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Article

Investigation into the Properties of Alkali-Activated Fiber-Reinforced Slabs, Produced with Marginal By-Products and Recycled Plastic Aggregates

by
Fotini Kesikidou
,
Kyriakos Koktsidis
and
Eleftherios K. Anastasiou
*
Laboratory of Building Materials, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
*
Author to whom correspondence should be addressed.
Constr. Mater. 2025, 5(3), 48; https://doi.org/10.3390/constrmater5030048
Submission received: 19 May 2025 / Revised: 13 July 2025 / Accepted: 21 July 2025 / Published: 24 July 2025
(This article belongs to the Special Issue Advances in the Sustainability and Durability of Waste-Based Construction Materials)
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Abstract

Alkali-activated building materials have attracted the interest of many researchers due to their low cost and eco-efficiency. Different binders with different chemical compositions can be used for their production, so the reaction mechanism can become complex and the results of studies can vary widely. In this work, several alkali-activated mortars based on marginal by-products as binders, such as high calcium fly ash and ladle furnace slag, are investigated. Their mechanical (flexural and compressive strength, ultrasonic pulse velocity, and modulus of elasticity) and physical (porosity, absorption, specific gravity, and pH) properties were determined. After evaluating the mechanical performance of the mortars, the optimum mixture containing fly ash, which reached 15 MPa under compression at 90 days, was selected for the production of precast compressed slabs. Steel or glass fibers were also incorporated to improve their ductility. To reduce the density of the slabs, 60% of the siliceous sand aggregate was also replaced with recycled polyethylene terephthalate (PET) plastic aggregate. The homogeneity, density, porosity, and capillary absorption of the slabs were measured, as well as their flexural strength and fracture energy. The results showed that alkali activation can be used to improve the mechanical properties of weak secondary binders such as ladle furnace slag and hydrated fly ash. The incorporation of recycled PET aggregates produced slabs that could be classified as lightweight, with similar porosity and capillary absorption values, and over 65% achieved strength compared to the normal weight slabs.

1. Introduction

In recent times, there has been growing attention on alkali-activated materials because of their eco-friendliness and sustainability in using industrial by-products instead of ordinary Portland cement (OPC). To ensure that alkali-activated mortars attain the necessary strength and durability for their applications, it is essential to have the right combination of binders, aggregates, and alkaline solutions [1,2]. Researchers have investigated the use of various alternative binders such as different types of slag [3,4] and fly ash [5,6], marble waste [7], concrete sludge [8], red mud [9], etc. In each case, the type of activator differs depending on the chemical composition of the binder. In general, the most common activators are alkali hydroxides and silicates, which generate the highest pH values and accelerate the dissolution of the aluminosilicates for the activation process [10].
High-calcium fly ash (HCFA) is a by-product of burning lignite instead of hard coal, and it is produced in large quantities worldwide [11]. HCFA is currently used by the cement industry as an ingredient for the production of cement mixes, but the utilization rate is low, mainly due to the significant variations in chemical composition. In addition, due to the large quantities produced annually, the amount of HCFA that is not used is being stockpiled. Before storage, HCFA is mixed with 30% water by weight to prevent air pollution with particulate matter due to wind. This stockpiled HCFA has become an increasing environmental burden, and solutions for its potential use are currently being investigated [12]. Ladle furnace slag (LFS), on the other hand, is a marginal binder that has been studied by several researchers as a cement substitute and has been classified as a latent hydraulic and weak pozzolanic material [4]. All these three alternative binders (HCFA, hydrated HCFA, and LFS) can be categorized as marginal by-products, and the investigation of their potential use in alkali-activated systems is of great interest.
In addition to the use of by-products as binders, much research has also been carried out into the replacement of natural aggregates with recycled aggregates. Gismera et al. [13] investigated the rheology of alkali-activated mortars as a function of the type of aggregate. The authors compared the use of natural aggregates with recycled aggregates from demolition waste and concluded that alkali-activated mortars were found to be more sensitive to different aggregates than OPC mortars. Papayianni et al. [14] investigated the use of recycled glass cullet as an aggregate in alkali-activated fly ash in the production of slabs. The authors found that the finer fraction of the glass cullet is dissolved and participates in the geopolymer matrix, which contributes to an increase in mechanical strength. The huge quantities of plastics used and disposed of worldwide require effective solutions for their recycling. Although the construction industry can be a potential destination for recycled plastics due to its size, many types of recycled plastics perform poorly in cement-bound mixtures [15]. Recycled polyethylene terephthalate (PET) aggregates have shown comparatively better performance, making them a potential alternative for lightweight mortars [16]. Previous work of the authors has proven that recycled PET can substitute conventional aggregates up to 80%, affecting; however, the compressive strength of the mortars. Lower rates can be used without particularly adverse effects in the mechanical performance of the material [17].
It has been reported that alkali-activated mortars exhibit brittle behavior with low flexural strength [18]. To eliminate this phenomenon, different types of fibers have been added to alkali-activated mortar mixes in the literature as follows: natural (hemp, jute, flax, kenaf, cotton, and sisal, etc.) or synthetic (carbon, ceramic, polyester, acrylic, and steel, etc.) [19]. Although the addition of fibers does not affect the compressive strength and modulus of elasticity of the material, it has a great influence on the flexural and splitting tensile strength, toughness, and ductility [20]. However, the effectiveness of fibers is influenced by several factors, such as type, length, geometry, distribution, orientation, and proportion in the concrete matrix [21]. Steel and glass fibers are among the most commonly used fibers for reinforcement. It has been proven that the addition of steel or glass fibers (1–2% per volume of concrete) has a positive effect on the flexural and tensile strength. However, their use can lead to reduced workability and accelerated stiffening of fresh mortar [22,23].
The aim of this work is to study the potential use of local by-products in the production of precast slabs. Alkali activation is a well-studied mechanism; however, the reaction depends on the type of the precursors, which vary based on the source. This paper is expanding on previous work [24] and focuses on the experimental investigation of alkali-activated mortars based on the use of marginal by-products such as fly ash, hydrated fly ash, and ladle furnace slag that are rich in calcium oxides. The optimum mixture is used for producing precast slabs with different types of fibers (steel and glass) and recycled plastic aggregates.

