1. Introduction
The concept of a circular economy highlights the minimisation of disposed waste by recycling it in various industrial processes. The recycling of bottom ash from the incineration of municipal solid waste (MSW) in cement-based materials is one of the trends in the development of a circular economy. The use of MSW has a unique potential to support sustainability while preserving natural resources. Greenhouse gas (GHG) emissions in cement manufacturing are very high, accounting for about 8% of the global carbon dioxide (CO
2) emitted [
1,
2]. The cement industry contributes to more than 85% of CO
2 and GHG emissions [
3]. The use of bottom ash can reduce the cement content in concrete and at the same time moderate environmental pollution and CO
2 emissions.
As waste reuse and recycling are more expensive alternatives to landfilling or energy recovery, 17.6% of municipal waste is still landfilled in Lithuania [
4]. However, the landfilling of waste causes social, economic, and environmental problems [
5,
6]. Waste incineration process generates bottom ash (slag) and steam boiler dust (boiler ash), which are classified as non-hazardous waste. Bottom ash has a high potential for reuse in cement-based products as a partial cement replacement, thus reducing the ash content in landfills and the cement content in cement products [
7].
Fly ash generated in various industries [
8,
9,
10] have been used in the production of concrete mixtures for many years [
11,
12,
13]. In the last decade, the use of ash as a fine and/or coarse aggregate in concrete has received considerable attention in various countries in order to widen the range of potential uses of fly ash in the construction sector [
14,
15,
16]. Researchers [
17] tested the possibility of replacing 100% of natural sand in mortars with 0/2 mm MSW bottom ash. The leaching, physical, and chemical properties of the ash were analyzed. The results of the produced cement mortars showed that the compressive strength of the specimens containing ash ranged from 19.4 MPa (without superplasticizer) to 33.3 MPa (with 3% of superplasticizer). Huynh and Ngo [
18] investigated the potential re-utilization of bottom ash as a replacement for crushed sand in cement-based mortars. It was found that the use of ash at higher levels (0%, 25%, 50%, 75%, and 100%) increases porosity, water absorption, and drying shrinkage, and reduces the compressive and flexural strength of the mortar specimens. After 56 days of curing, all mortars showed very good ultrasonic pulse velocity test results, with values ranging from 3483 to 4416 m/s. Authors [
19] described the substitution of sand with 0%, 10%, 20%, and 30% of ash in cementitious mixes. The partial replacement of sand by ash increased the early (3-day) compressive strength of the cement mortar. The optimal results were achieved at 10% and 20% replacement ratios.
Great attention has been paid to the use of bottom ash as a secondary raw material for the production of Portland cement clinker [
20,
21,
22]. One of the main reasons for using MSW bottom ash in the cement clinker manufacturing process is that chemical composition of bottom ash is similar to that of the raw cement clinker materials, including lime, silica, alumina, iron oxide/hematite, and calcium sulphate [
23]. Municipal solid waste incineration ash can partially replace Portland cement in concrete. Zhang et al. [
24] tested the replacement of cement with 15% bottom ash in concrete mixes. The results show that concrete specimens have much better strength and durability values when activated by carbonation. Dry-cast concrete with 20% cement replaced by bottom ash showed 18% higher strength [
25]. Cheng et al. [
26] found that the optimal ash content in concrete was between 10% and 15%. Within this range, the ash not only improves the durability of concrete, but also ensures that concrete strength meets the design requirements. Other researchers [
27] recommend replacing up to 15% of cement with ash to achieve low strength concrete. Although bottom ash has a porous structure, good pozzolanic properties of the ash were observed after grinding and thus bottom ash can be used as supplementary cementitious materials.
The composition of bottom ash depends on the incineration technology and the type of solid waste. The bottom ash consisted of silica, calcium, alumina, and iron [
28]. Aluminium leads to the formation of aluminium hydroxide and high porosity of concrete due to the release of hydrogen in the reaction with water [
29,
30,
31]. MSW incineration bottom ash, when added to the concrete mix directly, causes swelling and cracking due to the reaction between cement and metallic aluminium [
32]. To avoid early age in concrete flooring, steel and polypropylene fibres are added to concrete mixes [
33]. The main function of concrete fibres is to reduce the appearance and propagation of micro-cracks [
34]. There is an increasing use of fibre-reinforced polymer composites as an efficient alternative to traditional construction materials in civil engineering and infrastructure applications [
35,
36]. Therefore, polypropylene fibres (PP), which do not undermine the durability of concrete, were used in concrete mixes tested in this research work.
