocessed and processed BBA were studied for comparison. The results revealed that by applying combustion and crushing processes a relevant increase in the compressive strength of the mortar was achieved for substitutions in respect to unprocessed BBA mortars. This fact is very interesting because if additional physical processes are applied in BBA, it is possible to significantly improve their properties to manufacture mortar and concrete. Secondly, Fukasawa et al. (2017) proposed the application of the syntheses of potassium-type zeolites (K-zeolites) from BFA via a hydrothermal route, which represents a novel use for this material. The syntheses of potassium-type zeolites (K-zeolites) from BFA via a hydrothermal route were applied. An aqueous solution extracted from biomass incineration with a high concentration of potassium was employed to substitute the KOH solution in the synthesis of K-zeolites. In this work they were combined this extract of BFA with coal FA. Thirdly, Alkali activation of BFA combined with metakaolin (MK) has been investigated in recent years. Several works have been carried out in this sense, and will be discussed in a later Section 2.4 of this chapter.

2.3

Utilisations of biomass fly ash (BFA) and biomass bottom ash (BBA) in concrete design

Major challenges will arise relating to the efficient management of BFA and BBA by-products or waste. The primary concerns are ash storage, ash disposal, ash usage and the presence of unburned carbon. The use of ash from the combustion of biomass is conditioned by its physical-mechanical properties. There is not much literature on the use of ashes coming from the combustion of biomass in construction materials (especially in cement-based materials) and its use is limited to materials of medium resistance, partially replacing some of its components (cement or aggregates) so that the properties are not affected.

Biomass fly ash and biomass bottom ash

41

Go´mez-Barea et al. (2009), affirmed that the ashes can be used for soil stabilisation, as a substitution of cement in concrete, synthetic aggregate, road base and subbase, asphalt-based products and even as light bricks. In the past decade, FA was widely studied, however, there have been few studies on the use and management of BA.

2.3.1 The design of concrete with biomass ash: general properties for design More efforts have to be made to improve the sustainability of the concrete industry. Replacing traditional aggregates with BA can contribute to this goal. But current regulations, such as Standard Specification for coal FA and raw or calcined natural pozzolan for use in concrete (ASTM C618, 2017) about FA for concrete, do not allow the use of BA for the manufacture of concrete because there is not enough knowledge about the effects they can have on the properties of concrete. Until now, only partial replacements of cement by BA has been considered to maintain the workability, strength and durability of concrete under control. However, a deeper knowledge of the properties of BA is needed to overcome other problems. As BA is highly absorbent, the demand for concrete water is high. This problem of viability can be solved by submitting the fine fraction BA to a grinding operation to remove the porous elements as well as previously wetting the fine and coarse fractions of BA in a controlled manner. Regarding long-term behaviuor, heavy metal leaching and exposure to freeze thaw are not problematic, although there is a susceptibility to acetic and lactic acid attack and perhaps a greater sensitivity to the alkali silica reaction (ASR). A lot of research has been carried out to analyse the physical, chemical and mechanical characteristics of the ashes coming from the combustion of biomass with the aim of being used in concrete, mainly due to the pozzolanic properties of some types of BA. The use of BA in the manufacture of concrete presents an important problem as it is a very heterogeneous waste presenting a very variable size and with different percentages of unburnt particles in their composition. According to Rajamma (2011), the main aspects that influence the characteristics of BA are: G

G

G

The properties of the original biomass, that is, if it is of herbaceous origin, wood or bark. The technology used for combustion. And whether BA or FA are used.

2.3.2 Influence of BA on concrete’s fresh-state properties: density and slump When BA is applied in concrete, the replacement of aggregates by BA can produce a negative effect on the workability in the fresh state. This is mainly due to the greater water absorption of BA. However, when it uses fairly high ratios of water

42

New Trends in Eco-efficient and Recycled Concrete

with respect to cement (W/C), workability is not the real problem. To obtain an adequate workability, W/C ratio of 0.60 0.65 is recommended. In addition, the use of a superplasticizer can easily improve the workability (Beltra´n et al., 2014). Previous studies showed that the density of concrete, mortar or soil cement mixtures depends mainly on the density of the aggregates used (Matias et al., 2013). In general, the data for dry bulk density decreases with the use of BA (Rosales et al., 2017) slightly as Portland cement is replaced by more BA due to the lower density of BA compared to the ordinary cement. The decrease in the apparent density shows an increase in the porosity of the samples, therefore, the water penetration increases due to the hydration products and pozzolanic materials that fill the voids in the cement paste (Wongkeo et al., 2012). The use of highly porous industrial by-products, such as BA, significantly increases workability due to its high water uptake.