2. Materials and Methods

The experimental part is divided into two parts. In the first part, seven different mortar mixtures were produced, and their mechanical and physical properties were tested at different ages. The results were evaluated, and the best mixture was chosen to produce the alkali-activated slabs. The second part of this work regards the experimental testing on slabs reinforced with different types of fibers (steel or glass) to improve their mechanical performance. In the slabs tested, 60% of the siliceous sand aggregates were replaced by recycled PET aggregates to produce lightweight slabs. The slabs were tested for their homogeneity, flexural strength, fracture energy, porosity, density, and capillary absorption.

2.1. Materials

A high-calcium fly ash (HCFA) derived from a powerplant in Greece was used as a precursor. Ladle Furnace Slag (LFS), industrial waste of the Electric Furnace Arc Steel Industry, produced during the secondary refining of steel, was also used as a binder with high calcium oxide content. Previous experimental work indicated that alkali-activated mortars based on Greek fly ash and LFS have lower mechanical properties compared to literature results on fly ashes and blast furnace slags [3,6]. Thus, Ordinary Portland Cement type CEM I42.5 R was added in some mixtures to potentially improve their mechanical properties.
HCFA was used in two different forms in the mortars: (i) as derived from the powerplant and (ii) processed with water 24 h before mixing of the mortars. Based on previous research [12], this process is followed for stockpiling the fly ash in the field for future reuse. As a result of the hydration, the amount of free calcium oxide content in HCFA is reduced. Thus, twenty-four hours before the production of the mortars, the fly ash was mixed with 30% water (by mass of the fly ash) and left to dry. The chemical composition of all binders, measured by atomic absorption spectroscopy, is presented in Table 1. The fly ash, rich in calcium oxide (CaO = 36.78%), has a free calcium oxide content of 9.41%, while LFS also had a high calcium oxide content (CaO = 39.18%). Ordinary Portland cement type CEM I42.5 R was added in some mixtures to potentially improve their mechanical properties.
The pozzolanicity index of the two materials and ladle furnace slag was tested according to ASTM C593 [25], to investigate the reactivity of the binders. The results are given in Table 2, indicating the pozzolanic behavior of the fly ash that exceeded the strength limit of the standard (4.1 MPa) at both 7 and 28 days. The hydrated fly ash, on the other hand, reached 4.70 MPa at 28 days, which implies a lower pozzolanic reactivity of the binder after the curing with water, as expected due to the lower amount of free CaO. LFS cannot be classified as a pozzolanic material based on the results of the test.
Previous work of the authors on the investigation of the production of precast products with Greek by-products has proven that waterglass can be used as an activator. This was tested in an effort to find a safe and easily accessible activator for industrial applications [26]. Based on this, a solution of commercial waterglass (30%) and tap water (70%) was used as an alkaline activator. Siliceous sand was used as aggregate in the mixtures. The maximum aggregate size of the sand was selected to be 2 mm to achieve the optimum compaction for the compressed slabs. Recycled plastic aggregates (PET) (Figure 1), originating from waste plastic bottles, were used as a substitute for sand in some mixtures to reduce the density of the produced mortars. PET aggregates were crushed to reduce their granulometry to 0–4 mm, as the material was received in the form of thick plates. PET and natural aggregate have different granulometry, but finer PET particles were difficult to obtain due to the hardness of the material. The granulometry of the aggregates is given in Figure 2, according to ASTM C33 [27]. The PET/sand ratio (60:40) was determined to reach an acceptable distribution of the combined aggregate particles and at the same time to maintain an adequate utilization rate, based on previous experimental results [17]. Different proportions were tested, and this was the most acceptable granulometry for the mixed aggregates. The size, shape, and texture of PET aggregates can influence the water demand and affect the workability and the mechanical properties of the concrete [28]. Coarser PET aggregates need less paste for better workability. Yet, the addition of sand is necessary to improve the granulometry of the material and also maintain the homogeneity of the mortar.
Finally, steel and glass fibers were added to the slabs to investigate their performance (Figure 3). The properties of the aggregates and the fibers are given in Table 3.

