1 Introduction

With the development of modern civilization, man began to emit increasing amounts of carbon dioxide (\(\hbox {CO}_{2})\) into the atmosphere. It has a negative consequence for nature and man. The cement industry produces a significant amount of \(\hbox {CO}_{2}\). It is estimated that this industry is responsible for approximately 3.5% of \(\hbox {CO}_{2}\) emissions related to human activities [1, 2]. The search for alternatives to Portland cement that will reduce the environmental impact of the cement industry is crucial for the implementation, sustainable development, and circular economy. One of the alternatives is geopolymers. The production of geopolymers will generate less \(\hbox {CO}_{2}\) emissions and is also a real alternative to waste management from the energy or metallurgical sector (e.g., fly ash, slag) [3,4,5].

Geopolymers are a class of synthetic inorganic aluminosilicate materials typically formed by reacting aluminosilicate (e.g., fly ash, metakaolin) with a silicate solution under a highly alkaline environment or sometimes in acidic conditions [6, 7]. Products based on geopolymers are characterized by very good properties such as: compressive strength, thermal stability, acid resistance, and fire resistance or dimensional stability [8, 9]. This makes it possible to use them in construction, to immobilize toxic and hazardous waste, for example asbestos or radioactive waste, and to use the material as refractory coatings in the aviation industry [9, 10]. However, the relatively low fracture toughness limits the use of these materials in many areas. For this reason, the topic of strengthening geopolymers with the use of fibers is developing more and more dynamically [3, 8].

The addition of fibers may improve the mechanical properties of the obtained material and change the fracture nature from brittle to more ductile, which allows for new applications for the composite [8, 11]. Reinforcing geopolymers with fibers gives the opportunity to improve the bending strength and increases the amount of energy absorbed by the material before damage occurs [12, 13]. The addition of fibers reduces the number of cracks and their dimensions in the material, which in turn minimizes the damage caused by cracking and keeps the material cohesive for a longer period of time under the given load. Such behavior is important, especially in emergency situations, such as fires or earthquakes, in which a person has more time to leave the endangered place. For this reason, reinforcing geopolymer matrices with fibers is a very interesting design solution that stands out from other currently available on the market [14, 15]. As a reinforcement for geopolymer composites, different types of fiber are used, artificial fiber such as steel or polymer fibers [8, 16] as well as natural [3, 17]. The main advantages of natural fibers include their availability, low price, low density, renewal, and relatively high strength properties. For this reason, natural fibers can be an attractive and environmentally friendly alternative to synthetic fibers used as reinforcement in composites [18, 19]. However, these fibers are not without their drawbacks, such as low durability, poor interaction of the fibers with the warp, poor resistance to moisture absorption, low-dimensional stability, and high fiber content, which reduces the workability of the fresh composite [19, 20]. For this reason, some scientists are focusing on improving the compatibility of natural fibers with a geopolymer matrix by modifying their surface, for example, using chemical treatment [21].

Among the geopolymers distinguished are geopolymer materials intended for use as lightweight materials. The production of light building materials entails a number of benefits, such as reduction of the load-bearing capacity of the structure, better thermal and acoustic insulation of buildings, and reduction of transport and assembly costs [22, 23]. Structural elements made of them may have reduced cross-sectional dimensions and at the same time high durability, allowing reducing the occupied storage space. Many lightweight concrete structures are built in seismically active areas. In this case, the advantages of using these materials are: their low density, high fatigue strength, and resistance to dynamic loads [14, 22]. Concrete and geopolymer density can be reduced using lightweight and porous aggregates [23, 24]. Material is considered light when its dry density is within the range (800–2000) kg/\(\hbox {m}^{3}\) [24, 25]. The use of lightweight geopolymers as insulation materials appears promising due to their complete non-flammability and excellent strength, but the main limitations are their complex manufacturing process and the lack of stability of the geopolymer foams, as well as difficulties in achieving such good insulation properties possessed by polyurethane foams, polystyrene, and wool [26].