Considering the long-term prospects for the recycling of MSW ash, there is a clear trend in the replacement of traditional aggregates with bottom ash and fly ash in concrete mixtures. The analysis of the literature about the use of bottom ash in concrete shows that future research needs to consider in great detail the workability, tensile strength, and drying shrinkage characteristics of concrete modified with bottom ash.
The aim of this study is to investigate the effect of bottom ash, generated from a cogeneration plant incinerating municipal solid waste, on the physical and mechanical properties and durability of cement concrete in terms of frost resistance, and to assess the possibility of replacing part of the cement in the mixes with the MSW incineration ash. This concrete can be used for industrial flooring applications.
2. Materials and Methods
2.1. Raw Materials
The following binding materials were used: commercial Portland cement CEM I 42.5 R (containing 95–100% of clinker) produced by AB Akmenės cementas, Naujoji Akmenė (Lithuania); and bottom ash from a cogeneration power plant UAB Gren Lietuva, Klaipėda (Lithuania). The chemical composition of the cement and bottom ash (BA) is given in
Table 1 and the physical and mechanical properties of cement are given in
Table 2. The mineral composition of the cement was as follows: 62.5% of C
3 S, 16.9% of C
2 S, 7.1% of C
3 A, 11.5% of C
4 AF, and 2% of other materials (alkali sulphates and CaO). BA was obtained from the cogeneration plant, where municipal waste is incinerated at a temperature of ~1100 °C. The ash was stored for one year in the Klaipėda regional landfill in Dumpiai under natural conditions. Afterwards, it was dried at 105 °C and ground in a ball mill. BA had the bulk density of 0.91 g/cm
3 and the particle fineness (residual on a 90 μm sieve) of 4.0%. The bottom ash is attributed to Class F, as the total SiO
2 + Al
2O
3 + Fe
2O
3 content exceeds 70%.
0/2 fraction sand with the particle size distribution given in
Figure 1 (EN 933-1) was used as a fine aggregate in the tests. The physical properties of the sand are given in
Table 3.
5/16 fraction granite crushed stone (bulk density 1360 kg/m3) was used as a coarse aggregate in the tests (50%—5/8 fraction and 50%—11/16 fraction).
Polypropylene fibre (PP) Crackstop F6 Adfil NV (Belgium) with a density of 0.905 kg/dm3, equivalent diameter of 0.78 mm, length of 6 mm, and tensile strength of 110 MPa measured according to EN 14889-2 was used to prepare the concrete.
Superplasticiser Master Glenium ACE 560 (SP) MBCC Group, Master Builders Solutions (Poland), was used in the mixes. It is a high-performance liquid admixture based on the new generation of polycarboxylate ether polymers, with a density of 1.06 g/cm3 and pH value 5.6 at T = 20 °C.
Tap water was used to prepare the samples.
2.2. Mix Design and Sample Preparation
The cement content in the concrete mixes varied up to 12%. The maximum ash content was chosen by taking into account the data already presented by other authors [
37], who pointed out that 5% and 10% of ash was the optimal amount to be added to cement mortars. The compositions of concrete mixes with bottom ash are given in
Table 4. It can be seen that the ash to binder ratios of concrete mixes differ. The water to binder ratio (V/B) 0.55 was kept constant for all the concrete mixes with ash tested. The superplasticizer was added at 1% of the cement content.
15 specimens were cast for each concrete mix. Concrete cubes with the dimensions of 100 × 100 × 100 mm were cast in reusable stainless-steel moulds. Concrete pastes were compacted on the laboratory vibrating table (MATEST S.p.A., Arcore, Italy). The formed specimens were stored for one day in covered moulds, then demoulded and cured according to LST EN 12390-2. Three specimens from each batch were used to test the durability, and the physical and mechanical properties of concrete.
