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Article

Mechanical and Thermal Insulation Properties of rGFRP Fiber-Reinforced Lightweight Fly-Ash-Slag-Based Geopolymer Mortar

by
Mo Zhang
1,2,
Xinxin Qiu
1,
Si Shen
1,3,
Ling Wang
1,2,* and
Yongquan Zang
1,*
1
School of Civil and Transportation Engineering, Hebei University of Technology, Tianjin 300401, China
2
Smart Infrastructure Research Institute, 5340 Xiping Road Beichen District, Tianjin 300401, China
3
Investment Company of China Railway 18th Bureau Group Co., Ltd., Dagu South Road, Shuanggang Town, Jinnan District, Tianjin 300222, China
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(9), 7200; https://doi.org/10.3390/su15097200
Submission received: 2 March 2023 / Revised: 4 April 2023 / Accepted: 23 April 2023 / Published: 26 April 2023
(This article belongs to the Special Issue Life Cycle and Sustainability of Building Materials)
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Abstract

:
As a lightweight cementitious material for thermal insulation, the mechanical performance of foamed geopolymer is always compromised by its density reduction. In this study, recycled-glass-fiber-reinforced plastic (rGFRP) fiber was used to reinforce the fly ash-slag based foamed geopolymer, and vitrified micro bubbles (VMB) were applied to further decrease the thermal conductivity and modify the resistance of the lightweight mortar against drying shrinkage. The results revealed that the density, compressive strength, and thermal conductivity of the foamed geopolymer with/without VMB decreased with the increase in foaming agent content. By adding 2~6% of rGFRP fiber, the compressive strength was increased by 25~165%, and the drying shrinkage was reduced the most, by 55%. After the addition of 10% of VMB, the density, thermal conductivity, and drying shrinkage of foamed geopolymer mortar were further decreased, with the highest reductions of 8%, 26%, and 64%, respectively, due to the reduced pore volume and increase proportion of closed pores. With 6% of rGFRP fiber and 25% of foaming agent, the lightweight geopolymer mortar had the optimum performance, with compressive strength of 1.343 MPa, thermal conductivity of 0.134 W/(m·K), and drying shrinkage of 0.095%. This study developed a sustainable lightweight mortar with multiple types of industrial by-products, which benefit both the development of thermal insulation materials and reuse of solid wastes.