Other challenges for foamed geopolymers are the new areas of applications such as corrosive environments. The increased durability of this material in salt and fresh water allows it to be applied in new directions, such as filters, sorbents, or artificial reefs [27,28,29,30]. Research shows that this material could be applied to protect against corrosion of the marine infrastructure [27, 31]. Challenges include corrosion and deterioration of materials. Regardless of their resistance to corrosion, materials can be damaged due to environmental factors [31, 32]. Additionally, the presence of water and air pollution, such as soluble salts from greenhouse acid gases, sulfur oxide, nitric oxide and carbon oxide, can accelerate the corrosion and trough that weakened structural barriers as well as accelerating the destruction process [33, 34].

The main motivation of the research was the development of new foamed composites based on a geopolymer matrix and the investigation of its durability, especially against degradation in corrosive water environments. Currently, the main challenge for these foamed geopolymers is to investigate their durability. It requires standards definitions and appropriate data from long-term research. This will define the possibilities of its applications in more advanced products [26, 35].

2 Research methodology

2.1 Materials

Fly ash, FA, was used as a waste source of aluminosilicate—delivered from Skawina Combined Heat and Power Plant (located in Skawina, Lesser Poland, Poland). The oxygen composition was typical for class F fly ash [7, 36]. The type of fly ash used in this investigation consists mainly of silica and aluminum oxide and contains less than 4% calcium oxide [36]. The metakaolin, MK, was delivered from the Czech Republic [14, 15]. Both materials were mixed in the ratio 1:1 with silica sand [7]. For reinforcement composites, flax fibers were applied. Flax fibers were supplied by the Institute of Natural Fibers and Herbal Plants in Poznan [14, 15].

2.2 Sample preparation

Samples were prepared using sodium promoter, fly ash or metakaolin, silica sand (ratio: 1:1) and approximately 1% by mass of short flax fibers. To prepare 10 M alkaline solutions, technical sodium hydroxide in the form of flakes and an aqueous solution of sodium silicate—sodium glass R-145 with a molar module of 2.5 and density of about 1.45 \(\hbox {g}/\hbox {cm}^{3}\) and tap water, were used. First, the drying components were mixed with a laboratory mixer (LMB-s; standard mixer according to PN-EN 196) for about 5 min. Next, the alkaline solution was added and the mixing process was continued for about 15 min. To obtain foamed material, hydrogen peroxide \(\hbox {H}_{2}\hbox {O}_{2}\) 35% was used as a foaming agent. It was applied in the last stage of mixing in the amount of 2% by weight in relation to the weight of fly ash or metakaolin. After receiving a homogeneous paste, the mixture was molded. Molded geopolymer concretes were heated in a laboratory dryer for 24 h at \(75^{\circ }\hbox {C}\) in the atmospheric pressure. The two types of elements were prepared: bricks (solid material) and elevation plates (two layers—solid and foamed)—Fig. 1. The brick size was 25 cm \(\times \) 12 cm \(\times \) 5 cm, and the plate size was 40 cm \(\times \) 24 cm \(\times \) 5 cm.

Fig. 1
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Types of elements a a brick made from fly ash, b a brick made from metakaolin, c an elevation plate made from fly ash, d an elevation plate made from metakaolin [15]

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Initially, the samples were stored in laboratory conditions for 270 days (\(20^{\circ }\hbox {C}\) and 60% humidity). After 270 days, both products (bricks and elevation plates) were used to create the prototype wall (Fig. 2). It was tested for the next 90 days in a relevant environment. The wall was placed on the area of the Faculty of Materials Engineering and Physics of the Cracow University of Technology (Jana Pawła II Street, Cracow, Poland). The construction was regularly inspected for efflorescence, cracks, and other types of damage under changing weather conditions (with a temperature between − 5 and \(+\,20^{\circ }\hbox {C}\) and various humidity—open air).

Fig. 2
figure 2

The prototype wall during testing in the relevant environment

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Then it was demolished, and the chosen parts were investigated according to mechanical properties. Four groups of composites were chosen for the durability test (Table 1.).

Then, the samples for flexural and compressive strength test were prepared (Fig. 3). The samples have been cut from large plates (previously prepared wall elements).

Fig. 3
figure 3

Samples for compressive strength test a foamed composite based on metakaolin, b solid composite based on metakaolin, c foamed composite based on fly ash, d solid composite based on fly ash

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2.3 Methods

Compressive strength tests of geopolymer samples were performed 360 days after preparation. The samples have been cut from large plates previously prepared (wall elements). The test was repeated for the selected material composition also after the water absorption investigation. Compressive strength tests were carried out with the use of the Matest 3000 kN test machine according to EN 12390-3. The tests were carried out at a speed of 0.5 MPa/s. The tests involved cubic samples: 50 \(\times \) 50 \(\times \) 50 mm. Each time min. 5 samples were tested.

