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Article

Advancing the Sustainability of Geopolymer Technology through the Development of Rice Husk Ash Based Solid Activators

by
Olga Andriana Panitsa
1,
Dimitrios Kioupis
1,2,* and
Glikeria Kakali
1
1
School of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou St., 15773 Athens, Greece
2
Engineering School, Merchant Marine Academy of Aspropyrgos, 19300 Athens, Greece
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(17), 7243; https://doi.org/10.3390/su16177243 (registering DOI)
Submission received: 9 July 2024 / Revised: 7 August 2024 / Accepted: 18 August 2024 / Published: 23 August 2024
(This article belongs to the Special Issue Sustainability in Construction Materials)
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Versions Notes

Abstract

:
Rice husk ash (RHA), an agricultural waste byproduct, has already been tested as a component in geopolymeric binders, typically as part of the precursor solid mix, alongside materials like fly ash (FA), slag, and cement. This study presents a novel approach where RHA is employed to create a solid activator, aimed at entirely replacing commercial sodium silicates. The synthesis process involves mixing RHA, NaOH (NH), and water by applying a SiO2/Na2O molar ratio equal to 1, followed by mild thermal treatment at 150 °C for 1 h, resulting in the production of a solid powder characterized by high Na2SiO3 content (60–76%). Additionally, microwave treatment (SiO2/Na2O = 1, 460 W for 5 min) increases the environmental and economical sustainability of alkali silicates production from RHA since this processing is 12 times faster than conventional thermal treatment reducing at the same time the final product’s embodied energy. The efficacy of this new material as a sole solid activator for the geopolymerization of Greek FA is investigated through various techniques (XRD, FTIR, SEM). One-part geopolymers prepared with RHA-based solid activators demonstrated mechanical performance comparable to those prepared with commercial products (~62 MPa at 7 days). This research contributes to the advancement of sustainable construction practices emphasizing the importance of local materials and reduced environmental impact in achieving long-term sustainability goals.

1. Introduction

Geopolymerization technology is increasingly recognized as a key solution for sustainable construction, addressing both environmental and socio-economic challenges [1,2,3,4]. Some of the reasons for geopolymers’ market growth are the products’ advantages, such as structure performance, lower greenhouse gas emissions, and resilience to acid and fire [2,5,6]. Moreover, the possibility of producing them using local industrial by-products and wastes, without performing numerous prior processes, offers a significant economic advantage [7,8]. Thus, the geopolymers market is expected to grow at a CAGR of 29.6% during the period of 2022–2030, reaching an anticipated value of USD 57,700 million by 2030 [9].
A new class of geopolymeric binders, called one-part geopolymer mixes, has been created over the last few years, to make it easier to handle silicate solution-activated geopolymers and enhance the technology’s sustainability [4,10,11]. Aluminosilicate precursors can be used to create one-part geopolymers by combining them with solid activators instead of corrosive alkali solutions [12]. More particularly, while silicate solutions are typically used as activators in geopolymer binders, in one-part mixtures, solid activators exist in a dry mixture, similar to Portland cement, and the reaction starts when water is added to the solid mix. By utilizing this method, corrosive and viscous solutions are avoided during bulk production, enhancing the economic viability of geopolymer binders [8,13,14].
More specifically, several commercial reagents, including Na2CO3 [15,16,17,18,19,20,21], NaAlO2 [20,22,23], CaSO4 [24], Na2SO4 [24], and solid sodium silicates with a range of moduli (SiO2/Na2O molar ratio) [21,25,26,27,28,29,30,31], have been investigated either individually as solid activators or as part of a combination of solid activators. Based on the final geopolymers’ mechanical characteristics, anhydrous sodium silicates have been found to be the most effective [30]. However, alkali silicates are produced under extreme conditions and characterized by high embodied energy and high manufacturing costs.
In order to conserve the main geopolymers’ characteristics, researchers have examined the production of solid activators using silicon-rich products [32]. For example, Marios Soutsos and Raffaele Vinai created a sodium silicate product, based on waste glass, that can be utilized as a solid activator in an alkali activation procedure [33]. For its production, a mild thermal treatment was followed (150 °C). Additionally, our team developed a solid activator based on silica fume, that was used as the only solid activator in the matrix for Greek FA geopolymerization [34]. The solid activator was subjected to heat treatment at 150 °C or brief microwave treatment (two minutes). The outcomes show that geopolymers with similar mechanical properties to those made from commercial products (~60 MPa) were produced when the developed solid activator was employed.
The introduction of RHA in geopolymeric binders is not new, as many researchers have used RHA as part of the precursor solid mix, combined with FA, slag, metakaolin, cement, etc. [1,35,36]. However, in this study, RHA is used to create a solid activator capable of totally replacing the sodium silicate commercial reagents. During the synthesis process, RHA, NH, and water are mixed. The slurry undergoes a light thermal treatment (150 °C) to generate a solid powder with a high percentage of Na2SiO3. In order to accelerate the production process and reduce the energy footprint of the finished waste-based product, the application of microwave treatment was also considered. Lastly, a number of techniques are used to examine the use of this novel material as the sole activator in the geopolymerization of Greek FA. The development of these RHA-based solid activators supports the transition to greener construction practices, offering a viable alternative to traditional cement-based materials and promoting resource efficiency.