1. Introduction

Geopolymer is a type of aluminosilicate cementitious material synthesized by alkali or acid activation of materials rich in aluminosilicates, which has been widely studied as an important alternative to Ordinary Portland Cement (OPC) thanks to its low embodied CO2 and excellent performances (e.g., higher heat and chemical resistance and compressive strength) [1,2]. In addition, geopolymer can be synthesized from various industrial wastes, such as fly ash (FA), granulated blast furnace slag (GBFS), red mud, etc., providing a sustainable solution to environmental pollution induced by industrial by-products. However, the full potential of geopolymers in construction industry has not yet been fully exploited due to technical barriers and legislation [3]. To facilitate the utilization of geopolymers, their application should be extended to the fields in which OPC does not meet the requirements.
Foamed geopolymer has attracted more attention than OPC due to its higher thermal and acoustic insulation [4]. Foamed geopolymer is a new type of green thermal insulation material prepared by introducing bubbles into geopolymer materials by physical foaming or chemical foaming [5]. In addition to excellent sound and thermal insulation properties, foamed geopolymer exhibited many other advantages, such as low carbon emission, excellent mechanical properties, good durability, as well as high temperature resistance [6,7]. Foamed geopolymer with high strength and low thermal conductivity has been successfully synthesized with alkali-activated FA and GBFS, and researchers made great efforts to further reduce the self-weight and improve the mechanical strength of foamed geopolymer, such as adjusting calcium content, adding lightweight raw materials, etc. [8,9]. The thermal insulation ability of foamed geopolymer is provided mainly by tiny bubbles (generally less than 3 mm) inside the material, which reduce the density at the same time [10]. However, the proportions of interconnected pores and large-aperture pores can be increased [11], impacting the mechanical and thermal insulation properties [5,12,13], causing structural deterioration and reducing the service life of structural components. This considerably restricts the wide application of foamed geopolymer [14].
To overcome the above-mentioned problem, the lightweight aggregate is often added to geopolymers or foamed geopolymers to improve the overall thermal insulation and mechanical performance of materials [4,15,16]. Sengul et al. used expanded perlite to reduce the thermal conductivity of foamed concrete from approximately 0.6 (W/mK) to approximately 0.15 (W/mK) [17]. Sofia et al. found that the addition of expanded clay reduced the thermal conductivity of concrete by 40–53% [18]. Gong et al. studied the influence of different contents of vitrified micro bubbles (VMB) on the thermal conductivity of foamed concrete [19]. Results show that the thermal conductivity of foamed concrete was reduced by 18.14%. Pasupathy and Shi added expanded perlite and expanded polystyrene into the foamed geopolymer, which reduced the thermal conductivity of the foamed geopolymer by 12.1% and 37.4%, respectively [20,21]. Shi used expanded polystyrene particles as lightweight aggregate to manufacture foamed geopolymer mortar and significantly reduced the density (300~650 kg/m3) and thermal conductivity (0.122~0.195 W/mK) of geopolymer mortar and maintained the compressive strength in the range of 2.0~5.5 MPa [21]. Although lightweight aggregates can effectively reduce the thermal conductivity of the foamed geopolymer [22], they may have negative influence on the strength of the foamed geopolymer.
Fibers can be incorporated into the geopolymer matrix to improve the strength of the foamed geopolymer [23,24]. The addition of fibers can inhibit the generation of internal micro-cracks and the development of macro-cracks through the bridging effect of fibers when the matrix is under tensile load, thereby improving the strength of materials [25]. Abdollahnejad et al. [26] found that the addition of 6 mm polypropylene fibers reduced drying shrinkage by 50% of the foam geopolymers. Senff applied the glass fiber waste from wind blade production in foamed geopolymer and increased the flexural strength and compressive strength by 40% and 27%, respectively [27]. However, they pointed out that the addition of fiber can be detrimental to the thermal insulation properties of geopolymer. Dawood compared the effect of glass fiber and polypropylene fiber on the compressive strength of foamed geopolymer and found that 0.6% of glass fiber increased the strength by 61%, which had the most significant enhancement [28]. Triwulan improved the compressive strength of the foamed geopolymer from 2.85 MPa to 3.05 MPa by adding 0.8% natural banana fibers [29]. Wang used polypropylene fiber with the length of 3~19 mm to reinforce foamed fly-ash-based geopolymer, which improved the mechanical strength up to 71%, while impacting the thermal conductivity [30]. The main testing results on the foamed geopolymer and lightweight geopolymer mortar reviewed in this study are summarized in Table 1.
Glass-fiber-reinforced plastic (GFRP) has high specific strength, chemical resistance, and distinguished electrical properties (e.g., the maximum AC breakdown strength of 25 kV/mm) [31]. It has been widely applied in shipbuilding, vehicle manufacture, wind power systems, and pipe production [32]. With increasing annual production and consumption, increasingly more GFRP waste is generated from the manufacturing, processing, and decommissioning at the end of service life. To address the environmental pollution problem, GFRP wastes are mechanically crushed into fiber- and powder-shaped recyclates. Recycled GFRP (rGFRP) can be used as reinforcement and aggregate in concrete to improve the ductility and reduce cracking of cement-based materials [33]. Zhou et al. added rGFRP fibers in cement mortar and found that the 28-day flexural strength increased by 47.8% [34]. Suganya et al. [35] improved the compressive strength and splitting tensile strength of cement mortar by adding rGFRP powder into cement mortar. The compressive strength increased by 11.8% from 34 MPa to 38 MPa, and the splitting tensile strength increased by 25.7% from 5.38 MPa to 6.36 MPa. These studies have shown that the irregular resin coating on the surface of the rGFRP fibers increased the connection between fibers and cement-based materials and improved the strength of concrete [36]. However, the effect of rGFRP fibers on the strength improvement of foamed geopolymers has barely been explored. Adding rGFRP fibers to the foamed geopolymer may have a negative effect on its thermal conductivity [37], which may be offset by adding the lightweight aggregate. Therefore, the synergistic effects of adding rGFRP fibers and lightweight aggregates are investigated in this study for the first time to develop a sustainable type of fiber-reinforced inorganic lightweight mortar and explore a new way for the reuse of decommissioned GFRP.
In this study, fly ash (FA) and granulated blast furnace slag (GBFS) were selected as precursors to synthesize the geopolymer matrix, in which foam was introduced by mixing with the foaming agent. The rGFRP fiber obtained from crushed wind power blades was added to the foamed geopolymer for improving the compressive strength and drying shrinkage of the foamed geopolymer, while vitrified micro bubbles (VMB) were added to improve the density and thermal conductivity. This article is organized as follows: the materials, sample preparation, and testing methods are introduced in Section 2; the testing and characterization results are discussed in Section 3, including the workability, bulk density, thermal conductivity, compressive strength, drying shrinkage, and porosity visualization of the rGFRP fiber-reinforced foamed geopolymer and lightweight geopolymer mortar with VMB; and Section 4 presents the conclusions drawn from this study and suggestions for the future work. This study provides reference for the subsequent research on fiber-reinforced lightweight geopolymer materials.