Table 1 Composition of the samples
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Flexural strength tests of geopolymer samples were carried out 360 days after preparation. The samples have been cut from large plates previously prepared (wall elements). The flexural strength tests were carried out with the use of the Matest 3000 kN testing machine according to EN 12390-5. The tests were carried out at a speed of 0.05 MPa/s. The tests involved prismatic samples: 50 \(\times \) 50 \(\times \) 200 mm (space between supporting points 150 mm). Each time min. 3 samples were tested.

Porosity measurements were made using a Quantachrome Instruments Poremaster mercury porosimeter. Research was carried out on four samples of solid and foamed geopolymers (in case of foamed materials only the foamed part was investigated), each of them was subjected to a high-pressure and low-pressure test. The maximum value during the low-pressure test is about 50 PSI, and during the high-pressure test this value is about 35,000 PSI. The difference is also the volume of mercury injected into the sample. This volume is approximately 0.005 \(\hbox {cm}^{3}\) for the low-pressure test, where the mercury only reached the most accessible spaces. In case of high pressure, the volume is about 0.5 \(\hbox {cm}^{3}\). In the high-pressure test, mercury could penetrate the less accessible pores; hence the amount of mercury injected is larger. Mercury porosimeters allow for pore measurement with a size of about 250–0.003 \(\upmu \hbox {m}\).

Water absorption was made using fresh (distillate) and salty water. The samples have been cut from large plates previously prepared (wall elements). The samples were placed in water for 12 weeks. The measurements of the weight and dimensions were made every week. The samples were then dried in laboratory conditions for 2 weeks. Weight stabilization occurred after 1 week.

3 Results and discussion

3.1 Mechanical properties

The results of the compressive and flexural strength tests are presented in Table 2.

Table 2 The results of compressive and flexural strength tests
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The highest values for mechanical properties are for the geopolymer composites based on fly ash (FAS). It is above 75 MPa for compressive strength and above 8 MPa for flexural strength. The values allow this material to be applied for building purposes, even for demanding applications. The obtained results for the geopolymer composites based on metakaolin (MKS) are also sufficient for application this material for construction purpose. The composite has almost 43 MPa for compressive strength and more than 7 MPa for flexural strength.

The obtained values for foamed materials are much lower than tor solid one. This behavior is in the line with the other in results presented in the literature [26]. The mechanical properties are correlated with the porosity of the material (their density). In the case of foamed materials, the results obtained were quite similar for both composition. The compressive strength of the metakaolin-based geopolymer composites (MKF) was slightly better than that of the fly ash-based geopolymer composites (FAF). The values were 6.8 MPa and 5.9 MPa, respectively. The flexural strength was higher for the foamed composite based on fly ash (1.6 MPa) than for the composite based on metakaolin (0.9MPa).

In comparison with other geopolymer composites based on fly ash, the received value for compressive and flexural strength is above the average values [8]. The higher value of flexural strength is an effect of fiber addition [14, 15]. The results achieved for solid metakaolin-based geopolymer are typical for this kind of composition reinforced by fiber [3, 6, 8, 37]. The large difference between fly ash and metakaolin-based solid geopolymers according to the compressive strength is quite surprising. Significant differences are observed between foamed and solid materials. However, the results achieved for the foamed geopolymers are very good comparison with other foamed materials and above the average comparison with other foamed geopolymers [26, 38, 39]. Nevertheless, the results have been achieved for the mix structure solid and foamed used investigated as a whole element of elevation plate. In the case of foamed samples, the values for fly ash and metakaolin-based geopolymers are quite similar. The better results in compressive strength are for samples based on metakaolin, almost 7 MPa compared to about 6 MPa for fly ash samples. The reverse situation is for flexural strength, where the fly ash-based geopolymers obtained better results, approximately 1.5 MPa, and 1 MPa, respectively.

3.2 Porosity measurements

The devices used allow information to be obtained about different kinds of porosity, including total interparticle porosity and total intraparticle porosity. To obtain this information, two different measurements are required in low- and in high-pressure conditions. The summation of these values provides information about the total porosity for the investigated samples. Additionally, on the basis of this measurement, the theoretical porosity of the material is calculated. The results of the porosity measurements are presented in Table 3.