2. Materials and Methods

Rice husk (RH), a significant by-product of the rice-processing industry, was sourced from a Greek food company (AGRINO, Agrinio, Greece). The RH underwent thermal treatment at 550 °C for 3 h, resulting in RHA, which was subsequently used to develop solid activators. FA—type F, an aluminosilicate by-product of coal combustion in thermal power plants, was obtained from the Megalopolis Power Plant in Greece. The chemical compositions of RHA and FA were determined using X-ray fluorescence spectroscopy (XRF) with an Epsilon 1 Model (Malvern Panalytical, Malvern, United Kingdom), as presented in Table 1. Both RHA and FA were finely ground, achieving mass median particle sizes (d50) of 76 μm and 20 μm, respectively, as measured by the Malvern Mastersizer Micro (see Figure 1).
For the development of solid activators, NH (≥98.0%, CAS No. 1310-73-2) was utilized. The performance of the prepared RHA-based solid activators (RSAs) was evaluated against commercial solid activators commonly used in geopolymer technology, namely sodium silicate—Na2SiO3, CAS No. 6834-92-0 (1SS_COM) and sodium disilicate—Na2Si2O5, CAS No. 6834-92-0 (2SS_COM).

2.1. Solid Activators Synthesis

For the RSA synthesis, precise amounts of RHA powder, NH, and water were combined to make a pulp. The amount of added water was enough to totally dilute the NH pearls and was determined by taking into account the water solubility of NH at 20 °C (109 g NH/100 mL H2O). The mixtures were then heated, either thermally or by using a conventional microwave oven, to produce dry powdery products.
Following the investigation method introduced in previous research papers [33,34], three parameters were investigated during thermal treatment: SiO2/Na2O molar ratio (1 and 2), treatment period (0.5, 1, 2, and 3 h), and temperature (150, 250, 330 and 450 °C). The use of a microwave oven for household use was studied as an alternate treatment approach. The examined parameters during microwave treatment were treatment time (5, 12, and 20 min), microwave power (460 and 700 W), and SiO2/Na2O molar ratio (1 and 2).
The parameters’ levels were decided based on the following:
  • The shortest treatment time was required for both treatment types to produce a dry product. More precisely, the temperature and time ranges were chosen to allow for the complete solubilization of the NH reagent and the dehydration of the reactants’ pulp.
  • The household microwave oven’s technical features limited the microwave power to certain values (120, 460 and 700 W).
  • The value range of SiO2/Na2O molar ratio was chosen taking into account the practice used so far in geopolymerization in relation to alkali silicate. In particular, SiO2/Na2O molar ratios with an upper limit value equal to 2 were applied, since alkali silicates prepared with higher values exhibit very low solubility in water and pH [31].
In Table 2, the mixtures’ composition, as well as the different synthesis parameters examined, are presented.
Lastly, the solubility of RSAs was determined by diluting 1 g of sample in 150 mL of water for 1 h while stirring, followed by filtering and weighing of the solid residue [37]. The solubility of RSAs is critical in one-part geopolymer synthesis, since high solubility indicates higher alkalinity in the pastes through the availability of hydroxyl groups and silicon species, maximizing the dissolution rate of the aluminosilicate precursor and the geopolymerization reactions.