2. Material and Methods

2.1. Materials

Fly ash (FA) and granulated blast furnace slag (GBFS) provided by Jintaicheng Environmental Resources Co. LTD, China, were used as precursor materials for synthesizing geopolymer, as shown in Figure 1(a) and (b), respectively. The chemical composition tested with X-ray fluorescence is listed in Table 2. The particle size distribution of FA and GBFS were measured by a laser diffraction particle size analyzer and plotted in Figure 2. The average particle sizes of FA and GBFS were 50.1 μm and 18.0 μm, respectively, while their specific surface areas were 1.32 m2/g and 1.64 m2/g, respectively. The sodium silicate powder with SiO2/Na2O ratio of 1.0~1.6 and purity of 99% and the NaOH pellets with the purity of 98% were used as alkali activators. The foaming agent was composed of water, sodium dodecyl sulfate (SDS), sodium polyacrylate (SP), lauryl alcohol (LA), and silicone polyether (SPE), and the mixture composition is shown in Table 3. The Ca(OH)2 powder with the purity of 98% was used as the foam stabilizing agent. These chemical agents were all from Shandong Yousuo Technology Co., Ltd. The vitrified micro bubble (VMB), purchased from Jiayue Construction Materials Co., Ltd., Hengshui City, China, was selected as the lightweight aggregate (Figure 1c). The dry density, thermal conductivity coefficient, and linear shrinkage ratio were 0.24~0.30 g/cm3, ≤0.085 W/m·K, and ≤0.30%, respectively. The rGFRP fibers with the fiber length ranging from 1.45 mm to 4.75 mm were obtained from a decommissioned wind turbine blade through the process of cutting, crushing, and sieving by Anshu Langqing Environmental Protection Equipment Co., Ltd., Hengshui City, China, as shown in Figure 1d.

2.2. Mixture Design and Sample Preparation

The foamed geopolymer were prepared with the foaming agent contents of 10%, 15%, 20%, and 25% at different dosages of rGFRP fiber, i.e., 0%, 2%, 4%, and 6%. It should be noted that the dosage of the foaming agent was determined on the basis of the preliminary experiments, where 10% was the lowest dosage that could decrease the bulk density of geopolymer to 1000 kg/m3; the geopolymer paste cannot set with 25% of foaming agent. In this case, 10~25% was selected as the content range of the foaming agent, within which 15% and 20% were applied to investigate the effect of the content of foaming agent on the performance of the geopolymer. The dosage of rGFRP fiber was determined on the basis of a previous study on the rGFRP fiber-reinforced cement mortars by the authors [38]. As a comparison, the alkali-resistant glass (AR) fiber-reinforced foamed geopolymers were also prepared with the fiber content of 1%. On the basis of these paste groups, 10% of VMB was used to prepare the lightweight geopolymer mortars. A total of 21 mixture proportions were tabulated and are shown in Table 4. In this study, one-part mix process was adopted for the synthesis of foamed geopolymer paste and lightweight geopolymer mortar [39,40]. For rGFRP fiber-reinforced foamed geopolymer, the solid materials, including the rGFRP fiber, FA, GBFS, solid sodium silicate, and solid sodium hydroxide, were stirred for 2 min at 65 r/min. Water was then added and stirred for another 2 min at the speed of 65 r/min. Finally, the foaming agent was added, the composite was stirred at 65 r/min for 2 min, and then at 296 r/min for 10 min. The lightweight geopolymer mortar was prepared bythe same process, where VMB was mixed with the solid materials in the first step. For the unconfined compression tests, drying shrinkage tests, and thermal conductivity tests, the samples with the size, of 70.7 mm × 70.7 mm × 70.7 mm, 25 mm × 25 mm × 180 mm, and 300 mm × 300 mm × 30 mm were prepared. The samples were sealed with plastic films for 24 h and then cured at ambient temperature and relative humidity of 40~50% until testing. The study program is illustrated in Figure 3, including material synthesis, sample preparation, and tests.

2.3. Test Methods

2.3.1. Workability and UCS

In this study, the spread diameter and the setting time of the foamed geopolymer and lightweight mortars were tested according to GB/T 2419-2005 [41] and GB/T 1346-2001 [42], respectively. The unconfined compression strength (UCS) of the samples cured for 7 days and 28 days was tested using a WAW200 electro-hydraulic servo universal mechanical testing machine. A relatively low loading rate of 500 N/s was adopted to distinguish the strength difference between the foamed geopolymer sample sets since the loading rate should be in the range of 250~1500 N/s for building mortar, as regulated in JGJ/T70-2009 [43]. Three replicates were tested for each set.

2.3.2. Density and Drying Shrinkage

The density of samples was measured according to GB/T 9966.3-2020 [44]. During the test, the weight of the sample was first measured, and then the sample was submerged in a container with water for 24 h to measure the volume. The bulk density of each sample was calculated. The drying shrinkage of foamed geopolymer and lightweight mortar was tested with a length comparator according to JGJ/T70-2009 [45]. The test was started after the first 24 h, and the daily length change was recorded curing until the shrinkage was constant (within 28 days).

2.3.3. Thermal Conductivity

The thermal conductivity of the foamed geopolymer and lightweight mortar was tested using a TR-DRH-300 thermal conductivity meter from Shanghai Torrent Equipment Co., LT as per GB/T10294-2008 [46], as shown in Figure 4. The panel sample was placed between the heating and cooling plate (5 and 6 in Figure 4) for 4 h until the temperature reached equilibrium before testing. The environmental temperature was room temperature, and the temperature difference between the heating and cooling plate was maintained at 10 °C. The heat flux density was obtained, and the thermal conductivity was calculated according to Equation (1). Each sample was repeatedly tested 20 times, and the average thermal conductivity was calculated. Three identical samples were tested for each sample set.
k x = q x ″ ∂ T ∂ x
where x is the height along the direction of heat flow; qx″ is the heat flux density along the direction of the heat flow (in W/m2); and ∂ T ∂ x is the temperature gradient of the material along the direction of heat flow (in K/m).