Table 3 The results of porosity measurements
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The measurements compare the values received for the high and low pressure testing. The high-pressure testing allows to obtain information about the intraparticle porosity. The information obtained depends on the characteristics of the material, especially the characteristics of the pores. In the case of closed pores, some information about porosity can be missing. Providing tests in low and high pressure gives the best results. The results show the theoretical porosity of the foamed materials as about 75% for fly ash geopolymers and 82% for metakaolin-based geopolymers. In the case of solid materials, the value for the fly ash-based geopolymer is almost 53% and for the metakaolin-based 67%. This difference can explain the differences in the results of compressive strength tests. The literature shows that the mechanical properties are strictly correlated with the material density and porosity [40, 41].

Figure 4 shows the relationship between normalized volume (\(\hbox {cm}^{3}\)/g) and pore size from high- and low-pressure tests. It gives an information about pores size distribution in the materials.

Fig. 4
figure 4figure 4figure 4

The relationship between normalized volume and pore size of: a foamed fly ash-based geopolymer in low pressure, b foamed fly ash-based geopolymer in high pressure, c foamed metakaolin-based geopolymer in low pressure, d foamed metakaolin-based geopolymer in high pressure, e solid fly ash-based geopolymer in low pressure, f solid fly ash-based geopolymer in high pressure, g solid metakaolin-based geopolymer in low pressure, h solid metakaolin-based geopolymer in high pressure

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Figure 4 shows the relationship between the normalized volume (cm\(^{3}\)/g) and the pore size. The graph 4a and 4b show measurements from a sample FAF from high- and low-pressure testing. During the low-pressure test, we may observe that pores ranging in size from 100 to 200 \(\upmu \hbox {m}\) are the dominant faction. The volume of the pores is about 0.1 \(\hbox {cm}^{3}/\hbox {g}\). The high- pressure test also shows the large amount of smaller pours—between 2 and 5 \(\upmu \hbox {m}\). The volume of the pores is greater than 0.08 \(\hbox {cm}^{3}/\hbox {g}\).

Figure 4c and d shows measurements from a sample MKF from high- and low-pressure testing. The measurement in low pressure shows the different pores distribution for metakaolin-based material than for fly ash. For the metakaolin-based formed composition, there is lack of one dominated fraction of pores. A lot of pores is in the following ranges: between 5 and 6 \(\upmu \hbox {m}\), between 10 \(\upmu \hbox {m}\) and 20, and between 100 and 200 \(\upmu \hbox {m}\). They volumes are approximately: 0.4 cm\(^{3}\)/ g, 0.38 cm\(^{3}\)/g, and 0.3 \(\hbox {cm}^{3}/\hbox {g}\), respectively. In the case of high-pressure measurements, the dominated fraction is 1 \(\upmu \hbox {m}\) and 3 \(\upmu \hbox {m}\) with a volume of approximately 0.18 \(\hbox {cm}^{3}/\hbox {g}\).

Figure 4e and f shows measurements from a sample FAS from high- and low-pressure testing. The low-pressure measurements show a much lower volume of pores in the material, and this is in accordance with expectations. The relatively large number of pores in the range of 100 \(\upmu \hbox {m}\) and 200 \(\upmu \hbox {m}\), but their volume does not exceed 0.0014 \(\hbox {cm}^{3}/ \hbox {g}\). It is much lower than for the both foamed materials. For the measurements under the high pressure, the obtained results also are much lower with the foamed materials. However , the dominated fraction is comparable to foamed materials - 1 \(\upmu \hbox {m}\) and 2 \(\upmu \hbox {m}\). Their volume is much lower, approximately 0.04 cm\(^{3}\)/g.

Figure 4g and h shows measurements from a sample of MKS from high- and low-pressure tests. The solid metakaolin samples is characterized by higher porosity than solid material based on flay ash. Similarly to FAS, there are a large number of pores in the range of 100 \(\upmu \hbox {m}\) and 200 \(\upmu \hbox {m}\). The volume of the pores is more than 0.003 \(\mathrm{cm}^{3}/ \hbox {g}\). The high-pressure test also shows the large amount of smaller pours, especially between 3 and 4 \(\upmu \hbox {m}\). Their volume is approximately 0.03 \(\hbox {cm}^{3}/\hbox {g}\).