2.2. Geopolymer Synthesis

FA and various proportions of RSAs were dry-mixed for 3 min using a mortar mixer (65-L0005 model, Controls S.p.A., Milan, Italy). Water was subsequently added to the mixture to attain the desired workability and form a consistent slurry. This slurry was then lightly compressed and cast into 50 × 50 × 50 mm cubic molds. The specimens were initially stored at room temperature for 2 h before being cured at 70 °C for 48 h. Compressive strength tests were performed on the seventh day post-casting, following ASTM C109 standards [38], using a uniaxial testing press (E181N model, Matest S.p.A., Arcore, Italy) with a loading rate of 1.5 kN/s [39]. The geopolymer synthesis methodology is described in detail in previous research paper [34,40], and it is based in the preparation procedures of ASTM C109 [38] and ASTM C305 [41].
For each mix, the average compressive strength of three specimens is reported. The development of the one-part geopolymer systems (G1P) was guided by a prior study on optimizing the synthesis of standard two-part geopolymers (G2P), as shown in Table 3 [39]. For FA geopolymerization, the optimized synthesis parameters were Si/Al = 2.2 and Na/Al = 1. These ratios were met using the 1SS activators. For the 2SS activator mixtures, additional NH was required to achieve these synthesis parameters, as indicated in Table 3.
For comparison, a sodium silicate solution composed of 28.0% wt. SiO2, 8.6% wt. Na2O, and 63.4% wt. H2O by Multiplass SA, Greece (Sol) was also used to create a two-part geopolymer, which served as a reference sample.

2.3. Characterization Techniques

Three analytical approaches were applied to properly characterize raw materials and products. A D8 ADVANCE x-ray diffractometer (Bruker, Billerica MA, USA), using CuKa radiation, was used for the X-ray diffraction (XRD) mineralogical characterization of raw materials and RSAs, under the following measuring parameters: 2θ range 10–70°, step size 0.1°/s. The RSAs crystallinity was determined by applying the TOPAS software (v 4.2). Prior to the measurement, zincate (commercial reagent, CAS No. 1314-13-2) was used in a known quantity (9.9% w/w) as a spike. Then, the XRD measuring parameters altered and were adjusted accordingly (2θ range 10–70°, step size 0.02°/s.). Diffrac Eva v3.1 software was used to analyze the data. A graphical output of the XRD quantitative analysis generated by TOPAS software is shown in Figure 2.
A Fourier transform IR spectrophotometer (Jasco 4200 Type A, JASCO Europe s.r.l., Cremella, Italy) was used to perform the FTIR measurements. When the KBr technique was performed, the FTIR spectra were produced with a resolution of 4 cm−1 and a wavenumber range of 400 to 4000 cm−1.
Finally, the solid activators were analyzed using scanning electron microscopy (SEM) on a SEM-EDAX/FEI Quanta 200 (FEI, Hillsboro OR, USA) equipped with a low-vacuum secondary electron (LFD) detector and a SEM-Jeol 6380LV (JEOL Ltd., Tokio, Japan) coupled with Oxford systems energy dispersive spectroscopy microanalysis system.
The aforementioned methods were used to test the commercial sodium silicate products with SiO2/Na2O molar ratios 1 and 2 as well.