2.3.3 Physical-mechanical properties of mortars and concrete with BA Different studies have shown that biomass ash, depending on its composition and combustion process, does not always have a pozzolanic character. However, many types of BFA have pozzolanic properties similar to coal FA, for example, sugarcane straw, rice husk, wood and wheat straw (Martirena et al., 2006; Yu et al., 1999; Naik and Kraus, 2003). Some of these types of ash have been used as a mineral admixtures, showing a good mechanical behaviour, in addition to reducing the use of natural resources used for construction (Kumar and Patil, 2006). However, biomass ash from olive trees has a low content of oxides (Al2O3, SiO2, and Fe2O3) that provide pozzolanicity and a high CaO content. This composition could limit the use in cement-based materials (Vassilev et al., 2010). Some researchers have evaluated the use of BBA for the manufacture of mortar (Gemelli et al., 2004; Maschio et al., 2011; Da Luz Garcia and Sousa-Coutinho, 2013; Modolo et al., 2013; Carrasco et al., 2014) in general as a substitute for small amounts of cement. In most studies, several researchers limit the cement substitution content with BA to 5% 10% (Da Luz Garcia and Sousa-Coutinho, 2013). Maschio et al. (2011) used 5% 30% of ashes coming from spruce chip, crushed to a maximum size of 0.30 mm and with 5% of BA. Table 2.4 shows that similar physical and mechanical properties were obtained in mortar with BBA in comparison with the control mixture. This premise justifies the importance of the particle size distribution to obtain appropriate mechanical behaviour. On the other hand, it should be noted that ashes with a high potassium content reduce the properties at advanced ages due to the development of ASR. Once the viability of its use in mortar has been verified, investigations were carried out using BBA as a partial substitution for sand. Substitutions of less than 20% sand by BBA do not produce significant decreases in the mechanical properties of concrete, especially at advanced ages.

Table 2.4 Physical chemical properties of mortar with BBA from spruce chips (Maschio et al., 2011) Material (mortar)

Absorption 28 days (%)

Compressive strength 28 days (MPa)

Expansion 28 days (%)

Absorption 128 days (%)

Compressive strength 180 days (%)

Control % Bottom ash

1.5 5 10 20 30

78 1.2 1.9 5.3 6.9

0.01 82 74 66 69

1.1 0.013 0.016 0.031 57

83 7.1 7.4 10.1 10.3

56 49 38 37

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New Trends in Eco-efficient and Recycled Concrete

Unlike bottom ash, BFA is widely studied for its versatility. Studies reveal BFA are effective in the development of the strength of the concrete. The behaviour depends mainly on the level of SiO2 in the ashes.

2.3.4 Durability-related properties: water absorption (by immersion and capillarity), carbonation resistance and chloride penetration, sulphate and acid attack Durability problems play an important role, in particular among which the ASR is the most frequent. They denote reactions between reactive aggregates and alkalis released by the hydration of cement or other sources, such as mineral mixtures or aggregates. These reactions lead to the expansion of concrete, cracks and even faults, which are very damaging to concrete structures. Cracking of the ASR (Fig. 2.10) generally occurs in areas with a frequent supply of moisture, such as retaining walls exposed to the entrance of groundwater, marine structures in direct contact with salt water and other areas with capillary suction. A reactive by-product, like some BA, forms an alkaline silica gel by absorbing the water present in the concrete (central zone in Fig. 2.10). This property is important because the C S H gel (hydrated calcium silicate, also called calcium silicate hydrate (CSH) gel with formula CaO
SiO2
H2O) produces an expansive pressure, activating the crushing process between the aggregate and the surrounding cement paste (Lindga˚rd et al., 2012). When biomass ash is used in concrete or mortar, the durability also depends on whether the ash is pozzolanic or not. It depends on the structure of the particles in terms of crystallinity and is essential to determine their pozzolanic activity, obtaining improvements in terms of the durability of those ashes with pozzolanic activity (Rajamma et al., 2009).