The results show significant differences for foamed and solid materials. In the foamed materials, the pores are larger than in the solid one. The unexpected results are relatively high values of the pores for solid materials, especially based on metakaolin.

The important point is also the repetitiveness of the foaming process [26, 42]. The number of elements that could influence the quality of the pores achieved is a complex problem [42]. It caused the properties of the obtained materials to be different from those declared in the literature [26].

3.3 Water absorption

Figure 5 shows the way of the changes of the samples during 12 weeks (3 months) submerged in water.

Fig. 5
figure 5figure 5

Water absorption behavior of: a foamed metakaolin-based geopolymer, b foamed fly ash-based geopolymer, c solid metakaolin-based geopolymer, and d solid fly ash-based geopolymer

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Figure 5 shows that the fastest weight change is during the first week. Next, the process of water absorption is much slower. There is no significant change between foamed and solid samples in the case of the mechanism of the process itself. Significant differences are in the percentage of water absorbed—Tables 4, 5, 6, and 7.

The water absorption in foamed metakaolin-based samples was between 123 and 160% (Table 4). It is a significant value. The water environment also caused some degradation of the samples, what can be observed on final weight after the drying process. The samples loss between 2 and 22% of their initial weight.

Comparison with foamed metakaolin-based samples the weight of foamed fly ash-based samples changed to a lesser extent (Table 5). The water absorption was between 119 and 144%, and the changes of final weight were between 3 and 17%.

In the solid metakaolin-based samples (Table 6), the water absorption is significantly lower than in both foamed materials. Water absorption was between 112 and 114%. The value was practically the same. Furthermore, the change in weight after drying was very slight compared to that of the foamed samples.

The smallest weight change was observed for the solid fly ash-based samples (Table 7). It was between 105 and 108%. This composite seems to have the best water resistance. After drying, the loss of the mass was between 2 and 4%. This value is very similar to solid metakaolin-based samples.

Table 4 The results of water absorption for foamed metakaolin-based samples (MKF)
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Table 5 The results of water absorption for foamed fly ash-based samples (FAF)
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Table 6 The results of water absorption for solid metakaolin-based samples (MKS)
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The investigation shows that there is no significant difference between samples placed in salty and fresh water. The investigation shows that the solid materials are more durable in water environment. The factor that could additionally influence on the water absorption is the natural fibers addition. Flax fibers are hydrophilic in nature and, because of that, they have a poor resistance to water absorption. This phenomenon was investigated by Alomayri et al. for cotton fibers [43].

Additionally, for the foamed fly ash-based samples the compressive strength tests were provided. The results achieved for five samples show a value of 7 MPa \(+\)/− 1.9 MPa (standard deviation). This result is slightly better than for the samples investigated without the water absorption process. However, it is still in the frame of statistical error for the foamed materials. Because of that, we cannot say that this difference is statistically important.

Nowadays, the durability is an important factor for foamed material implementation [44, 45]. Advanced engineering applications require not only environmentally friendly materials such as geopolymers [46, 47], but also durable materials with high resistance to environmental conditions.

Table 7 The results of water absorption for solid fly ash-based samples (FAS)
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4 Conclusions

This paper presents the possibility of the development of new foamed geopolymer composites to increase durability, especially against degradation in corrosive water environments. As raw materials, fly ash and metakaolin mixed with sand were used. The composites were reinforced by flax fiber to improve mechanical properties. Solid and foam samples were prepared. The results show that:

  • Geopolymer composites can be successfully produced with the addition of natural fibers and have good strength parameters even after long-term use (materials were tested after 360 days).

  • The composites were exposed to large amplitudes of temperature and humidity, and after 90 days showed no signs of degradation or decrease (visual observation).

  • The porosity measurements confirmed the high porosity of foamed materials and they show also significant micro-porosity of solid materials that potentially could have an influence on environmental degradation.

  • The water absorption in fresh and salt water is comparable for solid and foamed samples.

  • The water absorption for the foamed samples is significantly higher than that of the solid samples.

  • The compressive strength tests after water absorption for the foamed sample based on fly ash do not show any significant differences compared to the compressive strength provided by the control samples.