**1. Introduction**

Coal fly ashes are valuable raw materials for the building materials industry, and especially for the manufacturing of cement and concrete. As a result of the transformation of the energy industry, new types of ashes are now being produced; these are mixtures of products from simultaneous coal combustion and gas desulphurization processes (ashes from fluidized bed boilers). They often contain high amounts of SO3 and CaO from unburned coal. For these reasons, they are sometimes treated as waste that is unsuitable for use in traditional cement production technologies. It is therefore necessary to look for other applications for these materials, one of which may be the synthesis of geopolymers.

Due to its significant content of silicon and aluminum, fly ash is an attractive material for use in the synthesis of geopolymers [1,2]. The traditional precursor, based on metakaolin, can be effectively replaced by fly ash of type F. The usefulness of fly ash in the synthesis of geopolymers is determined by its content of silicon and aluminum, since the Si/Al ratio determines the formation of a desirable type of zeolite. According to Tanaka et al. [3], a ratio of 0.9 gives a material that can be identified as a single-phase Na–A zeolite. This material is also produced at a lower rate at a ratio of around 1.7, and its crystallinity increases sharply at a ratio of 4.3.

Many studies have proved the superb strength properties of fly ash-based GPC (geopolymer cement) [4–6], which are comparable to those of OPC (ordinary Portland cement). They also provide good durability, and typically have better sulfate [7] and acid resistance [4,8], and excellent fire resistance [9]. Since it is cured at high temperatures, GPC has acceptably low shrinkage [10].

Another factor that may affect the properties of this material is the presence of calcium. In early studies, a higher amount of calcium (above 20%) was recognized as a contaminant,

**Citation:** Krzywo ´n, R.; Dawczy ´nski, S. Strength Parameters of Foamed Geopolymer Reinforced with GFRP Mesh. *Materials* **2021**, *14*, 689. https://doi.org/10.3390/ma14030689

Academic Editor: Hubert Rahier Received: 29 December 2020 Accepted: 29 January 2021 Published: 2 February 2021

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which caused a decrease in strength [11] and reduced the rate of geopolymer synthesis [12], and which produced various hydrates [13,14]. Recent studies, however, have highlighted other features with a beneficial effect on the properties of fresh mixture and hardened geopolymer [15]. The simultaneous formation of calcium silicate hydrate compounds in addition to aluminosilicate products under certain conditions improves the strength properties [16]; one example would be synthesis at ambient temperature [17,18]. Although thermal curing can still improve the properties of lignite coal fly ash [19,20], its properties are still worse than those of low-calcium Class F geopolymer [18].

The entrapment of air in the structure of a foamed geopolymer improves its thermal and sound insulating properties [21–23]. According to Cui et al. [24], foamed geopolymers may have better thermal and mechanical properties than foamed OPC. The lower density means that the weight of the structure is reduced, which allows for an additional reserve of load capacity. Another important benefit is the increase in compressive strength after exposure to elevated temperatures, a phenomenon that can be explained as an effect of geopolymer polycondensation and sintering at high temperatures [25]. Furthermore, the pores provide space to counteract the damage by heat [26]. Geopolymer foams can resist temperatures of up to 1000 ◦C without decomposition, and their shapes do not suffer dimensional changes within a temperature range of 400–800 ◦C [27]. These features mean that foamed geopolymers are very suitable for industrial applications as refractories, including furnace insulation and chimney cladding [28].

The main negative effect of foaming geopolymers is the reduction in the strength parameters. This problem affects most lightweight materials, and results from the disturbance in the homogeneous structure due to the presence of pores [29,30]. The relationship between the abovementioned advantages of foaming and strength reduction always involves a compromise, and the optimum parameters can be found by the proper addition of a foaming agent. Among the most common foaming techniques are the entrapment of air during mixing with surfactants (organic and inorganic technical foams) [31], and chemical foaming by the addition of peroxides such as hydrogen peroxide [32], sodium perborate [23], sodium hypochlorite [33] and the alkaline oxidation of metals, such as zinc, metallic silicon or aluminum [22,34,35].

Foamed geopolymers are brittle, and the ratio of the tensile to compressive strength, although better than for OPC concretes, is still very unfavorable. One remedy for this disadvantage may be the use of reinforcement. The most frequently studied method of reinforcing a geopolymer is the introduction of dispersed fibers to its structure, and various types of fibers have been investigated, including natural fibers (e.g., hemp [36], abaca [37]), and both organic (e.g., PVA [38], PP [39]) and inorganic high-strength fibers (e.g., glass [40], basalt [35,41] and carbon [42]). The most important limitation on the efficiency of reinforcement of foamed materials is the bond. In solid concrete, a suitable bond is ensured by chemical adhesion between the cement paste and steel, as well as the mechanical interlocking between the ribbed surface of the rebar and aggregate particles. The contact surface in porous materials is limited by the presence of voids, and flat fiber reinforcement does not usually provide interlocking conditions. The low thickness of the walls separating the pores and the brittleness of cellular concrete causes crushing of the local contact with the rebar rib. Although there has been no research on the reinforcement of foamed geopolymers with steel rebars, a study of this type of reinforcement in foamed OPC concrete shows that at a density of 1200 kg/m3, the strength of the bond is reduced by a factor of eight [43]. According to the authors of this study, the effectiveness of the bond in foamed concrete can be successfully improved by expanding the contact zone. In practice, this could be realized through the application of textile [44] or mesh reinforcements [45,46], in which a perpendicular thread provides anchorage for the fibers in the direction of the internal forces. This concept has been applied for many years to thin-walled concrete structures; it is called ferrocement, and was developed five years before the reinforced concrete.