2.3.4. Porosity

The porosity of the cube sample with the size of 25 mm × 25 mm × 25 mm was tested using X-ray computed tomography (CT) with a YXLON FF35 X-ray detector. The sample was rotated 360 degrees around the vertical axis, and a total of 1080 consecutive scanned 2D images were obtained. These 2D images were reconstructed into 3D images with the VGSTUDIO software, and the porosity was quantitatively analyzed. In addition, the microstructure of the pores in the foamed geopolymer and lightweight mortar were visualized with a Zeiss Smart zoom 5 digital microscope.

3. Results and Discussion

3.1. Workability

3.1.1. Spread Diameter

The spread diameter of foamed geopolymer and the lightweight mortar with VMB are shown in Figure 5a,b. It can be seen that the spread diameter of the foamed geopolymers increased with the increase in the foaming agent for all rGFRP fiber contents. Particularly, a more obvious increase was observed at higher fiber contents. For the geopolymer without the rGFRP fiber and with 6% rGFRP fiber, the spread diameter increased by 5% and 9%, respectively, with the foaming agent increasing from 10% to 25%. This may be caused by the dispersant effect of the SDS in the foaming agent, which is a commonly applied water-reducing agent [47]. The SDS was adhered to the fiber surface, reducing the interfacial adhesion between the paste and rGFRP fiber and is absorbed on the surface of cement particles, repelling them from aggregation. Therefore, the flowability of the foamed geopolymer paste and mortar increased with the increasing dosage of SDS at the same water content in this study.
The spread diameter of the foamed geopolymer decreased with the increase in rGFRP fiber content, as shown in Figure 5a. For example, the spread diameter of the foamed geopolymer decreased by 13.89% from 180 mm to 155 mm, with the content of rGFRP fiber increasing from 0% to 6% at the foaming agent content of 10%. On one hand, the small-sized fiber might agglomerate, absorb, and trap free water within the fiber clusters, and the free water content was reduced [48]. On the other hand, the bridging and interlocking effect of rGFRP fibers restricted the mobility of the fresh paste [49]. A more obvious reduction effect was observed by adding 1% AR fiber than by adding 6% rGFRP fibers. The possible reason is that the specific surface area of AR fiber in the geopolymer composite was larger than that of the rGFRP fiber, while the diameter was much thinner, which increased the viscosity of the paste and reduced the flowability of the composite.
As shown in Figure 5b, the spread diameter of the lightweight mortar with VMB developed in the same pattern as that of the foamed geopolymer. The spread diameter increased with the increasing content of the foaming agent and decreased with the increasing content of the rGFRP fiber. The largest spread diameter (182 mm) was achieved at the rGFRP fiber content of 0% and foaming agent content of 25%, while the smallest spread diameter (130 mm) was obtained at the rGFRP fiber content of 6% and foaming agent content of 10%. It was found that the addition of VMB reduced the fluidity of foamed geopolymer due to the high water absorption of VMB [50].

3.1.2. Setting Time

As shown in Figure 6a, the final setting time of foamed geopolymer increased with the increment of the foaming agent content. The most considerable increase was observed when the content of rGFRP fiber was 4%. In this group, the final setting time was prolonged by 34.78%, with the foaming agent content increasing from 10% to 25%. The possible reason is that the particles of the raw materials were wrapped by surfactants in the foaming agent, which hindered the geopolymerization and hardening of geopolymer paste [51].
The addition of fiber, both rGFRP and AR, prolonged the final setting of foamed geopolymer. The prolongation was more significant for the foamed geopolymer with higher content of rGFRP fiber, with the highest increase of 1.5 h for the sample with 25% of foaming agent and 6% of rGFRP fiber. This is due mainly to the water-absorbing properties of the rGFRP fiber, which reduced the water in the geopolymer matrix and accelerated the flocculation of precursor materials. A relatively small increase in the final setting time (0–0.75 h) resulted from the AR glass fiber, due to the smoother surface and less hydrophilicity than the rGFRP fiber.
As shown in Figure 6b, the change pattern of final setting time of the lightweight mortar with VMB was similar to that of the foamed geopolymer. However, the final setting time of the foamed geopolymer with VMB was shorter than that of the foamed geopolymer counterpart. Similar to the influence on the flowability, the high water absorption of VMB reduced the free water in the matrix and accelerated the flocculation and setting of the mortar [52].