Figure 2.10 Cracking of the alkali silica reaction.

Biomass fly ash and biomass bottom ash

45

The permeability of the concrete is an indication of its performance with respect to its durability. Permeable concrete allows the entry of aggressive agents such as chlorides, and this puts the concrete at risk. Beltra´n et al. (2014) showed that water penetration increases with the addition of BBA as shown in Table 2.5. Concrete is a porous material and incorporating BA increases the porosity, therefore, the CO2 from the air can penetrate through its pores towards the interior, where a chemical reaction with calcium hydroxide is produced. Most of the carbonation models consider the amount of cement as an important parameter, due to its influence on the reaction of CO2 with Ca (OH)2, forming the hydration of the cement. This reaction (carbonation) can reduce the pH value of the pore solution to less than 9. When the alkalinity of the pore solution is lost, the properties of the concrete such as strength, permeability, shrinkage and resistance to chemical and physical attacks will be affected by carbonation. As the pore index is related to the particle size, it has been shown that the concrete manufactured with BA of smaller particle size have a low carbonation compared to those that use medium or coarse particle size.

2.3.5 Technological properties: thermal and acoustic insulation The characteristics of thermal insulation and fire resistance are favoured, with respect to the products with ashes of the BA combustion, when ash or slag is used that come from processes such as combustion, pyrolysis, gasification or other thermal treatment processes using biomass, together with various binders and a minor proportion of additives, materials with high thermal and acoustic properties, are obtained. The use of BA, mainly bottom ones, as a substitute for coarse aggregate for the development of porous concrete with sound absorption properties has been the main subject of several investigations carried out by the authors in recent years (Arenas et al., 2013; Leiva et al., 2012). The properties of a sound-absorbing, porous concrete composed mainly of BBA, showing that porous concrete recycled with ash presents properties similar to other conventional materials used for acoustic absorption applications (Leiva et al., 2012). In addition, the origin of BBA (pulverised coal combustion, co-combustion or gasification of coal and pet coke, biomass combustion) does not affect the Table 2.5 Durability properties of hardened concrete (Beltra´n et al., 2014)

Water penetration (mm) Chloride penetration (mm)

Age

With 0% BBA

With 3% BBA

With 6% BBA

28 28 56

36.3 10.3 18.1

61.5 19.7 23.1

75.3 21.8 27.2

46

New Trends in Eco-efficient and Recycled Concrete

acoustic absorption behaviour of recycled concrete, as long as the size distribution is similar. The use of BA depends on their physical and chemical properties. The ashes coming from the rice husk which, among its varied chemical composition, has a high content of amorphous silica. This mineral is known to be one of the minerals that intervenes in cementing reactions and, therefore, are susceptible to developing pozzolanic activity if combined with the appropriate alkaline activator additive (Behak and Peres, 2008). This high silicon content provides the ash, mixed with other additives, with good pozzolanic properties. In Bangladesh, researchers at the University of Dhaka have examined BA as a possible profitable ingredient in the development of a variety of building materials, such as bricks, low cost thermal insulators and pozzolanic cement (Farooque et al., 2009).

2.4

Biomass ash-based geopolymer: clean production, properties and applications

In recent years BA have been used in the manufacture of geopolymer products allowing opportunities for their application in construction. In the following subsections the use of biomass ashes in the production of sustainable concrete, besides the different applications and benefits achieved, are presented.

2.4.1 Geopolymer concrete. A green concrete The demand of ordinary Portland cement (OPC) to manufacture conventional concrete has been constantly increasing due to the increment of infrastructural activities in the world. In order to produce OPC, the cement industry demands large amounts of energy, in addition to high quantities of limestone and clay as natural raw materials, with the consequent environmental impact due to the depletion of raw materials, the economic cost and the emissions to the atmosphere (Suksiripattanapong et al., 2017). The development of green concrete as an alternative to Portland cement concrete has the aim of reducing the environmental impact of construction sector, both increasing the use of industrial waste or by-products and reducing the negative impacts of cement production industry (Ramujee and PothaRaju, 2017). In this sense, the use of waste ashes as a precursor instead of the traditional pozzolanic coal ashes also allows for improving concrete performance (Islam et al., 2017). Geopolymers are one of the most promising alternative cementitious materials, which are obtained by mixing several source materials with a high content in silica and alumina, as FA, ground granulated blast furnace slags (GGBFS) or MK, with a solution of potassium or sodium hydroxides and soluble silicates, which is strongly alkaline (Part et al., 2015). In this system, the alumina and silica dissolved into the solution experiment geopolymerisation by means of thermal curing at temperatures below 100 C and lower times of curing up to two days to form an amorphous

