# Long-Term Behavior of Concrete Containing Wood Biomass Fly Ash
## 1 Introduction  
In the European Union (EU), the tendency to reduce greenhouse gas emissions from coal use has led to increased use of wood biomass for energy production [1]. It was estimated that in 2018, about 11 million tons of ash were generated from biomass combustion in the EU-28 countries, most of which was ash from wood biomass [2]. It is very important that ash from wood biomass is well managed to avoid the possible pollution of air, soil, or groundwater. However, most of the ash produced is currently landfilled due to logistical problems, different ash properties, or the lack of legislation [3; 4; 5; 6]. Ash from the natural wood combustion contains valuable plant nutrients that are concentrated in the coarse ash fractions, so perhaps the best way to manage this ash is to return it to the forest from which it originated [7]. The fine ash fractions, fly ash, contain most of the volatile heavy metals and should be utilized in industry or disposed of [7].  
It has been shown that fly ash can be used as a raw material for the production of various types of construction products [8; 9; 10; 11; 12; 13]. Today, fly ash from coal combustion is one of the main cement replacement materials in cement production, and its use as a concrete constituent has been standardized [14]. In addition to coal fly ash, many new supplementary cementitious materials are being investigated to increase the sustainability of cement-based products either by increasing the durability or by reducing the cement consumption [15; 16; 17; 18]. The cement and concrete industry has also been identified as one of the main potentials for the use of wood fly ash (WFA) [6]. The properties of WFA differ from those of coal fly ash. The differences in the chemical composition of WFA are related to the wood species, the wood pieces used as fuel, and the season of biomass harvest [19; 20]. In addition, different combustion technologies have a great influence onthe physical and chemical properties of WFA particles. Although they are not as reactive as cement, WFA from grate combustors and fluidized bed combustors can exhibit hydraulic and/or pozzolanic properties [21; 22; 23]. Extensive research has been conducted to investigate the effect of using WFA as a cement replacement material [20; 21; 22; 23; 24; 25; 26; 27]. When used as a cement replacement material, WFA deteriorates the mechanical properties and increases the water demand, which is attributed to the amount of unburned carbon particles that have lower stiffness compared to cement particles. However, replacing cement with WFA can also increase the strength and improve the workability due to improved particle packing or activation by grinding [2; 23; 28].  
The utilization of WFA in the concrete industry requires further research, especially since most studies on the influence of WFA on the properties of cement composites have been tested on cement pastes and mortars, and only a limited number of experiments have been scaled up to the concrete level [2; 26; 29]. In this work, two types of WFA with different physical and chemical properties are used as partial cement replacements in concrete. The effects of the WFA admixture on the compressive strength, permeability, and drying shrinkage of concrete measured over a period of one year were studied.