**3. Technical feasibility and valorization of WBA use as supplementary cementitious material**

A review of the available literature leads to the conclusion that the application of WBA significantly depends on its properties, which depend primarily on the characteristics of the biomass used, i.e., the type of biomass, the plant technology, the combustion temperature and the location of ash collection and storage. For this reason, the chemical composition causes variation in the properties of the tested cement composites. The use of WBA in the cement composites leads to an increase in water demand, which may be related to the morphology of WBA (irregular particle shape and fineness), free CaO and alkali content, and LOI values [43, 53]. According to [23], increasing the content of WBA as a cement replacement resulted in decreased workability of cement pastes, while water treatment (washing of WBA) had a positive effect on the workability of cement mixtures due to physical modification: treatment by washing decreased the average ash particle size, porosity, and specific surface area of WBA. Increasing the proportion of WBA in mortar mixes prolonged the setting time, while cement pastes with a WBA content of 15% should be dimensionally stable despite the high content of CaO minerals in WBA (free CaO and MgO) [43]. The effect of WBA on the hydration of binders was studied by monitoring the heat release with isothermal calorimetry, where the induction period is prolonged by the addition of WBA regardless of the type and chemical properties of WBA [54]. Mixtures with WBA exhibit a slower increase in strength. However, with time the compressive strength increases so that after 28 days the compressive strength of the samples with 5 and 10% WBA is equal to or higher than that of the reference samples without ash (**Figure 5**). The effects of higher proportions of WBA on compressive strength have been shown to be unfavorable in studies. Therefore, it is not recommended to increase the proportion of WBA in structural concrete to more than 20% [16, 55, 56].

In addition to the mechanical properties of cement composites with different proportions of WBA, tests of durability properties are also important. Capillary absorption is defined as the transport of fluids due to surface tension that occurs in capillary pores. Capillary pores are the main pathway through which water and other aggressive substances penetrate cementitious composites and cause permanent problems. Therefore, capillary absorption testing is often used as one of the tests and quality assessments of cementitious composites to select a suitable concrete/mortar for the construction of structural elements exposed to liquids containing aggressive substances (usually

*Utilization of Wood Biomass Ash in Concrete Industry DOI: http://dx.doi.org/10.5772/intechopen.102549*

#### **Figure 5.**

*Compressive strength: (a) after 7 days; and (b) after 28 days [43].*

chloride or sulfate) during wetting/drying cycles. In studies [13, 43] that investigated the absorption of concrete with different proportions of WBA, an increase in absorption with WBA content was observed (an average increase in capillary absorption of up to 2.27% for mixes with 15% WBA content compared to the reference mix). The reason for the correlation between lower compressive strength and lower resistance to capillary absorption is the negative influence of the porous structure on these properties of the concrete [57]. The results of gas permeability showed the same trend as the capillary absorption coefficient: on average, the gas permeability of mortars with a WBA content of 5% decreased by 3.1%, while the cement replacement with 10 and 15% WBA increased by 12.41 and 24.31% compared to the reference mortar [58]. The researchers [59] suggested the addition of silica fume and they found that after 28 days, the gas permeability of mortar samples with 8% WBA and 7.5% silica fume decreased by


#### **Table 2.**

*Recommended WBA properties that to be checked before use in concrete production [60].*

6.6%. Chlorides are one of the main causes of corrosion and deterioration of reinforced concrete structures. Based on the results presented in [43, 58], a decrease in the chloride diffusion coefficient can be seen for all mixtures with fly WBA, except for the sample with one type of WBA, which is related to the WBA particle size.

To make WBA a valuable resource for the construction industry, technical requirements must be established. The purpose of these requirements is to enable concrete producers to ensure consistent quality and predictable behavior of the product without adverse effects on the durability and mechanical properties of the concrete [18]. Therefore, the overall effect of individual physical and chemical properties of the WBAs used on the mechanical properties and durability of cement composites was determined by evaluating the individual effects of the physical and chemical properties relative to the reference mix [58]. This study was carried out to provide concrete producers with a preliminary recommendation on the main WBA properties to be checked during reuse (**Table 2**).