Biomass fly ash and biomass bottom ash

47

aluminosilicate three-dimensional network in form gels CSH (Puertas et al., 2000), CASH (calcium aluminosilicate hydrate) (Rashad, 2013) or NASH (sodium aluminosilicate hydrate) (Ismail et al., 2014). These gels make concrete with similar or higher strengths to that of OPC systems (Part et al., 2015). Alkali-activated systems are also denominated in the literature as: G

G

G

Alkali-activated cements, such as slags due to its content in calcium (Palomo and Palacios, 2003). Inorganic polymer, obtained from industrial by-products such as coal FA, GBFS (granulated blast furnace slag) or mine tailings and contaminated soil (Sofi et al., 2007). Hydro-ceramic or low temperature inorganic polymer glass, obtained from the reaction between metakaolinite in an alkaline sodium silicate solution that leads to an amorphous silicate at temperatures below 100 C (Rahier et al., 1996).

Recently, geopolymers have attracted a considerable amount of attention with respect to OPC because of its behaviour as a binder with a high compressive strength at early ages, improved workability, reduced permeability, increased durability, resistance to acid attack, reduction of plastic shrinkage cracking and excellent fire resistance, in addition to the large amount of energy demanded, among other environmental and economic benefits (Duxson et al., 2007; Liew et al., 2017; Provis and VanDeventer, 2009; Davidovits, 1991). Thus, geopolymer concrete promotes sustainable and innovative use of waste materials leading to faster concrete production, with the reduction of the curing time and construction costs, minimising the maintenance and increasing the service life of construction projects (Liew et al., 2017). In this sense, the mixture of geopolymer binders and aggregates of natural sources (Albitar et al., 2018; Tennakoon et al., 2017), lightweight (Islam et al., 2017, Novais et al., 2018), recycled aggregates (Nuaklong et al., 2016), or even crumb rubber (Park et al., 2016), among others, is presented as an opportunity to improve sustainability of the industry of concrete (Liew et al., 2017).

2.4.2 Limits and opportunities of biomass ash to produce geopolymer concrete The use of biomass ashes in the manufacture of geopolymer concrete or alkaliactivated cementitious composites results in a promising sustainable concrete where the use of OPC can be eliminated totally. Biomass ash geopolymer is considered to be the latest contribution to the scientific field of studies on geopolymers. In the literature, recent studies regarding the utilisation of rice husk ash (RHA) or palm oil fuel ash (POFA) as biomass ashes to produce geopolymer concrete in partial substitution of the traditional precursors (FA, GGBFS or MK) can be found. The chemical composition and physic characteristics of RHA and POFA are summarised in Table 2.6. In particular, it can be seen that the chemical composition in most of the BA studied conform to the ASTM C618 (ASTM C618, 2017) with regard to the maximum content in sulphuric anhydride (#4.0%), maximum value of loss of ignition (#10.0%) and minimum content in pozzolanic compounds

48

New Trends in Eco-efficient and Recycled Concrete

Table 2.6 Characterisation of RHA and POFA

SiO2 Al2O3 Fe2O3 CaO Na2O K2O SO3

RHA

POFA

92.01 96.03 (Lim et al., 2018; Suksiripattanapong, et al., 2017) Nd-0.20 (Suksiripattanapong, et al., 2017; Lim et al., 2018) 0.13 0.80 (Suksiripattanapong, et al., 2017; Lim et al., 2018) 0.49 0.53 (Lim et al., 2018; Suksiripattanapong, et al., 2017) Nd-0.20 (Suksiripattanapong, et al., 2017; Lim et al., 2018) 1.67 (Suksiripattanapong, et al., 2017) 0.0 0.2 (Lim et al., 2018; Suksiripattanapong, et al., 2017)