3. Results

3.1. XRD Analysis

Figure 3 and Figure 4 display the XRD patterns of RSAs with SiO2/Na2O molar ratios of 1 and 2, respectively. Additionally, Table 4 details the quantities of crystalline and amorphous phases of these products, along with their water solubility.
RHA is characterized by its complete amorphousness, indicated by a broad peak at 2θ ≈ 21°, which corresponds to its amorphous silicon oxide content.
For RSAs with a SiO2/Na2O ratio of 1 (Figure 3), the RHA underwent alkaline fusion and transformed into a new product with increased crystallinity and a high sodium silicate (Na2SiO3) content, regardless of the production method and synthesis parameters. Their structure closely resembles that of commercially available 1SS_COM, with the primary difference being the variation in crystalline Na2SiO3 content. Moreover, the solubility of all products was very high (>96%), similar to the solubility of the commercial product 1SS_COM [34,37].
Furthermore, RSAs with a SiO2/Na2O molar ratio of 2 (Figure 4) exhibit complete amorphousness similar to the commercial product 2SS_COM. Both samples prepared with the aforementioned ratio exhibit a shift in the peak of RHA amorphous phase to 2θ angles ≈ 29°. This shift suggests the incorporation of sodium and silicon into a new amorphous phase. However, solubility tests revealed that the RSAs have a very low solubility (<15%) compared to the fully water-soluble commercial product (2SS_COM). This indicates that the amorphous phases formed during the processing of RSAs differ in chemical composition from those of 2SS_COM [34].
Concerning the effect of synthesis parameters on the thermally treated RHA-NH pulps, the results showed that a maximum crystalline Na2SiO3 content (76%) can be achieved at 330 °C. Higher processing temperature does not affect the crystallinity of the final product. The impact of the curing temperature on the crystallinity seems to be related to the melting point of NH (323 °C) [33]. By varying the processing time, it was seen that after one hour of processing, the crystallinity of the final product was maximized, while after two and three hours of processing, the crystallinity decreased by 13% in total.
During experimentation with microwave processing, it was found that a dry product can be obtained from the initial pulp after treatment for only 5 min at 460 W (M). An increase in microwave power from 460 W (M) to 700 W (H) resulted in solid activators of enhanced crystallinity (70.7%). A possible explanation of this phenomenon can be related with the suppression of NH melting point due to the nanosized NH particles deposited on RHA grains surface after the water evaporation [33]. Similarly to the thermally treated pulps, an increase of processing time, from 5 to 12 min, led to the enhancement of products’ crystallinity, while higher processing times did not affect the crystallinity of the products.
Comparing the processing methods, it can be observed that thermal processing can achieve solid activators of higher crystalline Na2SiO3 contents (59.9–76.1%) in relation to microwave processing (53.9–70.7%).

3.2. FTIR Analysis

Figure 5 displays diagrams for all RSAs with a SiO2/Na2O molar ratio of 1. For comparative purposes, diagrams of 1SS_COM and RHA samples are also included. RHA demonstrates characteristic vibrations of Si-O bonds. In particular, the asymmetric and symmetric stretching vibrations of Si-O-Si bonds are linked to the broad absorption peak with a center at 1100 cm−1 and the peak at 804 cm−1, respectively, whereas the bending bond vibrations of amorphous silicon are related with the peak at 468 cm−1 [42].
The 1SS_COM shows absorption peaks at 1040, 970, 880, 710, 590, and 515 cm−1. These peaks are ascribed to different vibrations of the O-Si-O and Si-O-T (T stands for Si or Na) bonds inside the sodium silicate network [34]. Additionally, the presence of the carbonate phases in 1SS_COM is indicated by the absorption at 1450 cm−1 [43,44].
The spectra of all RSAs with SiO2/Na2O = 1 demonstrate the successful conversion of the initial reactants into sodium silicate-rich materials, regardless of the processing conditions. Notably, the RSAs exhibit infrared spectra identical to that of the commercial material (1SS_COM). It is worth mentioning that the peaks of the RHA spectra are totally diminished in the RSA samples, indicative of its complete conversion to sodium silicate products. The development of sodium silicate is primarily evidenced by the shift of the 1100 cm−1 peak (Si-O-Si) to lower wavenumbers, reflecting the higher incorporation of Na by Si-O-T bonds, and the emergence of a ‘trident’ peak pattern at 1040, 970, and 880 cm−1.
The diagrams of RSAs prepared with SiO2/Na2O = 2 are presented in Figure 6, alongside the diagrams of 2SS_COM and RHA. The 2SS_COM sample exhibits stretching vibrations of asymmetric and symmetric nature of Si-O-T bonds at 1000 and 880 cm−1, respectively [34]. Moreover, the peak at 755 cm−1 is associated with the tension of the O-Si-O bond [1]. The spectrum of the commercial product lacks sharp peaks, highlighting its amorphousness, an observation that is consistent with the results of XRD quantitative analysis. The peak at 1450 cm−1 is ascribed to the vibrations of carbonate ions similarly to 1SS_COM.
The spectra of the RSAs with a SiO2/Na2O molar ratio of 2 seem to obtain features from both 2SS_COM and RHA samples. Indeed, the spectra of RSAs with a SiO2/Na2O molar ratio of 2 exhibit the absorption peaks at 880 and 755 cm−1 that are found in the spectrum of 2SS_COM, but, at the same time, the absorption area at around 1375–800 cm−1 is wider and centered to higher wavenumbers, indicating the presence of Si-O-Si bonds of the RHA material. Furthermore, the sample processed through conventional thermal treatment (RHA_2SS_TT_330C_1h) presents the bending vibrations of amorphous silicon bonds of RHA indicating that its conversion yield is lower than that of the microwave treated sample (RHA_2SS_MT_M_M_5m). The aforementioned observations suggest that the conversion of RHA to silicate compounds is partial when a SiO2/Na2O molar ratio equal to 2 is applied. This conclusion is further supported by the solubility tests of products RHA_2SS_TT_330C_1h and RHA_2SS_MT_M_M_5m, which showed low solubility in water, indicating the presence of unreacted silica in the products.
As previously mentioned, almost all products, regardless of the SiO2/Na2O molar ratio and processing conditions, contain carbonates. This is confirmed by the presence of a peak at 1450 cm−1. This phenomenon can be attributed to the hygroscopic nature of the alkali silicates [37]. However, the presence of carbonates was not detected in the XRD patterns of the RSAs, likely due to their amorphous nature or their low concentration, which falls below the detection limit of the measurement. Figure 5 and Figure 6 illustrate that products prepared by heat treatment exhibit sharper peaks compared to those prepared by microwave treatment. This difference can be attributed to the longer exposure times of the samples produced in the furnace compared to those prepared in the microwave oven.