2.2. Mix Design

2.2.1. Production of Mortars

Seven mortar mixtures were designed with a binder-to-sand ratio (1:2) (by volume) and were prepared using an automatic mixer. The produced mortars are presented in Table 4. Mixture F contains only fly ash as a binder. Fly ash and ladle furnace slag were used in equal parts to produce the FS mix, while the SC mixture was made with 90% LFS and 10% cement. The hydrated fly ash (FH) was used as a binder to produce the mortars (FH, FHS, and FHC), and the FH mixture contained only hydrated fly ash as a binder, the FHS had 50% hydrated fly ash and 50% LFS, and finally, the FHC mixture contained 90% hydrated fly ash and 10% cement.
Greek fly ash is classified as high calcium (Class C) due to its high content in calcium oxides (CaO) and lower content of silicon and aluminum oxides (SiO2, Al2O3). Moreover, the material has a high amount of free calcium oxide (9.41%) that has a direct impact on its workability and viscosity. All mixtures were prepared with a 0.8 liquid/binder ratio and the addition of 3% of a superplasticizer (polycarboxylic based), aiming at 160 ± 10 mm workability of the mortars. The high selected ratios were based on the low workability and viscosity of the material. The flow table test, according to EN 1015-3 [29], was used to measure the workability of each produced batch of mortar.
To determine the hardened properties of the mortars, prismatic (40 × 40 × 160) mm and cylindrical (50 × 100) mm specimens were cast and after demolding, they were kept in a climate chamber with 21 °C and 95% RH until testing. The flexural and compressive strengths of the mortars were measured according to EN 1015-11 [30]. Ultrasonic pulse velocity, according to BS 1881-203 [31], and elastic modulus according to BS 1881-121 [32] were also measured. The porosity of the mortars was determined according to RILEM CPC 11.3 [33]. All of the results were acquired by calculating the average of three testing samples.
Mixture F, containing fly ash (100%) (without hydration) presented the best results under flexure and compression up to 90 days of age. This mixture was reproduced with the addition of PET aggregates. Testing of P mixture samples were performed up to 28 days, as the addition of PET aggregates does not contribute to long-term properties of the material.

2.2.2. Production of Slabs

Based on the mechanical properties of the above mortars, mixture F that achieved the best mechanical performance under flexure and compression was selected to produce the slabs, and the same mixing procedure was followed adding different aggregates and types of fiber reinforcement. Table 5 presents the composition of the slabs. Mixture 1 is the unreinforced reference slab with natural river sand 0–2 mm. Mixture 2 contains steel fibers at 1% by volume of the mortar. Mixture 3 is reinforced with 1% (by vol.) of glass fibers. Finally, based on the results of the previous slabs, an unreinforced mixture was produced using a combination of natural sand 0–2 mm and PET aggregates 0–4 mm. As the high ratios of the alkaline solution and superplasticizer did not seem to improve the workability of the previously produced mortars, a lower liquid-to-binder ratio was selected, and no superplasticizer was added for the production of slabs. Therefore, to achieve the desired consistency, compaction under pressure was applied. The procedure mimics the production of the compressed mudbricks where very low water content is used in favor of mechanical performance of clay. After mixing the material was cast in (200 × 100 × 50) mm wooden beto-form molds with an applied load of 30 kN (Figure 4). Mixture 3, containing glass fibers, required higher pressure (70 kN) to form the slabs. Mixture 4 had no reinforcement; however, 60% of sand aggregates were substituted by recycled plastic aggregates (PET). The molding of the material was conducted in a hydraulic testing machine, as in the industrial production of paving blocks [31,32], to optimize the compaction of the slab. After loading, the specimens were demolded and kept in a humid environment until testing. For each mixture, two slabs were produced and tested.
The dry bulk density of the samples was recorded following the European Standard EN 1015-10 [34], while the capillary absorption of the slabs was measured according to EN 1015-18 [35] and porosity according to RILEM CPC 11.3 [33]. The homogeneity of the slabs was investigated by measuring the pulse velocity of the samples in five different places. An ultrasonic testing instrument was used to record the values.
The flexural strength of the slabs was measured, according to EN 1339 [36], at 14 and 28 days. The fracture energy Gf was calculated at the 14- and 28-day age, according to JCI-S-001 (2003) [37], by recording the values of load-crack mouth opening displacement (CMOD) during the test and by the following equations:
Gf = (0.75 × W0 + W1)/Alig.
W1 = 0.75 × [(S/L) × m1 +2 × m2] × g × CMODc
where Gf = fracture energy (N/mm2);
W0 = area below CMOD curve up to rupture of the specimen (N mm);
W1 = work performed by deadweight of the specimen and loading jig (N mm);
Alig = area of the broken ligament (b h) (mm2);
m1 = mass of specimen (kg);
S = loading span (mm);
L = total length of specimen (mm);
m2 = mass of jig not attached to the machine but placed on the specimen until rupture (kg);
g = gravitational acceleration (9807 m/s2);
CMODc = crack mouth opening displacement at the time of rupture (mm).
Finally, broken pieces of the slabs were monitored with a digital microscope (Dino-Lite) to observe the interface between the mortar matrix and the fibers.