46.00 67.72 (Yusuf et al., 2014; Bashar et al., 2016) 3.10 27.52 (Yusuf et al., 2014; Lim et al., 2018) 2.45 5.50 (Yusuf et al., 2014; Lim et al., 2018) 4.30 10.20 (Liu et al., 2016; Huseien et al., 2016) 0.10 0.40 (Huseien et al., 2016; Lim et al., 2018) 4.08 8.64 (Yusuf et al., 2014; Huseien et al., 2016) 0.09 1.07 (Huseien et al., 2016; Bashar et al., 2016) 3.78 3.95 (Kabir et al., 2017; Yusuf et al., 2014) 0.17 (Kabir et al., 2017; Islam et al., 2015) 0.40 5.90 (Lim et al., 2018; Huseien et al., 2016) 0.13 0.33 (Yusuf et al., 2014; Kabir et al., 2017) 6.54 (Islam et al., 2015) 0.45 (Islam et al., 2015) 0.60 21.60 (Islam et al., 2015; Yusuf et al., 2014) 2.14 6.20 (Islam et al., 2017; Bashar et al., 2016) 172 1720 (Kabir et al., 2017; Islam et al., 2017)

P2O5 MnO MgO TiO2 CuO Cl LOI Specific gravity (g/cm3) Specific surface area (m2/kg)

Nd-0.30 (Suksiripattanapong, et al., 2017 Lim et al., 2018) Nd (Suksiripattanapong, et al., 2017)

1.44 (Suksiripattanapong, et al., 2017) 2.05 2.10 (Suksiripattanapong, et al., 2017; Lim et al., 2018) 495 (Lim et al., 2018)

RHA, rice husk ash; POFA, palm oil fuel ash.

(SiO2 1 Al2O3 1 Fe2O3 $ 70.0%), which make them suitable for use as supplementary cementitious materials. POFA is a solid waste by-product obtained from the generation of power in the palm oil industry, particularly in Malaysia and Thailand (Khankhaje et al., 2016). POFA possesses pozzolanic properties (Yusuf et al., 2014), so it can replace precursors in geopolymer concrete reducing the environmental pollution of these materials (Islam et al., 2017). The inclusion of finely divided POFA in the GBFS geopolymer system contributed to the compressive strength of concrete by the pore filling effect, the formation of less-ordered and homogeneous microstructures and dual products of the combination of the highly polymerised unit of geopolymer and CSH gel (Yusuf et al., 2014).

Biomass fly ash and biomass bottom ash

49

The industry of edible rice produces rice husk as a waste material which is also used as fuel in boilers and power generators due to its high calorific power. RHA generated contains high percentage of silica (. 90%), hence it is a suitable cementitious material (Lim et al., 2018) and a feasible precursor for geopolymerisation (Suksiripattanapong, et al., 2017). Consequently, it has been incorporated in geopolymer concrete due to its pozzolanic effect (Kabir et al., 2017). Particularly, RHA possesses a low amount of alumina compound which makes it worse as precursor. Finally, another highlight is that the variability and heterogeneity in the properties of these biomass ashes make them difficult to obtain geopolymeric products with homogeneous behaviour. That is the reason why it is necessary to deepen its study, which is still in a very early stage.

2.4.3 Stabilisation of biomass ashes using geopolymerisation: leaching characteristics The ashes from combustion in power plants contains heavy metals that are considered as first pollutants in the world (Wang et al., 2017) as these heavy metals cause very serious world environmental problems. It has been scientifically proven that geopolymers can effectively solidify or stabilise heavy metal ions (Zhang et al., 2008; Guo et al., 2014). Besides that, heavy-metal pollutants can be locked into the three-dimensional network structure of geopolymers. According to Sun et al. (2014), geopolymer solidification presents many other advantages, such as: (1) geopolymer’s permeability coefficient being very low, thus it can effectively prevent the infiltration of pollutants elements; (2) in the synthesis of geopolymer Al (III) is comprised of a four-fold coordinated atom allowing Al04 the ability to negatively charge, (3) so it can absorb the positive charge of heavy metals; and finally, (4) geopolymers entails simple processing durable and resistant to weather. These findings are available in a few studies, such as that conducted by Wang et al. (2017) in which geopolymer showed a degree of solidification of Pb(II), Cd(II), Mn(II) and Cr(III) of 99.9%. Yunsheng et al. (2007) also reached an immobilisation efficiency of 98.5% in a slag-based geopolymer mortar when heavy metals are incorporated in the geopolymeric matrix in the range of 0.1% 0.3%.