3.3. Microstructural Analysis

Figure 7 presents SEM images of both commercial and RHA-based solid activators alongside the RHA material. The microstructure of RHA is characterized by a highly amorphous silica matrix consisting of irregularly shaped particles in the form of fragmented flakes [45]. A closer look reveals the skeleton-like inner structure of the heat-treated material, also reported in previous studies [46]. The 1SS_COM sample and which consists entirely of sodium silicate (Na2SiO3), is a product with irregularly shaped particles and angular surfaces. The particle size varies between 20 and 70 μm, while a closer investigation shows that the texture of the particles is relatively dense. These morphological characteristics cope well with a material of crystalline nature, where a more highly polymerized silicate network with more bridging oxygen atoms is formed. The commercial activator, with a molar ratio SiO2/Na2O equal to 2, consists of small, deformed, disc-shaped or slightly more spherical particles of with a particle size of 40–120 μm. The amorphous nature of 2SS_COM justifies the existence of particles with smoother surfaces. Of course, the irregularity in the texture of the commercial products with varying SiO2/Na2O molar ratio can also be a result of the type of the production process followed.
The morphology of the RHA_1SS_TT_150C activator, obtained through conventional thermal treatment, shows similarities to that of the corresponding commercial product (1SS_COM). In particular, the particles of the sample have a polygonal shape exhibiting more or less the same particle size as 1SS_COM. Therefore, the alkaline fusion is assumed to be successful.
The RHA_1SS_MT_M_5m sample, obtained through microwave treatment, is characterized by a higher porosity compared to its counterpart obtained through conventional thermal treatment. This fact is closely related to the faster evaporation of water during microwave treatment of the initial pulp. Indeed, the samples prepared by microwave processing swelled strongly inside the oven, forming a ‘dome’ shape at the end of the treatment. The aforementioned explains the formation of pores inside these samples. Summarizing, microwave treatment produces RSAs with less rigid structure that can be more reactive in relation to those produced by conventional thermal treatment.
It must be noted that both RSAs prepared with SiO2/Na2O = 1 (RHA_1SS_TT_150C and RHA_1SS_MT_M_5m) demonstrate distinct differences in their microstructure in relation to the maternal material (RHA) indicating major network reorganization.
The RSA prepared with a SiO2/Na2O = 2 (RHA_2SS_TT_150C) showed a completely different morphology in relation to the 2SS_COM enhancing the results of the characterization analysis. In particular, RHA_2SS_TT_150C sample consists of flake-shaped particles 20–80 μm in size, which do not resemble the disc-shaped particles of the corresponding commercial activator. On the contrary, the particles’ shape shows more similarities with that of the RHA. Therefore, the assumptions emerged from the characterization analysis regarding the partial conversion of the raw material into alkali silicate with SiO2/Na2O = 2 are supported by SEM analysis.

