Fly ash is a by-product constituting about 60–88% of the total combustion residues from coal-fired electric generating plants and is recognized as an environmental pollutant. Since the increase of energy production facilities due to rising energy demands in developing countries, FA is rapidly becoming a very serious environmental problem. From the environmental perspective, the recycling of FA is as important as energy production and thus, the disposal of this by-product has long attracted the attention of researchers (1, 2). Commonly used disposal methods have included depositing the FA in landfills or just dumping it as waste. Disposal of solid wastes in landfills requires more storage space and also causes additional carbon emissions, leading to serious environmental issues. Consequently, this method has recently become less attractive (1, 3, 4). Over the past few decades, various approaches have been studied to find the best solution for FA utilization.

Research carried out on the reuse of FA as an alternative to landfill disposal covers a wide number of fields, ranging from agriculture to engineering. Bicer (5) used FA as a sand replacement in concrete and plaster and investigated the impact of FA grain size on the thermal and mechanical performance of the composite material. Cheng et al. (6) used isotropically consolidated drained (CID) triaxial tests to examine the stress-dependent behavior of marine clay admixed with fly ash-blended cement (FAC). Martin et al. (7) studied the effect of FA on the hydration of calcium sulfoaluminate cement. Güllü et al. (8) experimentally investigated the rheological capability of geopolymer material for grouting in comparison with grouts including FA and geopolymer aggregate. Duan et al. (9) examined the relevance of ultrafine FA properties and mechanical properties in fly ash-cement gelation blocks via static pressure forming. Ren and Sancaktar (10) studied the use of FA as a filler in synthetic rubber for tire applications. Zhang et al. (11) investigated the flexural and ductile failure of carbonized concrete containing different proportions of FA and active magnesium oxide. Sun and Vollpracht (12) presented a study on the time-dependent behavior of sodium silicate-activated FA and metakaolin geopolymers. Wang and Lu (13) studied the resistivity of FA concrete and the rebar corrosion rate in FA-substituted concrete. Hadi et al. (14) examined the properties of FA-based geopolymer mortar. Hefni et al. (15) sought to solve the low early strength problem via chemical activation of Class F fly ash. Bahedh and Jaafar (16) studied some of the properties of ultra-high-performance concrete produced by utilizing FA via autoclaving. Gökçe et al. (17) reported on the properties of foam concrete containing FA. Uysal et al. (18) studied geopolymer paste composed of FA, ground glass fiber, and glass powder. Atis et al. (19) investigated the use of a non-standard high-calcium FA in concrete. Irassar and Batic (20) studied the sulfate resistance of ordinary Portland cement with FA. Fernández-Jiménez et al. (21) experimentally investigated the effect of the NaOH, KOH, Na₂CO₃ K₂CO₃, sodium silicate, and potassium silicate as alkaline activators in the microstructural development of thermal-alkali activated FA systems. Woszuk et al. (22) used Class F and Class C fly ash as a partial replacement for the filler in asphalt. Yan et al. (23) utilized FA from the municipal solid waste incinerator as a replacement for the traditional limestone mineral filler in bitumen mortar and found that the FA improved the high-temperature performance of the asphalt binder.

The recycling/utilization of FA has a number of environmental benefits, including reduced landfill disposal, reduced consumption of virgin resources, and reduced greenhouse gas emissions (24). Direct utilization is among the desirable methods for the recycling of hazardous industrial wastes like FA, and since direct utilization of FA is low in cost, it has become a prominent strategy for industrial companies. Fly ash utilization also reduces the need for additional landfill space and conserves natural resources (2). Experiences gained in recycling indicate that recycling wastes through incorporation of them into useful products is possible; however, the actions necessary to reach industrial-scale solutions for recycling of these wastes via effective technologies are very important. Reducing waste using mechanical, thermal, and chemical approaches is increasingly being considered by various industrial sectors (25).

Glass fiber-reinforced polyester (GRP) composite pipes are used in a number of civil engineering applications such as transporting clean and potable water, water for irrigation, for hydroelectric power plants, and for water treatment, water storage, and sewer systems, for storm water and seawater intake and discharge, and for trenchless technologies. Due to this wide area of application, the direct utilization by the GRP pipe sector of a large amount of by-products can be regarded as an industrial-scale solution (26, 27).