2.4.4 Properties of geopolymer concrete produced with biomass ash The recent references founded in the scientific literature related to the investigation of geopolymer binders with the incorporation of biomass ashes indicates that the use of geopolymer contributes to the improvement of the waste management problem in several industries, in addition to the environmental problems associated to the construction sector with respect to the manufacture and use of OPC (Part et al., 2015).

50

New Trends in Eco-efficient and Recycled Concrete

Table 2.7 introduces the main physical and mechanical properties of geopolymer concrete manufactured with both biomass ashes, as found in the literature. In general, it can be seen that POFA has turned out to be the best partial substitute of precursors used in alkali-activated concrete, due to its capacity to contribute to achieving faster and higher physic and mechanical requirements. Particularly, in the fresh state, the addition of both biomass ashes in geopolymer concrete plays an important role in the workability as they have a high loss of ignition and an irregular shape (Islam et al., 2015). The increase in mixing water needed is due to the ashes having a high amount of micro-porosity and pore fluid (Suksiripattanapong et al., 2017). At the hardened state, geopolymer concrete with densities less than 2000 kg/m3 could be categorised as lightweight concrete (BS EN 206-1, 2008). On the other hand, in comparison with OPC concrete, the compressive strength developed, much higher for POFA geopolymer concrete than RHA geopolymer concrete, has been mainly achieved by the pozzolanic activity of the mix binder (Lim et al., 2018). Compressive strength also depends on the quality and ratio of alkaline activators, curing time, curing temperature, quantity and finesse of solid compounds and relative ratio of the reactive base materials. (Islam et al., 2015; Kabir et al., 2017; Lim et al., 2018). For its part, flexural behaviour exhibit strengths lower than traditional OPC concrete, that usually range between 8% and 11% of compressive strength (Islam et al., 2015). This result is attributed by Lim et al (2018) to the lubricant effect of the spherical particles of the biomass ash and to the high proportion of precursor in the mixing design. Finally, Yusuf et al. (2014) concluded that higher compressive strength is directly related to the higher modulus value, which is also directly allied to the higher Si/Al ratio of the reaction products.

Table 2.7 Characterisation of geopolymer concrete

Temperature curing ( C) Time curing (h) Density oven dry (kg/m3) Compressive strength (MPa) Flexural strength (MPa)

RHA

POFA

21 50 (Suksiripattanapong et al., 2017) 7 90 days (Suksiripattanapong et al., 2017)

25 90 (Islam et al., 2015; Yusuf et al., 2014) 6 48 (Yusuf et al., 2014; Liu et al., 2016) 1784 1935 (Bashar et al., 2016; Islam et al., 2015) 28.0 80.0 (Islam et al., 2015; Lim et al., 2018) 1.23 4.50 (Islam et al., 2015; Bashar et al., 2016; Lim et al., 2018) 6.25 11.12 (Yusuf et al., 2014; Islam et al., 2015)

0.29 3.00 (Suksiripattanapong et al., 2017)

Modulus of elasticity (GPa) RHA, rice husk ash; POFA, palm oil fuel ash.

Biomass fly ash and biomass bottom ash

51

2.4.5 Adsorption characteristics of BA-based geopolymers towards micropollutants Micropollutants such as mercury, lead, copper, zinc, nickel, chromium and cadmium are a major concern in the world as they become hazardous for human health, animals and the ecological environment. They are toxic and carcinogenic elements which accumulate over time. Geopolymers have been investigated as an adsorbent to remove these heavy metals from wastewater or aqueous solution (Kara et al., 2017). This capacity is guaranteed by the considerable amount of meso-porosity in geopolymers with size range of 10 50 nm ( . 40 vol%) (Landi et al., 2013) and their ability to reduce the mobility of most heavy metals ions when contained within their structure (Muˇzek et al., 2014). In the limited number of studies dealing with the use of geopolymers for heavy metal removal from aqueous solutions, Novais et al. (2018) developed a highly porous and lightweight BFA-based geopolymer used as an adsorbent for the removal of methylene blue from synthetic wastewater. The results showed a value in the absorption of 15.5 mg/g when the porosity was 80.6%, a threefold increase that was observed in the geopolymer with 40.7% of porosity. Barbosa et al. (2018) experimented with a meso-porous geopolymer made with MK and RHA as sources of silica and alumina and soybean oil as a meso-structure-directing agent attaining the adsorption equilibrium of methyl violet 10B dye from the aqueous solution valued in 276.9 mg/g within 120 min.