3.4. Compressive Strength

The RSAs chosen for one-part geopolymer production were based on their minimal energy requirement during preparation. Thus, the samples used for geopolymer synthesis were RHA_xSS_TT_150C_1h and xSS_MT_M_5m (where x = SiO2/Na2O = 1 or 2). It is important to note that the quantities of RSAs required were calculated assuming they consist entirely of sodium silicate phases.
Figure 8 shows the mechanical performance of the geopolymers that were prepared in terms of uniaxial compressive strength (averaged over three specimens). Geopolymers synthesized using the traditional activating solution (G2P) achieved a compressive strength of 62.3 MPa. The substitution of the activating solution with commercial solid activators (1SS_COM or 2SS_COM) yielded products with comparable mechanical strength, regardless of the activators’ SiO2/Na2O molar ratio. Further substitution of commercial solid activators with RSAs yielded promising results. Geopolymers produced using activators with a SiO2/Na2O molar ratio of 1, regardless of the processing method, achieved 90 to 100% of the reference (G2P) sample compressive strength. The slightly lower mechanical strength could be attributed to the presence of carbonate phases, which may reduce the reactivity of the RSA.
The geopolymers using the RSAs, with a SiO2/Na2O molar ratio equal to 2, despite the processing method, showed a remarkably lower compressive strength, compared to the reference geopolymer G2P. As presented in the characterization section these activators were not successfully prepared and they even have a very low water solubility, due to the increased SiO2/Na2O molar, reflecting in a low alkalinity of the geopolymer pastes.

4. Conclusions

This paper explores the development of eco-friendly, rice husk ash-based solid activators for the one-part (just add water) geopolymer synthesis. The goal was to enhance the sustainability of geopolymer technology by substituting the traditional activators of high embodied energy with solid mixtures prepared in mild conditions from wastes and by-products.
This research has led to the following conclusions
  • A simple synthesis procedure for the preparation of RHA-based solid activators was developed. This procedure involves the mixing of RHA and NH with water to form a pulp and then the treatment of the pulp by either conventional or microwave heating at mild conditions. Indeed, solid activators were successfully prepared by thermal or microwave processing after treatment at 150 °C for 1 h or 460 W for 5 min, respectively.
  • The alternative method of microwave treatment is 12 times faster than the average time of conventional heat treatment in a laboratory oven and, therefore, more economically and environmentally advantageous.
  • The SiO2/Na2O molar ratio proved to be a crucial factor for the successful preparation of RHA based solid activators. The application of SiO2/Na2O molar ratio equal to 1, successfully converts RHA to activators mainly consisting of crystalline Na2SiO3 (~60–76%) that obtain similar water solubility (>97%), microstructure and branched silica network to the commercial sodium silicate product. Higher SiO2/Na2O molar ratios did not succeed to achieve high conversion yields of RHA.
  • The other synthesis parameters (temperature, microwave power and time of processing) do not significantly differentiate the activators’ composition. However, they affect the Na2SiO3 crystalline content. In general, conventional thermal processing produces activators of higher crystalline Na2SiO3 contents (59.9–76.1%) in relation to microwave processing (53.9–70.7%).
  • The type of processing had a great impact on the microstructure of the produced solid activators. Microwave processing leads to more porous activators that can be more reactive compared to their counterpart obtained through conventional thermal treatment.
  • The efficiency of the developed solid activators was tested through the preparation of geopolymer pastes from FA. The results showed that the RHA activators can effectively replace both the conventional activation solution and the commercial solid activators (sodium silicates and disilicates) of the geopolymer technology, since the prepared geopolymer pastes achieve 90 to 100% of their compressive strength values (62 MPa).
The investigation of the microwave heating mechanisms taking place during the preparation of RHA based solid activators will be the next step of our research to efficiently improve their production process.

