**1. Introduction**

Globally, 0.74 kg of solid waste is generated per capita per day, with national rates varying between 0.11 and 4.54 kg per capita per day depending on urbanization rates and income levels [1–3]. The Europe and Central Asia regions, with 20% (392 million tons per year), rank second in solid waste generation [4–6]. The overall composition of waste mainly corresponds to organic and green waste (44%); paper and cardboard (19%); other materials (14%); plastics (12%); glass (5%), metal (4%); wood (2%); and rubber and leather (2%). As for waste treatment, it mainly focuses on recycling (20%) and incineration (17.8%), providing the possibility of giving a new useful life to the materials after their use and ensuring

**Citation:** Martínez-García, R.; Jagadesh, P.; Zaid, O.; S, erb ˘anoiu, A.A.; Fraile-Fernández, F.J.; de Prado-Gil, J.; Qaidi, S.M.A.; Gr ˘adinaru, C.M. The Present State of the Use of Waste Wood Ash as an Eco-Efficient Construction Material: A Review. *Materials* **2022**, *15*, 5349. https://doi.org/10.3390/ ma15155349

Academic Editor: Dumitru Doru Burduhos Nergis

Received: 7 July 2022 Accepted: 2 August 2022 Published: 3 August 2022

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adequate final disposal [7]. This is in line with the adaptation of a circular economy as a novelty and eco-friendly production model. In the specific case of the construction industry, part of the environmental impact is due to the demolition of structures, which generates different types of solid waste. On the other hand, the use of cement in the production of bricks/block and concrete, which is used in the latter to make it more resistant [8], implies a significant anthropic emission of carbon dioxide (CO2) of 5–8% worldwide, which could increase, according to projections, to 27% by the year 2050, especially taking into account that one cubic meter of concrete is produced annually per person [8–11]. Based on this reality and the projected scenario, the cement and concrete industry has been developing a series of strategies and innovations to reduce CO2 emissions. One of these innovations is the production of geopolymers to be used as alternative materials to replace all or part of the ordinary Portland cement used in construction, which is obtained either from metakaolin or from industrial, forestry, and agricultural waste with a high aluminosilicates content [12–15]. A geopolymer is a binder of mineral origin (inorganic) obtained from the dissolution [16–19] and subsequent polycondensation of ashes rich in aluminosilicates in the presence of an alkaline solution (hydroxides and silicates of alkali metals, Na and K) [13,20,21]. Additionally, the use of mixed geopolymers, which are generated by the combination of two or more types of chemically stabilized industrial wastes or ashes, has been considered [22]. The use of this type of materials can reduce CO2 production by up to 90%, while preserving or even improving their mechanical properties (e.g., porosity, structure, compressive strength, water absorption, and durability) [12,23].

Several researchers have devoted themselves to using different raw materials for the production of concrete, for example, agricultural residues such as rice husk ash and palm oil ash [24], sugar cane bagasse [25,26], and corn cob ash [27], finding good results in the properties of concrete [28]. On the other hand, wood waste ashes [29,30] have emerged as a good option for the fractional replacement of binder and kaolin used in the formation of geopolymers, since in addition to increasing workability, porosity, and drying shrinkage, these wastes are given an alternative use, and potential environmental pollution [29–33] is reduced by their entry into the environment, contributing directly to sustainable development [34,35]. Ekaputri reported [36] obtaining a concrete (geopolymer) with high compressive strength (48.5 MPa to 48.5 MPa) from class F ash with 10 mol/L NaOH due to the generation of hydroxide ions that significantly influence the dissolution of the Si and Al atoms of the source material. Despite the advantages of using high concentrations of alkali (NaOH, between 8–10 M) to obtain a high compression strength product of 104.5 MPa and 71 MPa for the paste and mortar, as well as a lower change in length due to temperature and water evaporation that have the lowest shrinkage percentage [37], it has been proven that the use of ashes from forest biomass (wood) can decrease the requirements of alkaline activators by up to 20% without the loss of properties [38–41]. However, when the substitution level of these ashes is higher than 10% by mass, the mechanical properties of the geopolymer are affected [42–45], proportionally reducing the compressive and flexural strength of the mortars, for all curing times [35]. Likewise, it is highlighted that different conditions can be used during the process of obtaining geopolymers, such as the type of curing, humidity control, temperature, concentration and proportions of alkaline activators, type and quantity of raw material or proportions of starting materials (in case of mixtures), which will influence the properties of the final product. Among the findings, it can be mentioned that the increase in SiO2/Al2O3 ratios positively influences the mechanical compressive strength of geopolymers [15], and it was found that the inclusion of 5–15% wood ash in the process can generate greater strength and durability depending on the age (aging time) of 3–7 days as a consequence of the formation of gels and minerals that increase alkalinity [12]. Research has also been conducted on the effects of the solid–liquid ratio and the alkaline activator in the synthesis of pure geopolymers. Alves et al. [46] used as precursor material ground blast furnace slag with a solid–liquid ratio between 1.5 and 2.2, and as activator solutions (a) a sodium hydroxide/sodium silicate/water mixture and (b) a potassium hydroxide/potassium silicate/water mixture, finding that the resulting

geopolymer possessed high compressive strength depending on the solid–liquid ratio and the percentage of water added to the mixture, which is further impacted by the composition of the activating solution. They also noticed that the strength increases with aging [46]. Currently, the addition of plastics to the optimized wood ash-based geopolymer is being tested; for example, in the case of polypropylene (PP), it has been reported that the addition of 1% PP fiber generates an increase in compressive, tensile, and flexural strength by 3.7%, 15.6%, and 10%, respectively [47]. Other types of materials are also being developed. Kristály et al. [48] produced a composite of geopolymer foam and glass to obtain a lightweight and environmentally friendly concrete from waste materials (secondary raw material), which is a valuable building material useful for thermal and acoustic insulation of walls that is also heat-, fire-, and acid-resistant [48].

Cement consumption in the world currently amounts to approximately 3 billion tons, which translates into 1.5 billion tons of carbon dioxide emitted into the environment [34]. According to the United Nations, the world population has increased in recent years, from 5300 million inhabitants in 1990 to 7300 million inhabitants in 2015; with a projected increase by the year 2050 of 24.74%, the requirements for cement, concrete, and other types of construction materials will increase significantly [49]. In this sense, the development of new and better alternative materials for the efficient substitution of cement for other materials at a global level will reduce production costs while reducing emissions, contributing to goals 11 and 13 of the 2030 Agenda for Sustainable Development, "Make cities and human settlements inclusive, safe, resilient and sustainable" and "Climate action", respectively. This literature review focuses on the approach to the processes for obtaining geopolymers from the use of wood ash, as well as the physical and chemical effects that take place under different production conditions. As per the authors' best knowledge, no significant review study exists on the physical, chemical, strength, durability, and microstructural analysis of concrete, which points to the originality of present work.