In the experimental stage, GRP pipes were produced using silica sand as a filling material, glass fiber as a reinforcing material, cobalt octoate as an accelerator, methyl-ethyl ketone peroxide as an initiator, and orthophthalic unsaturated polyester resin (with the characteristic of thermoset) as a binder. Two types of resins were used in the production of the inner and outer layers of the GRP pipes. The liner resin, having high chemical resistance, was used in the first 1-mm thickness of the inner surface. The orthophthalic polyester resin was used to produce the other layers of the GRP pipes. Properties of the resins used in this study are presented in Table 1.

E-glass fiber with a length of ~25‒50 mm was used as a reinforcing material. In the production of the GRP pipes, silica sand was used as a filler material to increase the rigidity. Table 2 summarizes the properties of the silica sand (maximum size, 0.85 mm).

The FA, a by-product obtained from the Çatalağzı thermal power plant (in Turkey), was substituted for silica sand in the mixtures by amounts of 10% and 20% to form the structural layers of the GRP pipes. The FA used in the study contained the following oxides: SiO₂ (55.16%), Fe₂O₃ (5.70%), TiO₂ (1.11%), Al₂O₃ (26.79%), CaO (1.91%), MgO (1.15%), Na₂O (0.43%), K₂O (4.94%), SO₃ (0.001%) and P₂O₅ (0.24%). Prior to the production process, the silica sand and FA were mixed (135 kg silica sand + 15 kg FA for the 10% FA substitution and 120 kg silica sand + 30 kg FA for the 20% FA substitution) by mechanical mixing methods in a large container and placed into the bulk trailer of the centrifugal casting (CC) system. Three types of GRP composite pipes were produced in this study. One of them was the reference pipe (REF) produced without FA, while the others contained 10% and 20% FA. The GRP pipes were produced on a one-to-one scale under PN 6 bar pressure class and SN 5000 N/m² stiffness class. All pipes were manufactured by the CC method operating with a PLC-PC fully automatic controlled system (Figure 1a). In the production of the pipes, the materials were fed to the rotating mold via a feeder (Figure 1b). Initially, the mold was rotated relatively slowly. When the feeding of all the raw materials was completed, the cast rotation speed was increased in order to achieve adequate compression. By increasing the rotation speed of the mold, full compression and transition to a solid material (curing) were ensured. The wall thickness of the pipe was manufactured layer-by-layer by the reciprocating motion of the feeder within the mold. Glass fibers, acting as reinforcing material, were placed on both sides of the neutral axis on the pipe wall, and the gap remaining in-between was filled with silica sand + FA, resin, and glass fiber. After production using the CC method, the pipes were removed from the rotating mold. The GRP pipe samples manufactured with FA are shown in Figure 1c.

The resin was designed to be non-polymerized during the pipe production. For the pipe production, the fibers used were cut in lengths of 25‒50 mm. The 10% FA-substituted GRP pipe produced in this study is given in Figure 2 with cross-sectional details. In the cross-section, the external surface protective layer (numbered as 1) and the body core of the structural layer (numbered as 5 and 6) were producing using the FA- silica sand admixture.

After producing the pipes, test samples were prepared according to the ISO 7685 standard for SM, ISO 8521 standard for HTS, and ISO 8513 standard for ATS. Details of all tests applied to the GRP pipes are given in Table 3.

After preparing the samples detailed in Table 5, HTS (Figure 3a), ATS (Figure 3b) and SM (Figure 3c) tests were performed on the produced pipes according to the related standards. Some dimensional details of test samples are also given in Figure 3d-e.

By using the ultimate forces (F) obtained from the test machine, the HTS and ATS were calculated using Equations [1] and [2], respectively.

To calculate the stiffness modulus [S_m], Equation [3] was used. The parameter f in Equation [3] can be calculated using Equation [4].

f: Deflection coefficient

y: Deflection of the ring at maximum force (m)

F: Force (N) used to give the test specimen a deflection of 2.5 – 3.5%, based on mean diameter

L: Average ring length (m)

Since natural raw material resources are limited, the use of recycled materials in industrial production is very important for sustainability. Various types of recycled materials are currently being used in many branches of engineering. Civil engineering is one of the leading engineering fields in the utilization of large amounts of solid wastes. According to this experimental investigation carried out to evaluate the potential reusability of FA in the production of GRP pipes, the following conclusions can be drawn:

All findings of this experimental study indicate that the use of FA in GRP pipe production is possible. Low-lime FA (Class F, having CaO<10% according to ASTM C618) was used in this study. Because of its high pozzolanic activity, low-lime FA is commonly used rather than high-lime FA (Class C, having CaO>10% according to ASTM C618). It is recommended that future research should focus on the potential use of high-lime FA, which has a relatively low pozzolanic activity, in GRP pipe production.