Author Contributions

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

Funding

This research was co-funded by Greece and the European Union (European Social Fund- ESF) by the Operational Programme Human Resources Development, Education and Lifelong Learning 2014–2020, grant number MIS 50 5049183.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Particle size distribution of and RHA (a) and FA (b).
Figure 1. Particle size distribution of and RHA (a) and FA (b).
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Figure 2. Graphical output of a full-profile fit, from TOPAS software, for XRD quantitative phase analysis of RHA_1SS_TT_450C_1h. Blue line: the experimental XRD pattern, red line: the fitted pattern after the end of the analysis, gray line: the difference between experimental and fitted XRD pattern.
Figure 2. Graphical output of a full-profile fit, from TOPAS software, for XRD quantitative phase analysis of RHA_1SS_TT_450C_1h. Blue line: the experimental XRD pattern, red line: the fitted pattern after the end of the analysis, gray line: the difference between experimental and fitted XRD pattern.
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Figure 3. XRD patterns of RSAs prepared with a SiO2/Na2O ratio = 1 through thermal (TT) and microwave (MT) processing. The patterns of the commercial product (1SS_COM) and RHA are also presented in the diagram.
Figure 3. XRD patterns of RSAs prepared with a SiO2/Na2O ratio = 1 through thermal (TT) and microwave (MT) processing. The patterns of the commercial product (1SS_COM) and RHA are also presented in the diagram.
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Figure 4. XRD patterns of RSAs prepared with ratio SiO2/Na2O = 2 through thermal (TT) and microwave (MT) processing. The patterns of the commercial product (2SS_COM) and RHA are also presented in the diagram.
Figure 4. XRD patterns of RSAs prepared with ratio SiO2/Na2O = 2 through thermal (TT) and microwave (MT) processing. The patterns of the commercial product (2SS_COM) and RHA are also presented in the diagram.
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Figure 5. FTIR spectra of RSAs prepared with SiO2/Na2O ratio = 1 through thermal (TT) and microwave (MT) processing. The spectrum of the commercial product (1SS_COM) and RHA are also presented in the diagram.
Figure 5. FTIR spectra of RSAs prepared with SiO2/Na2O ratio = 1 through thermal (TT) and microwave (MT) processing. The spectrum of the commercial product (1SS_COM) and RHA are also presented in the diagram.
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Figure 6. FTIR spectra of RSAs prepared with SiO2/Na2O ratio = 2 through thermal (TT) and microwave (MT) processing. The spectrum of the commercial product (2SS_COM) and RHA are also presented in the diagram.
Figure 6. FTIR spectra of RSAs prepared with SiO2/Na2O ratio = 2 through thermal (TT) and microwave (MT) processing. The spectrum of the commercial product (2SS_COM) and RHA are also presented in the diagram.
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Figure 7. SEM micrographs of different magnification of commercial and selected RSAs alongside RHA material.
Figure 7. SEM micrographs of different magnification of commercial and selected RSAs alongside RHA material.
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Figure 8. Compressive strength of geopolymers prepared by the use of RSAs prepared in this study. The mechanical performance of the geopolymers prepared by activation solution (G2P) and commercial sodium silicates (G1P_1SS_COM and G1P_2SS_COM) are also presented for comparison.
Figure 8. Compressive strength of geopolymers prepared by the use of RSAs prepared in this study. The mechanical performance of the geopolymers prepared by activation solution (G2P) and commercial sodium silicates (G1P_1SS_COM and G1P_2SS_COM) are also presented for comparison.