3.2. Density

Figure 7a shows the density of foamed geopolymer without VMB. It can be seen that the density decreased with the increase in the foaming agent content. As the foaming agent increased from 10% to 25%, the density of the foamed geopolymer decreased by 52%, 44%, 50%, and 50% with the rGFRP fiber contents of 0%, 2%, 4% and 6%, respectively. This phenomenon is caused by the fact that air was introduced into the paste by adding the foaming agent, which increased the porosity of the geopolymer [53]. It can also be noted that the density of the foamed geopolymer decreased by 14%, 17%, 13%, and 18% with the increasing in the content of the rGFRP fiber from 0% to 6% at the foaming agent contents of 10%, 15%, 20%, and 25%, respectively. The possible reason is that adding fibers in the paste introduced more air during the mixing process, and, thus, the porosity of the composite increased [54]. The minimum density of AR fiber-reinforced foamed geopolymer was 0.457 g/cm3, close to that of 6% rGFRP fiber-reinforced foamed geopolymer. This suggests that the AR fiber also had significant influence on the porosity of the foamed geopolymer.
From Figure 7b, it can be seen that the change pattern of the density of the geopolymer with VMB was consistent with that of geopolymer without VMB. However, the addition of VMB further reduced the density of the foamed geopolymer at the same foaming agent content and fiber ratio. The lowest density of 0.431 g/cm3 was achieved after adding VMB at the rGFRP content of 6% and foaming agent content of 25%. This is attributed mainly to the lower density of the VMB (0.300 g/cm3).

3.3. Drying Shrinkage

As shown in Figure 8a, the effect of the foaming agent content on drying shrinkage of the foamed geopolymer without VMB is not obvious. It is known that the drying shrinkage was induced by the evaporation of the free water in the pores of the matrix [55]. On the one hand, the addition of the foaming agent increased the porosity of the geopolymer, which accelerated water evaporation and, thus, increased the drying shrinkage. On the other hand, the sodium polyacrylate in the foaming agent could combine the free water, reducing the evaporation rate of the pore water and, thus, reducing the drying shrinkage rate. Therefore, these two effects reached a compromise, causing minor changes of the drying shrinkage with foaming agent content.
However, the drying shrinkage of the foamed geopolymer decreased with the increase in rGFRP fiber content. The sample without rGFRP fiber and with 15% foaming agent achieved the highest drying shrinkage rate of 0.46%, while the smallest drying shrinkage rate of 0.10% was obtained by the sample with 20% foaming agent and 6% rGFRP fiber. The possible reason is that the interlocking and bridging effect between the fiber and the matrix might inhibit the shrinkage of the geopolymer [56].
Compared with Figure 8a, Figure 8b shows that the addition of VMB reduced the drying shrinkage rate of the foamed geopolymer at the same foaming agent content and rGFRP fiber ratio. This may be attributed to the fact that the larger porosity of the VMB resulted in its better water-absorption ability. The water confined in the VMB was released slowly during the geopolymerization process, which reduced the drying shrinkage caused by the rapid water loss in the matrix.

3.4. Unconfined Compressive Strength

It can be seen from Figure 9a,b that the 7 d and 28 d UCS of the foamed geopolymer decreased with the increasing foaming agent content, which might be caused by the increased porosity of the geopolymer due to the addition of air from the foaming agent [57]. In addition, the UCS of the foamed geopolymer increased with the increase in the rGFRP fiber content. The highest UCS values at 7 d (3.3 MPa) and 28 d (5.3 MPa) were achieved at the foaming agent content of 10% and the rGFRP fiber content of 6%. The addition of AR fibers also increased the 7 d and 28 d UCS of the foamed geopolymers. It is well known that fibers can bridge the micro-cracks that appear in the geopolymer matrix, which inhibits the generation and development of cracks, and, thus, the UCS is improved [58].
Compared with the foamed geopolymer, the lightweight geopolymer mortar with VMB presented more enhanced mechanical performance. The compressive strength decreased less with the increase in foaming agent content, and increased more with the increased rGFRP fiber content, as shown in Figure 9c,d. This was due mainly to the fact that VMB had higher strength and stiffness than the geopolymer porous portion, which improved the mechanical strength of the matrix and alleviated the detrimental impact of the foaming agent, which induced open pores on the mechanical strength. In addition, the lightweight geopolymer mortar matrix can better incorporate with rGFRP fiber during loading and obtained higher increase in strength.

3.5. Thermal Conductivity

As shown in Figure 10a, the thermal conductivity of the foamed geopolymer decreased with the increase in the foaming agent content. The thermal conductivity of the foamed geopolymer and the lightweight geopolymer mortar was decreased with the increase in fiber content, for both the rGFRP fiber and AR fiber-reinforced sample sets. The lowest thermal conductivity of 0.118 W/m·K was achieved at the foaming agent content of 25% without rGRRP fiber, while the highest thermal conductivity of 0.232 W/m·K was obtained at the foaming agent content of 10% and with the rGRRP fiber content of 6%. This was because the higher dosage of foaming agent introduced a larger amount of foam in the matrix during the stirring process, thus forming a greater number of closed pores. The thermal conductivity of air was 0.026 W/m·K [59], far less than that of the geopolymer matrix, resulting in the decrease in thermal conductivity of the foamed geopolymer. In addition, the thermal conductivity of fibers (rGFRP fiber or AR fiber) is larger than the geopolymer matrix, and the fiber formed heat-conductive paths in the matrix, facilitating the heat flow of geopolymer. This research result is consistent with the previous studies that the increase in fiber content might lead to the increase in geothermal coefficients of the foamed geopolymer [27,37,60].
It can be seen from Figure 10b that the addition of VMB reduced the thermal conductivity of the foamed geopolymer. The thermal conductivity of the foamed geopolymer with 10% VMB was the lowest (0.095 W/m·K) without the addition of rGFRP fiber at the foaming agent content of 25%. This may be caused by the lower thermal conductivity of the VMB [61,62], which reduced the thermal conductivity of the geopolymer matrix. However, the increase in the foaming agent enhanced the reduction effect of the VMB on the thermal conductivity. The dispersion agent in the foaming agent might increase the dispersion of the VMB in the matrix, reducing the heat conductivity of the geopolymer matrix. The trend of thermal conductivity of the lightweight geopolymer mortar against foaming agent dosage and rGFRP fiber content followed the same patterns as those of the foamed geopolymer without VMB.

