Several power plants burning residual forest biomass were installed in the past few years in Portugal as a strategy to reduce forest fire risk and to mitigate climate change (Conselho de Ministros, 2010) as a result of carbon dioxide emissions being considered neutral. With the power plants fuelled by forest biomass currently in operation in Portugal, between 100,000 and 200,000 tons of ash are produced annually (Cruz et al., 2017). However, this percentage is projected to increase with the construction of new residual forest biomass plants, and the concession of more 60 MWe of installed capacity for the plants already in operation (Diário da República, 2017). In view of the transition to a circular economy, an adequate end-of-life management of ash is important, since ash from the combustion of woody biomass is typically disposed in landfills (Modolo et al., 2015; Tarelho et al., 2015).
Recycling ash alternatives should be pursued, not only due to the rising cost of the landfill disposal which, in turn, is reflected on the cost of the energy produced, but also as a consequence of the ‘zero-waste’ objective policies (Maschio et al., 2011). The rate of ash recycling in European countries varies. For example, in countries such as Germany, the Netherlands, Denmark, France and the United Kingdom between 50-90 % of ash is recycled, while countries such as Austria, Switzerland, Portugal, Italy and Norway recycle less than 10 % of ash, being the majority sent to landfill (Williams, 2013).
A possibility for the woody biomass ash valorisation includes its incorporation in construction materials, through the addition in cement mortar, adhesive mortar, concrete blocks and bituminous asphalts (Ahmaruzzaman, 2010; Chowdhury et al., 2015; Coelho, 2010; Dahl et al., 2010; Modolo et al., 2014; Ribbing, 2007; Vassilev et al., 2013a, 2013b). The incorporation in construction materials can depends on the ash variety (e.g., fly and bottom ash) and on the technology of the combustion process (e.g. fixed-bed, fluidised bed). For the overview of the main studies concerning the use of biomass ash in construction materials, see Table S1 of the supplementary material.
Utilization of biomass ash is technically feasible for the manufacture of cement mortars. In some studies, cement mortars were made by substituting part of the cement with biomass ash (Esteves et al., 2011; Esteves, 2010; Maschio et al., 2011; Rajamma et al., 2015, 2009; Ramos et al., 2013; Rosales et al., 2017; Tosti et al., 2018), while in other studies, cement mortars were made with ash total or partially substituting the aggregates (Coelho, 2010; Modolo et al., 2015, 2013; Ukrainczyk, 2016). Results show that biomass ash used as an additive to cement mortars has a number of positive effects, such as decrease of both water demand and expansion due to the alkali-silica reaction (Esteves et al., 2011). Modolo et al. (2015, 2014) also assessed the use of biomass ash in adhesive mortars and observed similar results for water demand than the ones obtained for cement mortars. Furthermore, an increase in the tensile adhesion strength was observed.
Several studies have been also performed regarding the use of biomass ash in concrete production (Barbosa et al., 2013; Bastos, 2014; Beltrán et al., 2014; Berra et al., 2015; Dias, 2011; Garcia and Sousa-Coutinho, 2013; Lessard et al., 2017). The utilization of biomass ash in concretes is partly based on economic aspects, since the cost of concrete is reduced due to the partial substitution by ash, and partly based in the technical benefits, such as, lower water demand, reduced bleeding, and lower heat evolution (Chowdhury et al., 2015; Gunaseelan and Ramalingam, 2016). In addition, Barbosa et al. (2013) showed that the compressive strength of concrete made with biomass ash was superior compared to the standard concrete.
The utilization in bituminous asphalt has also been a significant output for biomass ash (Scheetz and Earle, 1998). The ash can be used for soil stabilization, subgrade base course material, aggregate filler, bituminous pavement additive, and mineral filler for bituminous concrete (Alves, 2013; Dias, 2011; Melotti et al., 2013; Pasandín et al., 2016; Pinho, 2014; Yoshitake et al., 2016). Ash used along roadways has proven to be a beneficial practice because in some areas the cover materials are scarce and the biomass ash can be used, since the pollution due to its manipulation in road works is negligible (Ahmaruzzaman, 2010).
On one hand, biomass ash valorisation minimizes the demand for standard materials, but on the other hand, they entail environmental impacts. This trade-off can be evaluated using life cycle assessment (LCA) methodology. LCA has been widely applied to quantify the environmental impacts of waste disposal (Borghi et al., 2018; Demertzi et al., 2015; Hossain and Poon, 2018; Huang et al., 2017; Huber et al., 2017; Xuan et al., 2016). There are some LCA studies focusing on ash valorisation (Habert et al., 2016; Huang and Chuieh, 2015; Huber et al., 2017; Muñoz et al., 2015; Ondova and Estokova, 2014; Schepper et al., 2014; Seto et al., 2017; Teixeira et al., 2016), but they only refer to ash from coal or municipal solid waste.
The goal of this study is to evaluate and compare the trade-offs between the environmental impacts and the benefits of woody biomass fly and bottom ash incorporation in construction materials to support future decision-making regarding the best management options for ash valorisation. The alternatives selected were the production of cement mortar, adhesive mortar, concrete blocks and bituminous asphalt. Besides, the ash landfilling was also evaluated as a base scenario.