3. Results

3.1. Testing of Mortars

The results of the flexural strength of the produced mortars at 3, 7, 28, and 90 days of age are given in Figure 5. Mortars containing only fly ash as a binder (F) reached the level of 0.86 MPa, 2.36 MPa, and 2.62 MPa at 3, 7, and 28 days, respectively. The addition of LFS in mixture FS seemed to increase early strength development by around 68% (at 3 days); however, this increase did not continue over time. SC mortars with 90% LFS and 10% cement resulted in the same level of strength at 3 days (0.81 MPa) compared to mixture F. Nevertheless, at later ages, flexural strength was around 50% of the reference mixture F.
Regarding the use of the hydrated fly ash in FH and FHS, it seems that the process diminished the development of strength as the two mixtures exhibited the lowest results with 0.53 MPa and 1.00 MPa at 28 days, respectively. Yet, the addition of LFS improved the performance of the mortars. Similarly, the addition of cement in the FHC mixture enhanced the properties of the mortar, reaching 2.13 MPa at 7 days. This increase, however, was not maintained over time, as the flexural strength presented a drop of 18% and 45% at 28 and 90 days. The use of PET aggregates (P) reduced the flexural strength by around 50% compared to mixture F with sand. However, the results were comparable to mixtures SC (with LFS and cement) and FHS (with the hydrated fly ash).
The results of the compressive strength of the mortars are shown in Figure 6. A similar pattern to flexural strength outcomes was observed also here. Overall, the best values were reached for mixture F (10.63 MPa at 28 days and 15.0 MPa at 90 days), where only fly ash was used as a binder. The combination of fly ash and LFS in the FS mixture indicated good strength results at early ages (4.62 MPa at 3 days) and comparable to F strength development at later ages. At 7 days of age, compression was slightly lower than mixture F (around 14%) but rose again at 28 days at 11.63 MPa. The mixture ended up again with a faintly lower strength (13.61 MPa) compared to mixture F (15.00 MPa). Mortars containing LFS and cement exhibited good results at early ages, but at 28 and 90 days the compressive strength was around 50% down compared to F and FS. Again, the hydration of fly ash had a negative effect on strength development for mixtures FH and FHS, and although 10% of cement addition in FHC mortar indicated a better behavior at 3 and 7 days, this did not seem to continue over time. As expected, the use of recycled plastic aggregates (mixture P) reduced the compressive strength of the mortars to 4.00 MPa at 28 days. The gained strength, although lower than mixtures F, FS, and SC, was higher than the mixtures with the hydrated fly ash (FH, FHS, and FHC).
Overall, the reactivity of the hydrated fly ash was reduced significantly compared to HCFA, which is explained mostly by the reduction in free CaO. Some improvement was noted when replacing hydrated fly ash with 10% cement or when mixing fly ash with LFS. LFS, on the other hand, provided some early strength development acceleration, probably acting as a filler and some strength development at later ages due to its pozzolanic activity. Thus, the 10% replacement of HCFA with LFS increased early strength and resulted in levels of strength higher than 10 MPa at 28 and 90 days, compared to 100% HCFA. The combination of 90% LFS with 10% cement (SC) seemed to perform better than the combination of 90% hydrated fly ash with 10% cement (FHS), despite the opposite results in the reactivity testing of the materials (Table 2), which shows that LFS can be activated in a mixed binder system. The substitution of 60% of sand for PET aggregates reduced the mechanical properties of the mortars, as expected. The reduction was up to 50% in reference to mixture F, but the results were comparable to mixtures with the hydrated fly ash.
Ultrasonic pulse velocity (UPV) of the produced mortars, given in Figure 7, followed the results of compressive strength. One more time, the mixtures F and FS exhibited better results, and the mixtures with the hydrated fly ash (FH, FHS) had the lowest values.
Figure 8 describes the test of the static modulus of elasticity at 28 days, measured at cylindrical specimens 50 × 100 mm. Overall, the mortars exhibited values under 0.19 GPa. The results followed the pattern of the compressive strength test results, which was expected due to the known relationship between strength and elasticity. However, the addition of cement in mixtures SC and FHC clearly affected the elastic modulus, reaching the same levels as the FS mixture.
Shrinkage was measured by recording the length change in the mortars caused by moisture evaporation to the environment. The specimens were kept in a climate chamber with stable dry conditions (21 °C and RH60%) for up to 30 days. The results are shown in Figure 9. At early ages, up to 5 days, all mixtures presented a small expansion of up to 0.5%, while the mixture with 90% of LFS and 10% of cement showed the biggest increase in length, up to 0.9%. The length increase may be attributed to the hydration of free CaO during the first days, but all mortars seemed to stabilize after 7–8 days. Nonetheless, it should be noted that specimens FH and FHS, containing hydrated fly ash, as well as specimen P, with PET aggregates, cracked on the eleventh day of the test, probably due to their very low flexural strength. The long-term volume stability of the alternative binders tested should also be monitored in future research.
The porosity of the mortars at 28 days is given in Table 6. The values were relevant concerning the strength rates. Mixtures F and FS, which had the highest strength results, presented the lowest porosity values. Likewise, mixtures FH and FHS, with the highest porosity, had the lowest strength rates. Overall, the porosity of the mortars varied from 11.98% to 25.82%.
Table 7 shows the pH values of the alkali-activated mortars at different ages. All mortars have a pH over 14 at 3 and 7 days of age. At 60 days the values have a small drop; however, they still are over 13.5, varying between 13.56 and 13.96. As stated by other authors [10,38], to start the dissolution and reaction of the solid aluminosilicate precursors, a high pH is required (over 10.5) for most alkali-activated binder systems. However, this cannot necessarily result in faster and more extensive reactions. Moreover, Shi et al. have observed that an increase in pH leads to an increase in the viscosity of the material [39].