3.6. Porosity

As shown in Figure 11, more pores were observed in the foamed geopolymer with increasing the foaming agent content. It can also be observed in Figure 11e,f that the proportion of larger pores in the foamed geopolymer increased significantly as the foaming agent content increased from 20% to 25%. It should be noted that the size of pores in the foamed geopolymer also was reduced after the VMB was added due to the pore-filling effect of VMB, as shown in Figure 11g. The porosity of the foamed geopolymer was further characterized by X-ray CT, as shown in Figure 11i. The porosity of the geopolymer matrix (CG) was the lowest (1.96%). The addition of 10% foaming agent significantly increased the porosity of the geopolymer to 12.05%. The addition of 6% rGFRP fiber in F10-R6 further increased the porosity of the foamed geopolymer to 14.16%. This may be caused by the introduction of the air voids due to the introduction of rGFRP fibers. It can also be observed that the pore volume was increased from 12.05% to 72.75% as the foaming agent increased from 10% to 25%, which also resulted from the increasingly introduced air due to the increase in the foaming agent. Compared with F10-R6, the porosity of the F10-R6-V was increased. This suggests that although the addition of VMB reduced the large-size pores, and the proportion of the small-size pores increased, increasing the overall porosity of the geopolymer.
As shown in Figure 12a, a total of 2295 pores were collected in the control group (CG), of which 86.37% had a pore volume greater than 1000 μm3. After adding 10% foaming agent (Figure 12b), the number of total pores increased to 8,063; however, the proportion of pore volume greater than 1000 μm3 was reduced to 71.51%. Compared with F10, the addition of 6% rGFRP fiber (F10-R6) in Figure 12c slightly reduced the number of pores from 8,063 to 7,944, while the proportion of pore volumes less than 1000 μm3 increased to 80.27%. The possible reason is that the closed pores with smaller sizes might be pierced by fibers and gathered into larger pores during the mixing process [63].
It can be observed in Figure 12c–e that the number of pores increased considerably from 7,944 to 37,548, while the proportion of pore volumes less than 1000 μm3 increased from 80.27% to 93.74%, with the foaming agent content increasing from 10% to 20% at the rGFRP fiber content of 6%. Further, the proportions of pores less than 100 μm3 accounted for 15.17%, 17.90%, and 33.69% at the foaming agent contents of 10%, 15% and 20%, respectively, indicating that the proportion of smaller pores increased as the foaming agent increased. For F25-R6 (Figure 12f), the total number of pores and the proportion of pores smaller than 1000 μm3 decreased to 22,583 and 85.06%, respectively, compared with those of F20-R6 (37,548 and 93.74%). However, Figure 11i shows that the total pore volume of F25-R6 was larger than that of F20-R6, suggesting that a greater number of smaller pores were combined into larger pores, as the foaming agent content was larger than 20%. In comparison with F10-R6, the addition of 10% VMB (Figure 12g) increased the total pore number from 7994 to 30,008 and increased the proportion of the pores smaller than 1000 μm3 from 19.13% to 74.39%. This resulted mainly from the large volume of inner pores in the VMB, which significantly increased the porosity of the foamed geopolymer, and, thus, the density and thermal conductivity were considerably reduced.