In this paper, we develop the concept of reinforcing discussed above. Tests were carried out on 18 sets of prisms made of foamed fly-ash-based-geopolymer, reinforced with glass fiber mesh. The aim was to assess the impact of the bulk density on the mechanical properties of the foamed geopolymer and the flexural behavior of beams strengthened with GFRP (glass-fiber-reinforced polymer). The parameters investigated here included changes in the content of foaming agent (1%, 2%, and 3%), the origin of the fly ash, and localization of the reinforcement (none, external, and internal).

#### **2. Experimental Program**

Our research program was divided into two parts. The first involved material research, and the goal was to determine the mechanical properties of the geopolymers by carrying out testing on cylinders and beams. To assess the compressive and flexural strength, prismatic beams (40 × <sup>40</sup> × 160 mm3) were cast according to EN 196-1:2016 [47], and for each mixture, three cylinders (60 × 120 mm2) were simultaneously made to test the modulus of elasticity and cylindrical compressive strength (Figure 1a,b).

**Figure 1.** Dimensions of the sample and location of reinforcement: (**a**) beam with internal reinforcement; (**b**) beam with bottom reinforcement; (**c**) cylinder with strain sensors; (**d**) beam with bottom reinforcement. Unit: mm.

Using the same mixture, glass-mesh-reinforced beams were made for bending resistance tests. Figure 1c,d show the dimensions of the beams and the location of the reinforcing mesh. The meshes were arranged so that each beam contained eight bundles of fibers.

The applications of glass fiber meshes in geopolymers have not yet been studied. Based on an analogy to OPC, there may be a danger that the penetration of alkaline solutions can severely damage glass fiber in terms of a loss of toughness and strength, and embrittlement [48] (p. 160). On the other hand, fiberglass has a beneficial chemical composition [49], and most studies show that dispersed glass fibers are effective in increasing the strength of a geopolymer without giving rise to embrittlement problems [50,51]. To ensure resistance to alkalis, a fiberglass lathing mesh was used. This was originally designed for cement plaster reinforcement, and is based on C-glass fiber impregnated with an alkaliresistant dispersion. The mesh size was 4.5 × 5 mm2, and the weight was 160 g/m2. The breaking strength was 25 kN/m.

#### *2.1. Materials and Methods*

The foamed geopolymers tested here were synthesized from fly ash, drawn from the three largest coal-fired power plants in Poland: the Jaworzno power plant, which is fired with anthracite coal, and the Belchatow and Turow power plants, which are fired with lignite coal. The chemical compositions of the fly ash materials were identified by X-ray photoelectron spectroscopy (Thermo Scientific™, Waltham, MA, USA) (using the K-Alpha™ X-ray Photoelectron Spectrometer XPS System), and are shown in Table 1.


**Table 1.** Chemical composition of fly ashes (mass %).

The densities of the coal fly ashes were determined based on the EN 1097-6 [52] standard, and are presented in Table 2. The results indicate that fly ashes from the incineration of lignite coal have a higher density than those from anthracite coal.

**Table 2.** Densities of the fly ashes (kg/m3).


An activator based on a sodium silicate solution (Na2O 8.6%, SiO2 27.8%, water 63.2%) and 10 M sodium hydroxide was used. The components of the activator were mixed before use. The mass ratio of fly ash/sodium silicate/sodium hydroxide was constant for all mixtures, and was equal to 3/1.5/1 (Table 3). The mixture proportions were optimized in the strength tests of samples based on a not-foamed geopolymers.


**Table 3.** Composition of foamed geopolymer mixtures.

The geopolymers were foamed by the addition of a 30% solution of hydrogen peroxide. To vary the densities of the geopolymers, three different concentrations of hydrogen peroxide with respect to the precursor were used: 1%, 2%, and 3% of the total weight ratio. In an alkaline environment, hydrogen peroxide decomposes into water and oxygen [53]; the introduction of oxygen into the mixture causes it to grow in volume. The advantage of using hydrogen peroxide for foaming is the generally homogeneous distribution of the macro-pores [29] and the increase in viscosity of the geopolymer paste [54]. However, there are also disadvantages, such as a reduction in the rate of the geopolymerization process [29]. The growing process is restricted by the hardening of the sample, which makes it relatively difficult to predict the final volume.

The same production conditions were used for all samples. The mixing procedure was as follows: the alkaline activator components were mixed for two minutes, while the precursor was simultaneously poured into the mixing bowl. After adding the activator, the components were mixed for three minutes at a constant stirrer speed of 100 RPM. Finally, hydrogen peroxide was added, and the materials were mixed for a further minute. Immediately after mixing, the samples were poured into molds containing fixed reinforcing meshes. After 10 min of growing, the excess geopolymer paste was removed, and the molds were sealed and cured in a heat chamber at 40 ◦C for 24 h. All the specimens were then de-molded and kept at room conditions until the test.

A total of 27 types of beams were cast, each set consisting of six identical beams made from the same mix. In addition, three cylinders were made for each mix, in order to determine the strength properties.

The following system is used here to describe the types of samples. The first letter represents the origin of the ash (J-Jaworzno, T-Turow and B-Belchatow), and the content of the foaming agent is then specified (1%, 2%, and 3%). In the case of the prismatic samples, the last two letters indicate the type of reinforcement (no-pure geopolymer (Figure 1a), in–internal mesh (Figure 1c), and bt-mesh at the bottom (Figure 1d)). For example, J\_2%\_in represents a beam sample made of fly ash obtained from the Jaworzno Power Plant and reinforced with an internal mesh.