3.2. Testing of Slabs

The produced slabs (mixtures 1 to 4, as shown in Table 5) were tested for their homogeneity, porosity, density, and capillary absorption. The results of the tests are given in Table 8 and Figure 10.
Table 8 cites the average pulse velocity (UPV) values at five different points of the produced slabs at 14 days and 28 days of age. For each slab, values did not vary more than 10%, which indicated the homogeneity of the matrix of the specimens and the good distribution of the fibers throughout the samples. However, the pulse velocity of reinforced mixtures 2 and 3 was lower than the reference unreinforced mixture 1, indicating that the addition of fibers increased the amount of air in the mixture. This was also confirmed by the porosity of the specimens, which is also given in Table 8. Porosity values of the unreinforced mixture 1 were around 19.92–21.72%. Likewise, mixture 4 with PET aggregates and no reinforcement varied from 20.62 to 21.91%. However, mixture 2 with steel fibers had an increase up to 21.86–23.43%, and mixture 3 with glass fiber was even higher, 28.44–29.19%. Density rates, on the other hand, remained at the same levels for mixtures 1, 2, and 3, with mixture 2 having slightly higher values due to the high specific gravity of the steel fibers (7850 kg/m3). Mixture 4, where 60% of sand was replaced with PET aggregates, had the lowest density rates, from 1400 kg/m3 at 14 days to 1320 kg/m3, because of the low specific gravity of PET aggregates (920–950 kg/m3).
Capillary absorption of the four mixtures of slabs at 28 days is shown in Figure 10. Mixtures 1 and 4, with no reinforcement, had the lowest absorption, with the highest value at 2.50 g/cm2. Steel fibers in mixture 2 increased capillary to almost 3.50 g/cm2, while mixture 3 with glass fibers reached the highest value at 4.00 g/cm2. These results correspond to the porosity measurements and confirm the pulse velocity performance of the slabs.
Flexural strength is given in Figure 11. Based on the results, the addition of the fibers tarnished the performance of the alkali-activated slabs under flexure. Mixture 1, with no reinforcement, reached 0.59 MPa at 14 days and 0.55 MPa at 28 days, while mixture 2 with steel fibers, received 0.52 MPa and 0.35 MPa at 14 and 28 days, respectively. Mixture 3, with glass fibers, had even lower values, not exceeding the rate of 0.34 MPa at 28 days. Finally, mixture 4 with PET aggregates, although having lower flexural strength than the reference mixture 1, is comparable to the reinforced mixtures 2 and 3.
Fracture energy of mixtures 1 to 4 is presented in Figure 12. The fracture energy, as calculated here, refers to the energy required until the first cracking of the slabs, and post-cracking behavior was not recorded. The fibers in the mixtures were not expected to contribute to the fracture energy recorded until the first cracking [40]. Once more, mixture 3 with glass fibers had the poorest performance, while mixture 4 with PET aggregates presented similar behavior to the reference mixture 1. Generally, fracture energy confirms the conclusion that the fibers did not seem to improve the pre-cracking performance of the alkali-activated slabs.
Microscopic images of the broken reinforced slabs are shown in Figure 13. The results of the mechanical tests are confirmed by the poor interface between the steel fibers (Figure 13a) or the glass fibers (Figure 13b) and the mortar matrix. In the case of mixture 3, glass fibers seemed to have no adhesion to the mortar, even creating an obstacle to the compaction of the slab.