4. Conclusions

In this study, foamed geopolymer and lightweight geopolymer mortar were manufactured with FA and GBFS through mechanical frothing by mixing 10~25% foaming agent. The influence of foaming agent dosage, recycled GFRP fiber, and VMB lightweight aggregate on the workability and physical–mechanical properties were investigated. The following conclusions can be drawn:
(1) The dosage of foaming agent presented the same impact pattern on the performances of foamed geopolymer and lightweight geopolymer mortar. The setting time and flowability were increased, while the density, compressive strength, and thermal conductivity were decreased with the increment in the dosage of foaming agent.
(2) The compressive strength of the foamed geopolymer and lightweight geopolymer mortar was significantly improved by the addition of rGFRP fiber. With 6% of rGFRP fiber, the highest increment of the 28 d strength was obtained, which was 125% and 100%, respectively, compared with the non-fiber counterparts. This was attributed mainly to the tensile–transfer bridge formed by the rGFRP fiber in the geopolymer and mortar matrix, which effectively decreased the drying shrinkage by more than 57% and 75% for the foamed geopolymer and lightweight geopolymer mortar, respectively. However, the high thermal conductivity of the rGFRP fiber formed a large number of thermal bridges and increased the thermal conductivity.
(3) Thanks to the large amount of closed inner pores introduced by VMB, the lightweight geopolymer mortar had lower density, thermal conductivity, and drying shrinkage than the foamed geopolymer with the same content of foaming agent and rGFRP fiber. By adding 10% of VMB, the thermal conductivity was increased by 8~23%. However, VMB had inconsistent influence on the compressive strength of the foamed geopolymer, which decreased the strength of the geopolymer with lower porosity and higher rGFRP fiber and increased the strength of those with higher porosity.
(4) The addition of VMB alleviated the detrimental influence of rGFRP fiber on the thermal insulation properties and that of the foaming agent on the mechanical strength of foamed geopolymer. The VMB and rGFRP fiber had the most significant synergistical effect on the lightweight geopolymer mortar with 25% of foaming agent and 6% of rGFRP fiber, in which the thermal conductivity was 0.134 W/(m·K) and the 28d compressive strength was 1.34 MPa.
The findings in this study provide a sustainable type of inorganic lightweight mortar and new route to synergistically reuse different types of massive industrial by-products. However, the mechanical strength and durability of the geopolymer mortar should be further improved in future work. Moreover, the industrial by-products with low resource utilization rate, such as steel slag, will be used to prepare the geopolymer matrix to further enhance the physical–mechanical performances and environmental benefit of the lightweight mortar.