2.4.6 Use of BA-based geopolymers for thermal and acoustic insulation Geopolymer concrete leads to an excellent new material that will save operational energy due to its low density and relatively lower thermal conductivity than normal-weight concrete (Zhang et al., 2014). Moreover, Liu and his colleagues (Liu et al., 2014, 2016) have introduced the foamed technique into geopolymer materials to improve thermal insulation. In their initial investigation regarding the behaviour of palm oil-shell foamed geopolymer concrete utilising industrial wastes such as palm oil shell as lightweight coarse aggregate and POFA and coal FA as binder mix in concrete, they produced a geopolymer with densities between 1300 and 1700 kg/m3 due its higher porosity. Despite the fact of a reduction in compressive strength, they concluded that a thermal conductivity of about 0.47 W/m K was 22% and 48% lower than blocks and bricks as conventional materials for walls. Hence this geopolymer could be categorised as structural concrete Class I (compressive strength more than 15 MPa) and structural and insulating concrete Class II (compressive strength between 3.5 and 15 MPa and thermal conductivity less than 0.75 W/m K) according to the RILEM (1983) classification. From the point of view of the acoustic properties of BA-based geopolymer, it is well-known that the sound absorption of a porous material is related to the loss of noise by friction in the wall of its pores (Park et al., 2005), so that geopolymer

52

New Trends in Eco-efficient and Recycled Concrete

concrete which presents a high open voids ratio will have a greater sound absorption coefficient than less-porous concrete (Kim et al., 2012). Nevertheless, no study has been found in the literature regarding to the use of biomass ash in geopolymers with the purpose of enhancing its acoustic properties. The development and application of geopolymers manufactured with biomass ashes as insulated materials can contribute to the environmental impact on buildings reducing the energy demand both during the construction and use.

2.5

Concluding remarks

Renewable resources such as biomass are used for heating and electricity generation, etc., and generate large amounts of ashes. Due to their pozzolanic or filler capacity, it is possible to determine these specific conclusions, related to the recycling of BA in cement-based materials: G

G

G

G

G

G

G

Biomass for energy can include a wide range of materials such as natural biomass, residual biomass and energy crops. Currently, the performance and technology of systems to produce energy with biomass are highly advanced and have the potential to be used for many applications due to different energy conversion technologies: transport, heat and electricity production, etc. As a result, a reduction of pollution emissions such as CO, HC and NO and protecting the environment would be achieved. The use of ashes from biomass combustion in cement-based materials are strongly conditioned by their physico-mechanical properties. The rapidly increasing amount of biomass ash generated implies the necessity of recycling, with their application in cement-based materials being a highly appropriate use. Two types of biomass ash are obtained, BBA and BFA. BBA is the portion of noncombustible residue found in the furnace or incinerator and it looks like artificial sand. BFA is the portion of ash that escapes through the chimney and is retained to prevent it from being released into the atmosphere, and it presents a dusty appearance such as coal FA. BBA presents high contents in CaO (17% 30%) and in some cases in SiO2 (until 72%), which could allow for its application in the manufacture of concrete and mortar, although it is recommended to apply some type of processing to reduce the organic matter content. Moreover, partial substitutions of cementitious materials by biomass ashes are frequently required due to their excess content in K2O and MgO. BFA can be effective in the development of the strength of recycled concrete, and its behaviour depends mainly on the level of SiO2 in the ashes, thus, it can be applied in substituting up to 50% of cement. On the other hand, substitutions of less than 20% of sand by BBA do not produce significant decreases in the mechanical properties of concrete.

In general, the use of biomass ashes in the manufacture of alternative cementbased materials is positive, however, it is necessary to continue carrying out specific studies regarding to the application of BBA and BFA, and legal adaptations should be made.

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Further reading Carrasco-Hurtado, B., Corpas-Iglesias, F.A., Cruz-Pe´rez, N., Terrados-Cepeda, J., Pe´rezVillarejo, L., 2014. Addition of bottom ash from biomass in calcium silicate masonry units for use as construction material with thermal insulating properties. Constr. Build. Mater. 52, 155 165.