4. Discussion

4.1. Testing of Mortars

Regarding the workability of the produced alkali-activated mortars, the results shown in Table 4, indicate that, although a high ratio of liquid was added along with a high percentage of a superplasticizer, the workability of the mortars was low (11–12 cm) and in some cases varied extremely. For example, the SC mixture had 20 cm in the first produced batch; however, in the following was only 11 cm. Even in the case of the hydrated fly ash (FH), the measurements had no repeatability. This could be attributed to multiple factors related to flash setting, high viscosity and high pH values.
According to relevant research [41], flash setting is defined when the setting starts at less than 30 min. This could be the result of several factors, such as overdosage of strong activators (NaOH, Ca(OH)2, and KOH), low water content, or the chemical composition of the precursors. As has been reported in previous works [8], Greek fly ashes, due to their chemical composition which is high in calcium oxides (CaO) content and free calcium oxide (CaOf), has a short setting period that begins after 20 min. The presence of CaO and especially free CaO can accelerate the setting time, particularly in alkali-activated materials, and increase early strength development [42]. Another factor that affects the lack of workability is the high viscosity of these mortars, which is often found in alkali-activated mortars containing high calcium binders and cannot usually be recorded by a standard rheometer [8]. Moreover, the high alkalinity of the mixtures, with pH values over 14, blocks the function of any admixtures (e.g., superplasticizers, retarders, and viscosity modifiers, etc.) [37,43]. Therefore, new products should be developed that could be functional in such complex environments.
Concerning the mechanical properties of the alkali-activated mortars, overall, mixture F, with fly ash, and FS, with 50% fly ash and 50% LFS, had the best performances. Mixture F had a higher strength development over time. The poorest results were seen in the mixtures with the hydrated fly ash (FH and FHS). The process of the hydration of the fly ash is known to decrease the amount of free calcium oxide content. However, in the case of HCFA, the decrease in free CaO results in the decrease in its reactivity [44]. This is confirmed by the pozzolanicity index of the hydrated fly ash. Although fly ash is a material with high pozzolanicity (7.66 MPa at 28 days), the hydrated fly ash (4.7 MPa) barely passed the limit set by ASTM C 593 (4.1 MPa). This could explain the lower values of strength for the FH and FHS mixtures. Ladle furnace slag had almost no pozzolanic behavior (1.49 MPa); thus, it can only be used in synergy with other materials, like fly ash or cement, in alkali-activated mortars. Despite the different reactivities of the binders and the different results in strength development, all mixtures maintain high pH levels, which is attributed not only to the activator but also to the chemical composition of the raw materials and especially CaO content [45].
The addition of 10% of cement in mixtures SC and FHC seemed to improve the mechanical performance of the mortars [46]; nevertheless, strength was lower than mixtures F and FS. On the other hand, the static modulus of elasticity of SC and FHC was greatly improved with the use of cement. The substitution of siliceous sand with recycled PET aggregates decreased the mechanical performance of the alkali-activated mortars, which has been observed by other researchers as well [47]. The gained strength of mixture P was about 50% of the reference F mixture. However, the results were at the same level as mixtures with the hydrated fly ash (FH, FHS, and FHC).
Porosity results followed strength results, with mixtures F and FS having the lowest values and mixtures FH and FHC having the highest values. Regarding length change, all mortars had an expansion during the first days of the test, but this change was smaller than 1%. After 10 days, the mixtures seemed to have been stabilized.