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Table 1. Raw materials’ chemical composition (% w/w).
Table 1. Raw materials’ chemical composition (% w/w).
SiO2Al2O3Fe2O3CaOMgOK2ONa2OSO3TiO2P2O5MnOL.O.I.
FA40.6020.583.3311.611.190.720.230.900.790.260.0616.70
RHA90.410.000.061.043.390.520.000.330.001.070.154.20
Table 2. Samples ID and synthesis parameters of the RSAs.
Table 2. Samples ID and synthesis parameters of the RSAs.
ID of RSAsRHA
(% w/w)		NH
(% w/w)		H2O
(% w/w)		SiO2/Na2O Molar RatioTreatment MethodTemperature (°C)Power (watt)Treatment Duration
RHA_1SS_TT_330C_0.5h30.436.632.91Thermal330 0.5 h
RHA_1SS_TT_330C_1h30.436.632.91Thermal330-1 h
RHA_1SS_TT_330C_2h30.436.632.91Thermal330-2 h
RHA_1SS_TT_330C_3h30.436.632.91Thermal330-3 h
RHA_1SS_TT_150C_1h30.436.632.91Thermal150-1 h
RHA_1SS_TT_250C_1h30.436.632.91Thermal250-1 h
RHA_1SS_TT_450C_1h30.436.632.91Thermal450-1 h
RHA_2SS_TT_330C_1h46.628.125.32Thermal330-1 h
RHA_1SS_MT_M_5m30.436.632.91Microwave-4605 min
RHA_1SS_MT_M_12m30.436.632.91Microwave-46012 min
RHA_1SS_MT_M_20m30.436.632.91Microwave-46020 min
RHA_1SS_MT_H_5m30.436.632.91Microwave-7005 min
RHA_1SS_MT_H_12m30.436.632.91Microwave-70012 min
RHA_2SS_MT_M_5m46.628.125.32Microwave-4605 min
Abbreviations: 1SS, SiO2/Na2O molar ratio equal to 1; 2SS, SiO2/Na2O molar ratio equal to 2; TT, thermal treatment; MT, microwave treatment; 150–450 °C, treatment temperature; 5 m–3 h, treatment duration in minutes or hours; M, H, microwave power (medium, 460 W; high, 700 W).
Table 3. Geopolymer mixtures developed in this study (% w/w).
Table 3. Geopolymer mixtures developed in this study (% w/w).
MIX IDFASol1SS_COM2SS_COMRSAsNHH2O
1SS2SS
G2P60.414.8----9.814.9
G1P_1SS_COM61.3-15.3----23.4
G1P_2SS_COM63.1--11.6--5.220.0
G1P_RHA_1SS_TT_150C63.4---15.8--20.8
G1P_RHA_2SS_TT_150C57.8----10.64.826.8
G1P_RHA_1SS_MT_M63.9-- - 16.0--20.1
G1P_RHA_2SS_MT_M58.0----10.74.826.5
Abbreviations: G, geopolymer sample; 1P, one-part; 2P, two-part; 1SS, SiO2/Na2O molar ratio equal to 1; 2SS, SiO2/Na2O molar ratio equal to 2; TT, thermal treatment; MT, microwave treatment; 150C, treatment temperature (150 °C); M, microwave power (medium, 460 W).
Table 4. Mineral composition and water solubility of commercial solid activators, waste based solid activators and RHA.
Table 4. Mineral composition and water solubility of commercial solid activators, waste based solid activators and RHA.
Sample IDNa2SiO3 (%)Amorphous (%)Solubility (%) *
Commercial
1SS_COM50.949.199.1
2SS_COM-100.099.2
Thermally Treated
RHA_1SS_TT_330C_0.5h59.940.198.3
RHA_1SS_TT_330C_1h76.123.997.1
RHA_1SS_TT_330C_2h67.332.797.9
RHA_1SS_TT_330C_3h66.933.198.5
RHA_1SS_TT_150C_1h66.833.298.3
RHA_1SS_TT_250C_1h68.931.198.2
RHA_1SS_TT_450C_1h75.524.596.5
RHA_2SS_TT_330C_1h-100.010.1
Microwave-treated
RHA_1SS_MT_M_5m56.843.295.9
RHA_1SS_MT_H_5m58.042.099.2
RHA_1SS_MT_M_12m57.142.999.6
RHA_1SS_MT_H_12m70.729.398.7
RHA_1SS_MT_M_20m53.946.198.7
RHA_2SS_MT_M_5m-100.015.2
RHA-100.06.5
* % sample percentage after dissolving 1 g of sample in 150 mL of water.
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Panitsa, O.A.; Kioupis, D.; Kakali, G. Advancing the Sustainability of Geopolymer Technology through the Development of Rice Husk Ash Based Solid Activators. Sustainability 2024, 16, 7243. https://doi.org/10.3390/su16177243

AMA Style

Panitsa OA, Kioupis D, Kakali G. Advancing the Sustainability of Geopolymer Technology through the Development of Rice Husk Ash Based Solid Activators. Sustainability. 2024; 16(17):7243. https://doi.org/10.3390/su16177243

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Panitsa, Olga Andriana, Dimitrios Kioupis, and Glikeria Kakali. 2024. "Advancing the Sustainability of Geopolymer Technology through the Development of Rice Husk Ash Based Solid Activators" Sustainability 16, no. 17: 7243. https://doi.org/10.3390/su16177243

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