2.3. Testing Methods
The compressive strength of concrete specimens was determined according to EN 12390-3 with a hydraulic press ALPHA 3-3000S test machine (FORM+TEST Seidner + Co. GmbH, Riedlingen, Germany). Three specimens of each mix were tested at 7 and 28 days of curing. The density of hardened concrete was determined according to EN 12390-7, the density of compacted fresh concrete was determined according to EN 12350-6, the slump was determined according to EN 12350-2, and the flow was determined according to EN 12350-5. Ultrasonic pulse velocity was determined using a Pundit 7 instrument (converter frequency = 54 kHz) calculating the ultrasonic pulse velocity (UPV, m/s) according to the literature [
38]. To determine the water absorption kinetics [
39], the specimens were dried to a constant mass, weighed, immersed in water, and weighed in air after 10 min, 30 min, 60 min, 24 h and 48 h.
The porosity parameters of concrete were determined by measuring water absorption kinetics according to the methodology presented in GOST 12730.4-78. This method is used to describe the open porosity (
Pa) (capillary pores), the total porosity (
Pt), and the closed porosity (
Pu) (air pores) of concrete. The open porosity was calculated from the following Equation (1):
where
W is water absorption after 48 h, %;
ρ is the density of concrete, g/cm
3,
ρw is the density of water, g/cm
3. The total porosity was calculated from the following Equation (2):
where
ρt is the density of concrete, kg/m
3;
ρp is the specific density of concrete, kg/m
3.
The closed porosity of concrete was calculated from the following Equation (3):
The frost resistance of concrete using the all-sided testing direction was tested using the Rumed 3301 climate chamber (Rubarth Apparate GmbH, Laatzen, Germany) in order to determine the effect of freezing and thawing (FT) cycles on the compressive strength of concrete when different amounts of fly ash were used in concrete mixes. The durability of concrete specimens was determined after 28 days of curing according to the requirements of LST 1428-17. According to the accelerated frost resistance test, water saturated specimens are subjected to cyclic freezing in air and thawing in water. For the accelerated test, the specimens are soaked in 3% aqueous sodium chloride solution (NaCl). The frost resistance class F100 corresponds to the resistance to 20 FT cycles. Prior to the FT test, the saturated specimens are removed from the water bath and placed in such a manner that the water can drain from them. After 2–4 h following the removal of the specimens from the water bath, the control specimens are tested for their initial compressive strength according to LST EN 12390-3. The remaining concrete specimens are placed in the freezing chamber. The specimens are air frozen for at least 2.5 h. The temperature in the centre of the freezing chamber are (−18 ± 2) °C during freezing. After determining the frost resistance, the specimens are removed from the freezing chamber and placed in the bath of aqueous NaCl salt solution at (18 ± 5) °C. The specimens are kept in the bath for at least (2 ± 0.5) h. The all-sided frost resistance of concrete specimens is determined not only by measuring the compressive strength but also by the mass loss. If, after the required number of FT cycles, the loss in compressive strength of concrete specimens does not exceed 5% and/or mass loss of the concrete specimens does not exceed 3%, the concrete is considered to have passed the frost resistance test. Two freeze–thaw cycles per day were carried out with the tested specimens. After 100 cycles, the specimens were crushed and the loss in strength and mass was calculated.
4. Conclusions
The tests conducted in this research work reveal positive prospects for the use of bottom ash in concrete. According to the test results, MSW incineration bottom ash has an effect on physical-mechanical properties and durability in terms of frost resistance of concrete.
6% is the optimal content of bottom ash in concrete according to density and compressive strength values at 28 days. The density of the specimens modified with 6% of bottom ash is 2323 kg/m3, and the average compressive strength is 36.1 MPa. A higher content of bottom ash reduces the slump flow and density of the mix. The compressive strength of concrete specimens after 28 days of curing corresponds to the design strength class C25/30. The quality of the concrete is good, as the average ultrasonic pulse velocity in the specimens is 3912 m/s.
It was found that water absorption of concrete increased significantly up to 4.1% after 24 h. Concrete specimens modified with 6% of bottom ash had a lower water absorption rate of 3.9% after 24 h and 48 h.
In summary it can be stated that concrete modified with 6%, 9%, and 12% of bottom ash has good durability in terms of frost resistance and can be used in construction work where resistance of concrete to 100 freeze–thaw cycles (F100) is required.