4.2. Testing of Slabs

Based on the pulse velocity measurements (Table 8), the alkali-activated slabs were homogeneous, with a good distribution of the fibers in the matrix. However, the addition of the fibers in mixtures 2 and 3 increased the porosity and capillary of the slabs, which was also confirmed by the lower rates of the measured velocity.
The mechanical tests (flexural strength and first crack fracture energy) of the alkali-activated slabs proved that the fibers added did not improve either of the properties which is expected according to the literature [48]. Instead, in some cases (mixture 3) the fibers seemed to deteriorate the performance of the material. These observations can be explained by the poor interface of the fibers with the mortar matrix, which can be seen in Figure 13. It seems as if there is no adhesion between the fiber and the mortar. The flash setting of the material, due to the high CaO and free CaO content, could potentially hinder the interaction between the fibers and the paste. This could influence the bond between the fibers and the matrix as rapid setting can reduce the workability and, thus, making more difficult for the fibers to fully interact with the binder [49]. Bhutta et al. [50] have also found that the addition of steel fibers in geopolymer mortars can lead to more brittle failure mechanisms, indicating that the shape of the fibers is crucial as the nature of the geopolymer matrix is very different. However, in general, the addition of fibers seems to be the way to improve the elastic properties of alkali-activated materials.
Finally, the addition of PET aggregates reduced the density of the slabs (1320 kg/m3), while mechanical properties were reduced at expected levels. The performance under flexure of the slabs with PET aggregates had a reduction of around 17% compared to the reference mixture. The fracture energy of the mixtures remained at the same levels. The homogeneity of the mixture did not seem to be affected by the use of recycled PET aggregates. The porosity and capillarity of the specimens did not change, although a large part (60%) of the siliceous aggregates were substituted.

5. Conclusions

Based on the experimental results of the alkali-activated mortars and slabs, the following conclusions can be made:
  • Alkali-activation of mortars containing Greek by-products such as fly ash and ladle furnace slag can result in mortars with a compressive strength of around 10 MPa at 28 days.
  • Hydrated fly ash mortars and ladle furnace slag mortars did not produce the same strength results; however, the addition of 10% of cement could enhance their mechanical properties.
  • Alkali-activated mortars have brittle behavior (low static modulus of elasticity). However, the addition of cement, even in low amounts (10%), can improve the results.
  • pH values are extremely high in all cases. Although high pH is needed to start the reaction process, it does not guarantee high-level strengths. Moreover, high pH seems to hinder the role of admixtures in the mixtures, decrease the setting time, and increase the viscosity of the mortars.
  • The incorporation of steel and glass fibers seemed to decrease the flexural strength and first crack fracture energy of the produced slabs. This could be attributed to the poor interface between the fibers and the mixture, possibly due to the low workability and flash setting of the material.
  • The addition of PET aggregates at 60% of the sand aggregates resulted in lightweight slabs (1320 kg/m3) with good physical properties (porosity and capillary) in relation to the reference mixture. Mechanical properties were reduced but at expected levels and presented comparable or even better behavior to the reinforced mixtures.
Although high-calcium by-products can be used for the production of alkali-activated mortars, several parameters could be further investigated to improve their rheological and mechanical performance. Future research can elaborate more on the influence of their chemical composition on the setting time, workability, and viscosity of the alkali-activated mixtures. However, low strength demands on many construction applications, such as slabs, must cling towards utilizing marginal by-products (e.g., hydrated fly ash, ladle furnace slag, and recycled PET aggregates).

