1. Introduction
Biomass is one of the most important renewable energy sources. Burning biomass, which is mostly derived from forests, creates solid waste, like slag and ash, which needs to be managed properly. Both human health and the environment are negatively impacted by fly ash. As biomass fly ash originates from biogenic material, it has a high carbon content and is considered a hard-to-utilize waste [
1]. Wood-based biofuels represent about 9% of worldwide energy production [
2,
3]. With the growing use of biofuel for energy generation, more waste ash is being generated every year. A crucial initial step in solving the world’s waste problem is to replace raw materials with waste and use natural resources sustainably. Biofuel combustion ash can be successfully utilized to produce building materials [
4]. Due to of its chemical, physical, and morphological properties, biofuel fly ash, as a pozzolanic additive, can be used to substitute part of the cement portion in cementitious materials [
5,
6]. As pointed out in the literature, various kinds of biomass fly ash derived from waste are also widely utilized in geopolymer concrete, even considering the various other kinds of biomass fly ash available [
7].
Ash is currently widely used in geopolymers, but it is not sufficient for complete waste utilization and management. However, the utilization of wood biofuel fly ash in geopolymers is not widespread due to the low aluminium content in and amorphous phase of this kind of ash [
8]. Therefore, the utilization of biomass fly ash in cementitious materials is still relevant. Biomass fly ash is mainly used in binder materials, such as Portland cement clinker, in the concrete industry [
8]. The possibility of using the biofuel fly ash used in the concrete industry to reduce the CO
2 footprint is also important. Studies have demonstrated the potential application of biofuel fly ash as a renewable waste material for cement substitution [
9]. Nevertheless, the currently applicable specifications of fly ash exclude biofuel fly ash from being added in concrete because it is not a coal combustion waste. A large number of studies have been conducted on the use of biofuel fly ash (BFA) in concrete or mortar [
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34].
The capacity of BFA to bind with calcium compounds makes it a valuable constituent of cement on par with mineral aggregates. Due to its pozzolanic properties, BFA is used as an active additive to modify the functional properties of a binder [
26]. BFA can be applied in the manufacture of concrete mixtures and prefabricated concrete elements [
33]. For instance, BFA in porous concrete acts as both a binder and a filler. The developed products with improved technical characteristics are produced at lower cost [
23].
BFA incorporated in concrete as a cement substitute may have a significant effect on the fresh and hardened cement paste properties. These properties depend on the burning process, the kind of wood and the pieces of trees turned into biomass, and local operating conditions [
11]. The most frequently examined properties of concrete mixtures containing 5–45% BFA by weight of cement are rheological properties, heat of hydration, 7- and 28-day compressive strength, porosity, and capillary absorption. Gabrijel et al. stated that the replacement of up to 15% of cement with very fine BFA accelerated cement hydration and raised the concrete’s compressive strength by up to 18%, whereas in samples where cement was substituted with coarser BFA, the heat released more gradually, and the compressive strength decreased by 5%. Although almost no changes were observed in capillary absorption values in samples where up to 45% of cement was substituted with BFA, only the minimum mechanical properties of concrete for use in construction were achieved. The rheological properties of concrete were found to improve when biomass fly ash was used as a filler and as a substitute for fine fillers in concrete [
16].
The authors of one study compared mortars modified with BFA and coal combustion waste fly ash and found similar compressive strength values, although their porosity, water absorption, and water vapour permeability were higher [
13]. Mortars wherein more cement is replaced with BFA usually have higher water demand and significantly lower compressive strength [
12,
13,
14,
15,
16,
18,
34]. Ban and Ramli [
22] studied structural concrete wherein certain parts of cement were replaced with BFA and concluded that BFA raised the water requirements and decreased the mechanical strength of the concrete, but it had an insignificant effect on durability and significantly reduced concrete shrinkage. Udoeyo et al. [
21] studied the substitution of biofuel fly ash by up to 30% by weight of cement in concrete and observed a significant drop in mechanical strength (from 9% to 38%), especially in the mixtures containing a high amount of BFA. Other authors claim that low amounts of BFA (added at up to 10% by weight of cement) do not change the mechanical properties of mortars.