Author Contributions

M.Z., supervision and writing—original draft; X.Q., investigation and writing—review and editing; S.S., writing—original draft and data curation; L.W., writing—review and editing and project administration; Y.Z., visualization and writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Natural Science Foundation of Hebei Province [Grant No. 20373803D] and Youth Top Talent Project of Hebei Province [Grant No. BJ2020001], Natural Science Foundation of China [Grant Nos. 52078181 and 52208240], and S&T Program of Hebei [Grant No. E2022202051].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to acknowledge the experiment support provided by the lab staff in Tianjin Key Laboratory of Prefabricated Building and Intelligent Construction.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Photos of as-received (a) GBFS, (b) FA, (c) VMB, and (d) rGFRP fiber.
Figure 1. Photos of as-received (a) GBFS, (b) FA, (c) VMB, and (d) rGFRP fiber.
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Figure 2. Particle size distribution of FA and GBFS.
Figure 2. Particle size distribution of FA and GBFS.
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Figure 3. Flowchart of the study program.
Figure 3. Flowchart of the study program.
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Figure 4. The sketch view of the thermal conductivity meter. Note: 1 and 2 are the thermostatic baths; 3 and 4 are the circulator pumps; 5 and 6 are the heating and cooling plates, respectively; 7 is the test specimen; 8 represents the heat flow meters; 9 is the height control lever; and 10 is the control center.
Figure 4. The sketch view of the thermal conductivity meter. Note: 1 and 2 are the thermostatic baths; 3 and 4 are the circulator pumps; 5 and 6 are the heating and cooling plates, respectively; 7 is the test specimen; 8 represents the heat flow meters; 9 is the height control lever; and 10 is the control center.
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Figure 5. Spread diameter of (a) the foamed geopolymer and (b) lightweight mortar.
Figure 5. Spread diameter of (a) the foamed geopolymer and (b) lightweight mortar.
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Figure 6. Final setting time of the (a) foamed geopolymer and (b) lightweight mortar.
Figure 6. Final setting time of the (a) foamed geopolymer and (b) lightweight mortar.
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Figure 7. Density of the foamed geopolymer (a) without and (b) with VMB.
Figure 7. Density of the foamed geopolymer (a) without and (b) with VMB.
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Figure 8. Drying shrinkage rate of the foamed geopolymer (a) without VMB and (b) with VMB.
Figure 8. Drying shrinkage rate of the foamed geopolymer (a) without VMB and (b) with VMB.
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Figure 9. UCS of the samples with different foaming content and rGFRP ratios without VMB at (a) 7 days and (b) 28 days and with VMB at (c) 7 days and (d) 28 days.
Figure 9. UCS of the samples with different foaming content and rGFRP ratios without VMB at (a) 7 days and (b) 28 days and with VMB at (c) 7 days and (d) 28 days.
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Figure 10. Thermal conductivity of the foamed geopolymer (a) without VMB and (b) with VMB.
Figure 10. Thermal conductivity of the foamed geopolymer (a) without VMB and (b) with VMB.
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Figure 11. Micrographs of (a) CG, (b) F10, (c) F10-R6, (d) F15-R6, (e) F20-R6, (f) F25-R6, (g) F10-R6-V, (h)VMB, and (i) porosity.
Figure 11. Micrographs of (a) CG, (b) F10, (c) F10-R6, (d) F15-R6, (e) F20-R6, (f) F25-R6, (g) F10-R6-V, (h)VMB, and (i) porosity.
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Figure 12. Pore distribution of (a) CG, (b) F10, (c) F10-R6, (d) F15-R6, (e) F20-R6, (f) F25-R6, and (g) F10-R6-V.
Figure 12. Pore distribution of (a) CG, (b) F10, (c) F10-R6, (d) F15-R6, (e) F20-R6, (f) F25-R6, and (g) F10-R6-V.
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Table
Table 1. Summary of Research Results on Foamed Geopolymer and Lightweight Geopolymer Mortar.
Table 1. Summary of Research Results on Foamed Geopolymer and Lightweight Geopolymer Mortar.
No.ReferencesMaterials28 d Compressive Strength (MPa)Density (g/cm3)Thermal
Conductivity (W/m·K)		Porosity
(%)
1G. Gu et al. [7]FA, GBFS, and Na2SiO3N/A1.71–1.87N/AN/A
2Y. Hu et al. [9]FA, ZSM-5 waste, and Ca(OH)26.80–12.900.74–1.39N/A29.4–41.8
3G. Liang et al. [8]GBFS, RHA Na2SiO3, and NaOH59.67–74.640.75–1.920.11–0.2929.1–75.2
4L. Su et al. [11]FA, GBFS, and Na2SiO31.2–2.2N/AN/A60.2–65.1
5A. Aziz et al. [16]EP, MK, Na2SiO3, and NaOH10.9–36.21.48–1.73N/A17.6–33.7
6O. Sengul et al. [17]EP, OPC, and sand0.1–28.80.70–2.020.13–0.6N/A
7K. Pasupathy et al. [20]FA, GBFS, and Na2SiO30.48–1.380.56–0.570.25–0.2867.2–74.1
8J. Shi et al. [21]MK, FA, and Na2SiO32.28–5.380.36–0.670.12–0.2020.3–50.2
9L.Y. Dwijayanti et al. [29]FA, abaca fiber, Na2SiO3, and NaOH44.65–65.50.19N/AN/A
10Y. Wang et al. [30]FA, PP fiber, sand, Na2SiO3, and NaOH0.97–7.200.50–1.720.10–1.09N/A
Note: RHA is rice husk ash, EP is expanded perlite, MK is metakaolin, and PP is Polypropylene.
Table
Table 2. Chemical Composition of Precursor Materials.
Table 2. Chemical Composition of Precursor Materials.
Chemical CompositionSiO2 (wt.%)Al2O3 (wt.%)FexOy (wt.%)CaO (wt.%)MgO (wt.%)SO3 (wt.%)Other (wt.%)
FA52.134.35.422.770.491.453.47
GBFS30.516.80.2838.98.82.332.39
Table
Table 3. The Proportion of the Foaming Agent.
Table 3. The Proportion of the Foaming Agent.
MaterialSDSWaterSPSPELA
Mass (g)1222841.21
Table
Table 4. Mixture Design of rGFRP Fiber-Reinforced Foamed Geopolymer and Lightweight Geopolymer Mortar.
Table 4. Mixture Design of rGFRP Fiber-Reinforced Foamed Geopolymer and Lightweight Geopolymer Mortar.
Sample SetFOArGFRP FiberAR FiberVMBFASlagCa(OH)2Na2SiO3NaOH
(g)(g)(g)(g)(g)(g)(g)(g)(g)
CG000050500134
F1010000454510134
F10-AR01
F10-R220
F10-R440
F10-R660
F-151500
F15-AR01
F15-R220
F15-R440
F15-R660
F202000
F20-AR01
F20-R220
F20-R440
F20-R660
F-252500
F25-AR01
F25-R220
F25-R440
F25-R660
F10-V100004545
F10-R2-V20104040
F10-R4-V40
F10-R6-V60
F-15-V1500
F15-R2-V20
F15-R4-V40
F15-R6-V60
F20-V2000
F20-R2-V20
F20-R4-V40
F20-R6-V60
F-25-V2500
F25-R2-V20
F25-R4-V40
F25-R6-V60
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Zhang, M.; Qiu, X.; Shen, S.; Wang, L.; Zang, Y. Mechanical and Thermal Insulation Properties of rGFRP Fiber-Reinforced Lightweight Fly-Ash-Slag-Based Geopolymer Mortar. Sustainability 2023, 15, 7200. https://doi.org/10.3390/su15097200

AMA Style

Zhang M, Qiu X, Shen S, Wang L, Zang Y. Mechanical and Thermal Insulation Properties of rGFRP Fiber-Reinforced Lightweight Fly-Ash-Slag-Based Geopolymer Mortar. Sustainability. 2023; 15(9):7200. https://doi.org/10.3390/su15097200

Chicago/Turabian Style

Zhang, Mo, Xinxin Qiu, Si Shen, Ling Wang, and Yongquan Zang. 2023. "Mechanical and Thermal Insulation Properties of rGFRP Fiber-Reinforced Lightweight Fly-Ash-Slag-Based Geopolymer Mortar" Sustainability 15, no. 9: 7200. https://doi.org/10.3390/su15097200

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