Author Contributions

Conceptualization, F.K. and E.K.A.; methodology, F.K. and E.K.A.; validation, F.K. and K.K.; formal analysis, F.K.; investigation, K.K.; resources, F.K.; data curation, F.K. and K.K.; writing—original draft preparation, F.K.; writing—review and editing, E.K.A.; supervision, E.K.A.; project administration, E.K.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Constrmater 05 00048 g001
Figure 1. Recycled PET aggregate granules.
Figure 1. Recycled PET aggregate granules.
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Figure 2. Granulometric curves of the aggregates.
Figure 2. Granulometric curves of the aggregates.
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Figure 3. (a) Steel fibers; (b) glass fibers.
Figure 3. (a) Steel fibers; (b) glass fibers.
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Figure 4. (a) Pressing of the slab at the hydraulic machine; (b) slab after demolding.
Figure 4. (a) Pressing of the slab at the hydraulic machine; (b) slab after demolding.
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Figure 5. Flexural strength of the produced mortars [24].
Figure 5. Flexural strength of the produced mortars [24].
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Figure 6. Compressive strength of the produced mortars [24].
Figure 6. Compressive strength of the produced mortars [24].
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Figure 7. Ultrasonic pulse velocity of the produced mortars [24].
Figure 7. Ultrasonic pulse velocity of the produced mortars [24].
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Figure 8. Static modulus of elasticity of the produced mortars.
Figure 8. Static modulus of elasticity of the produced mortars.
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Figure 9. Length change in the produced mortars.
Figure 9. Length change in the produced mortars.
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Figure 10. Capillary absorption of the produced mortars.
Figure 10. Capillary absorption of the produced mortars.
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Figure 11. Flexural strength of the produced slabs.
Figure 11. Flexural strength of the produced slabs.
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Figure 12. Fracture energy of the produced slabs [24].
Figure 12. Fracture energy of the produced slabs [24].
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Figure 13. (a) Steel fibers used in slabs; (b) glass fibers used in slabs [24].
Figure 13. (a) Steel fibers used in slabs; (b) glass fibers used in slabs [24].
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Table 1. Chemical composition of the binders (total oxides, % wt.).
Table 1. Chemical composition of the binders (total oxides, % wt.).
Raw MaterialSiO2Al2O3CaOFe2O3MgOSO3Na2OK2OL.o.ICaOf
Fly ash35.7011.0836.785.073.215.080.440.910.599.41
Hydrated fly ash45.8811.5624.495.461.792.400.541.161.014.50
Ladle furnace slag38.781.7439.182.193.330.120.630.0411.41.59
CEM I42.525.845.6759.193.231.531.240.840.892.812.52
Table 2. Pozzolanicity index of the binders.
Table 2. Pozzolanicity index of the binders.
Type of BinderCompressive Strength (MPa)
7 Days28 Days
Fly ash6.837.66
Hydrated fly ash3.884.70
Ladle furnace slag1.031.49
Table 3. Properties of aggregates and fibers.
Table 3. Properties of aggregates and fibers.
Type of AggregatesSpecific Gravity (kg/m3)Size (mm)
Siliceous sand26700–2
Recycled plastic aggregates (PET)920–9500–4
Steel fibers785013
Glass fibers285013
Table 4. Composition of the produced mortars (parts by volume).
Table 4. Composition of the produced mortars (parts by volume).
NoMixtureFly AshHydrated Fly AshLFSCementRiver Sand
(0–2) mm		PET Aggregates
(0–4) mm		L/BAverage Workability (mm)
1F1---2-0.80115
2FS0.5-0.5-2-0.80125
3SC--0.90.12-0.80135
4FH-1--2-0.80135
5FHS-0.50.5-2-0.80110
6FHC-0.9-0.12-0.80120
7P1---0.850.650.80155
Table 5. Composition of the produced slabs (parts by volume).
Table 5. Composition of the produced slabs (parts by volume).
MixtureFly AshSand
(0–2) mm		PET
(0–4) mm		Liquid/
Binder		Steel FibersGlass Fibers
112-0.5--
212-0.51%-
312-0.5-1%
410.81.20.5--
Table 6. Porosity of the mortars at 28 days.
Table 6. Porosity of the mortars at 28 days.
MixtureFFSSCFHFHSFHCP
Porosity (%)13.3115.0619.8524.3722.4016.6621.22
Table 7. pH values of the produced mortars [24].
Table 7. pH values of the produced mortars [24].
MixturepH
3 Days7 Days60 Days
F14.314.413.8
FS14.414.513.6
SC14.614.713.9
FH14.514.513.6
FHS14.614.713.9
FHC14.714.813.9
Table 8. Pulse velocity values at different points of the produced slabs.
Table 8. Pulse velocity values at different points of the produced slabs.
MixtureAge (days)Average UPV (km/s)Standard Deviation
(km/s)		Porosity (%)Density
(kg/m3)
1141.6940.08319.91520
1281.7060.05921.71460
2141.6940.08321.91590
2281.7060.05923.41490
3141.6940.08328.41520
3281.7060.05929.21470
4141.6940.08320.61400
4281.7060.05921.91320
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MDPI and ACS Style

Kesikidou, F.; Koktsidis, K.; Anastasiou, E.K. Investigation into the Properties of Alkali-Activated Fiber-Reinforced Slabs, Produced with Marginal By-Products and Recycled Plastic Aggregates. Constr. Mater. 2025, 5, 48. https://doi.org/10.3390/constrmater5030048

AMA Style

Kesikidou F, Koktsidis K, Anastasiou EK. Investigation into the Properties of Alkali-Activated Fiber-Reinforced Slabs, Produced with Marginal By-Products and Recycled Plastic Aggregates. Construction Materials. 2025; 5(3):48. https://doi.org/10.3390/constrmater5030048

Chicago/Turabian Style

Kesikidou, Fotini, Kyriakos Koktsidis, and Eleftherios K. Anastasiou. 2025. "Investigation into the Properties of Alkali-Activated Fiber-Reinforced Slabs, Produced with Marginal By-Products and Recycled Plastic Aggregates" Construction Materials 5, no. 3: 48. https://doi.org/10.3390/constrmater5030048

APA Style

Kesikidou, F., Koktsidis, K., & Anastasiou, E. K. (2025). Investigation into the Properties of Alkali-Activated Fiber-Reinforced Slabs, Produced with Marginal By-Products and Recycled Plastic Aggregates. Construction Materials, 5(3), 48. https://doi.org/10.3390/constrmater5030048

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