A significant effect is observed when 20% of cement is substituted with BFA. The compressive strength of mortars modified with BFA can increase even when a small amount of cement is replaced [
34]. Rajamma et al. reported a positive influence of biofuel fly ash on the rheological characteristics of cement paste. The researchers observed a higher rate of hydration and a shorter setting time [
6]. They observed an increase in both earlier (2- and 7-day) and later (28- and 90-day) strength. It was found that the speed of hydration and the formation of newly created products depend highly on the content of alkali and the W/B ratio in the pastes [
31]. Biofuel fly ash with high quantities of alkaline compounds provides extra alkalinity to cement pastes, accelerates the hydration process, intensifies newly created product formation, and consequently increases the mechanical strength of the specimens [
25].
Biofuel fly ash in a cement mixture lowers the rate of hydration [
29] and thus protects concrete from thermal cracks and stress [
30]. Wang et al. [
5,
13,
24] analysed the impact of the amount of BFA added on the properties of cementitious pastes, the mechanical strength of the specimens, and the durability of concrete. They discovered that adding BFA by weight of cement greatly increased the need for water, prolonged the setting time, increased the amount of air-entraining additive required, and decreased the concrete’s long-term durability. However, the effect on chloride penetration, early mechanical strength, and resistance to cyclic freezing and thawing was insignificant. The authors of [
9] stated that BFA improves the freeze–thaw resistance of concrete.
Wang S. et al. [
5] reported that concrete altered with BFA had the same, or even better, longevity and strength characteristics as ordinary fly ash concrete. Teixeira E. R. et al. [
25] came to a similar conclusion, stating that concrete that contains BFA has carbonisation resistance similar to that of ordinary fly ash concrete. The addition of fly ash allows for an increase in the chloride resistance of concrete [
26] and shrinkage resistance [
27] as well as a reduction in water absorption [
28]. One review [
35] highlighted that the optimal quantity of cement to be substituted depends on the chemical composition of BFA and the quantity of alkali compounds (K
2O and Na
2O). As generally indicated in several studies, the content of K
2O and Na
2O in BFA does not exceed 5% [
4]. There are few studies in which BFA with higher K
2O and Na
2O content (up to 10–13%) was used. In order to develop the guidelines for using BFA in construction concrete, the interdependency of BFA properties and the characteristics of fresh and hardened concrete should be determined [
11]. The data from the reviewed literature leads one to the conclusion that employing BFA as partial substitute for cement in construction materials has the potential to minimize the consumption of natural resources in cement production. BFA is a partially renewable resource: that is why the utilisation of BFA as a partial substitute for cement offers several environmental benefits, including supressing CO
2 emissions, reducing fuel consumption, and facilitating BFA’s valorisation [
36].
Based on the literature review, it can be concluded that the number of available byproducts has been rising recently, and this study’s focus is on the binding properties that can be obtained by substituting cement with byproducts. BFA with a high alkali content, varying from 0 to 30%, was studied in order to highlight the possibility of applying such BFA in concrete mixes. However, the effect of highly alkaline BFA on the properties of fresh and hardened cement paste is still insufficiently studied. This study examines the use of high-alkali BFA as a partial cement replacement and its impact on a number of concrete characteristics, including workability, mechanical properties, porosity, structure, durability, and sustainability. The aim of this study is to determine the potential of high-alkali BFA as a material for producing sustainable concrete and to ascertain when it is appropriate to use BFA in place of cement components to lower the cost of concrete.
2. Materials and Methods
The tests were performed using CEM I 42.5 R, which satisfies all standards specified in EN 197-1 [
37].
