###### Abstract  
Biomass fly ash is a growing challenge for combustion by-product (CBP) management. This research was conducted to investigate the influence of activation by sieving through a 63 \(\upmu\)m sieve and a 125 \(\upmu\)m sieve on fresh and hardened cement mortar properties. Sieving increased the CaO content by 9.3 percentage points (p.p.) in the oxide composition of the fly ash. The 28-day Strength Activity Indices increased by 24.9 p.p. A 25% replacement rate of cement with fly ash sieved with a 63 \(\upmu\)m sieve increased the 2-day compressive strength of mortars by 24% when compared with untreated fly ash. The 90-day compressive strength results of cement mortars with a 15% replacement rate of cement with fly ash sieved with a 63 \(\upmu\)m sieve were similar to the control specimen results. The utilization rate of biomass fly ash can be increased to 15% of binder mass without the detrimental effect of the mechanical properties of cement mortar. SEM and TG analyses showed that activated biomass fly ash promoted the growth of the C-S-H phase and ettrinegite. fly ash; cement mortars; activation; sieving; biomass Article Assessment of Sieving as a Mean to Increase Utilization Rate of Biomass Fly Ash in Cement-Based Composites  
Footnote †: journal: Applied Sciences  
of fly ash in the range of 10-30% binder mass (b.m.) has resulted in a small increase in the water absorption of concrete [8]. The cement matrix prevents the leaching of heavy metals from biomass fly ash particles [9; 10]. Biomass fly ash can be added as a sand substitute at up to a 30% replacement rate in concrete without reducing compression strength and freeze-thaw resistance [8]. The influence of woody biomass fly ash on mechanical properties can vary widely, which is related to its pozzolanic and hydraulic properties, grain size distribution, and the oxide composition of the additive. A mass replacement of 25% of the binder with fly ash from the combustion of wood biomass may result in a 7-day concrete compressive strength similar to that of the control samples, but from 28 days up to 1 year, modified concrete develops less strength than the control specimens [11]. In another study, up to a 25% replacement rate of biomass fly ash can lead to results similar to the control specimen compressive strength of concrete specimens [12]. Concurrently, the 28-day compressive strength of mortars with up to 20% b.m. of wood biomass fly ash can be 17% higher than that of the control series samples [13]. There is a positive correlation between the amount of reactive lime, reactive alumina with free lime and sulfates, and the compressive strength of mortar, with ettrinigite as the main hydration product [14]. At 25% b.m. replacement of cement with biomass fly ash in the binder, its effect on freeze-thaw resistance was not observed. Chloride ion migration studies performed as part of the same experiment showed that 25% fly ash replacement rate slightly increases the electric conductivity of the specimen [15]. Currently, the EN 450-1 standard prohibits the use of fly ash that was not collected from the combustion or co-combustion of coal [16; 17]. For any potential use in the industry, biomass fly ash could be preprocessed or activated to change certain insufficient properties of that material. The adequate treatment of combustion by-products can increase their usability in other industries and decrease the spaces needed for their landfilling [18]. Sieving, grinding, washing, or drying are potential methods that can modify the properties of fly ash for desirable utilization routes [19]. The chemical activation and modification of the aluminosilicate composition of fly ash-cement systems can be used to increase the efficiency of the fly ash pozzolanic reaction [20]. Washing can be an effective way to remove excess chlorides found in unburnt coal particles of biomass fly ash [21]. The electrodialysis of biomass fly ashes can be used to separate more than 70% of its initial cadmium content [22].