Figure 1 illustrates the particle size distribution of Portland cement. The measurements were conducted in the range of 0.10–500 μm. The average size of the particles was 15.05 μm.
BFA collected in electrostatic precipitators of biomass fluidised bed boilers was used to modify fine-grained concrete. To increase the percentage amount of BFA used to replace cement in concrete mix without compromising the properties of the cementitious composite, sieved BFA with a particle size of less than 100 μm was used [
10]. The measurement results are presented in
Figure 2. The sieved BFA particles were approx. 1.5 times bigger than the cement particles. The distribution of particle size was determined using a Cilas 1090 LD device. Water was used as a dispersion medium. The content of solids in the suspension was 12–14%. The particle size of BFA was measured in the interval of 0.04–100 μm. The average particle size was 23 μm.
The properties and chemical composition of BFA are presented in
Table 1 and
Table 2. According to research data [
6], BFA is characterized by large amounts of SiO
2 and CaO and lower amounts of Al
2O
3 and Fe
2O
3. BFA is expected to participate in pozzolanic and hydraulic reactions because it consists of more than 10% CaO [
10,
38]. It should be mentioned that the BFA used in this study was highly alkaline, since the total K
2O and Na
2O oxide content was 8.92.
Sand in a fraction of 0/4 corresponding to LST EN 12620:2003+A1:2008 requirements [
39] was adapted for the tests.
The bulk density of the sand was 1640 kg/m3, and the density of its particles was 2622 kg/m3.
Seven batches of sustainable concrete compositions were prepared. Mixing procedure employed is as follows: dry components, i.e., cement, BFA, and sand, were mixed for 5 min; then, water was added, and the blend mixed again for 5 min.
The batches differed in terms of the content of BFA in the concrete mixture, which ranged from 0% to 30% by weight of cement substituted. BFA was added into the mixture in 0; 5; 10; 15; 20; 25; and 30% portions by weight of cement. Compositions of concrete mixtures are presented in
Table 3.
Using a Bruker X-ray S8 Tiger WD X-ray fluorescence spectrometer, BFA was chemically analysed. Rh target X-ray tube was used, and an anode voltage up to 60 kV and a current (I) up to 130 mA were employed. Pressed samples were measured in a helium environment. The measurements were performed using the SPECTRA Plus QUANT EX-PRESS technique.
The structure of BFA was tested using a scanning electron microscope (SEM JEOL JSM-7600F). The microscopy parameters were as follows: the voltage applied was 10 kV and 20 kV, and the distance to the specimen surface ranged from 7 to 10 mm.
The properties of the specimens tested were determined according to applicable standards: LST EN 12350-5:2019 [
40] for the flow of the mixtures, LST EN 12390-7:2019 [
41] for the density of hardened sustainable concrete, LST EN 12390-3:2019 [
42] for compressive strength, and LST EN 12390-5:2019 [
43] for flexural strength.
Ultrasonic pulse velocity (UPV) test was carried out following the recommendations of standard LST EN 12504-4 [
44].
The durability of sustainable concrete predicted according to freeze–thaw cycles was measured based on porosity parameters. Frost resistance factor KF was calculated based on the assumption that concrete is frost-resistant when the volume of closed pores is higher than the increased volume of frozen water in capillary pores. Frost resistance factor KF was calculated using the following equation:
where
Pu is closed porosity, and
Pa is open porosity.
Using the known value of frost resistance factor
KF, the resistance of concrete to freezing and thawing cycles can be predicted from the function of freeze–thaw resistance and frost resistance factor
KF. Porosities were determined using Scheikins theory [
45].
ASR resistance of sustainable concrete was determined using the RILEM AAR-2 method. The test lasted fifty-six days. The cured samples were kept in 1 M NaOH solution at a temperature of 80 °C for fifty-six days. On day 56, the expansion of the specimens was tested. ASTM C 441 indicates that the expansion limit was 0.1%.