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Article

The Influence of Substitution of Fly Ash with Marble Dust or Blast Furnace Slag on the Properties of the Alkali-Activated Geopolymer Paste

by
Brăduţ Alexandru Ionescu
1,
Alexandra-Marina Barbu
2,3,*,
Adrian-Victor Lăzărescu
1,*,
Simona Rada
4,
Timea Gabor
4 and
Carmen Florean
1,4
1
NIRD URBAN-INCERC Cluj-Napoca Branch, 117 Calea Floresti, 400524 Cluj-Napoca, Romania
2
NIRD URBAN-INCERC, 266 Sos. Pantelimon, 021652 Bucharest, Romania
3
Doctoral School, Technical University of Civil Engineering Bucharest, 122-124 Lacul Tei Bvd., 020396 Bucharest, Romania
4
Faculty of Materials and Environmental Engineering, Technical University of Cluj-Napoca, 103-105 Muncii Boulevard, 400641 Cluj-Napoca, Romania
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(2), 403; https://doi.org/10.3390/coatings13020403
Submission received: 18 November 2022 / Revised: 4 February 2023 / Accepted: 8 February 2023 / Published: 10 February 2023
(This article belongs to the Special Issue Recent Progress in Sustainability and Durability of Concrete and Mortar Composites)
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Versions Notes

Abstract

:
Worldwide, it is now known that industrial by-products rich in silicon (Si) and aluminum (Al) can be transformed by alkaline activation into so-called “green concrete”, an efficient and sustainable material in the field of construction; the geopolymer material. In this work, geopolymer materials produced using fly ash and marble dust or blast furnace slag were studied to assess the influence of these substitutions on the performances of the final product. Geopolymer materials have been characterized by physico-mechanical methods, FTIR spectroscopy and microscopically. The analysis of the results indicates the reduction of the mechanical strength performance by substituting the fly ash as the raw material.

1. Introduction

Fly ash (FA) is a by-product resulting from the coal burning in thermal power plants. Millions of tons of fly ash are produced globally. Fly ash is partially eliminated in the dumps and waste storage, but also in the atmosphere, when stored unproperly. Current policies at the global level require, on the one hand, the reduction of pollutant emissions, and on the other hand, the identification of new possibilities for sustainable use of non-renewable raw materials, with special emphasis on the implementation of the main concept regarding sustainable development of construction Materials Circular Economy [1,2,3,4,5]. The use of fly ash as a raw material in the production of new, “ecological concrete”, by exploiting the alkaline geopolymerization mechanism, actively and efficiently responds to the need for sustainable development and the implementation of the Circular Economy principles. Moreover, when compared to the ordinary Portland cement concrete, one of the advantages of heat-treated geopolymers are a shorter gain of mechanical properties’ time (generally seven days), while for cementitious composites, the hydrolysis-hydrolysis reactions of cement are continuous [1,2]. The resulting geopolymer product has a three-dimensional structure, similar to that of natural silico-aluminous materials [1,2,3].
The ecological concrete concept includes a whole series of new techniques, the common feature of which is to minimize the impact on the environment, either directly in the manufacturing process, or indirectly, contributing through its use to the reduction of other types of errors, as well as reducing costs. Another common element is the nature of the performance of these techniques, which must ensure similar mechanical behavior to the technique they replace to be validated, as this will affect durability and subsequent conservation costs.
There is no doubt that recently, and increasingly in the future, the recycling of construction products is promoted by public administrations, both for economic and environmental reasons [1,2,3,4,5].
Ecological concrete plays an important role in the field of civil engineering. This technology can effectively improve the internal structure and improve the mechanical properties and durability of the material. Among the many advantages of using it in construction are the following: reduced costs for maintenance and repair, increased life of structures with lower costs, cement economy by not justifying such a large number of new constructions, reduction of green-house gas emissions, use of cheap and environmentally friendly materials, local (fly ash, blast furnace slag), as well as reducing the consumption of non-renewable resources. The reduction of costs will be made based on increasing the period between two repairs, without the safety of the structures suffering. Moreover, it will reduce the environmental problems related to the intervention works and the waste resulting from this activity [1,2,3,4,5].
The geopolymerization process is a heterogeneous chemical reaction between a silico-aluminum solid material and a hydroxydic solution, strongly alkaline [2] and is based, according to Duxon et al., [3] on the following stages: dissolution, oligomerization and geopolymerization. The alkaline elements involved in the geopolymerization process (Na and K) have the role of generating a pH high enough to activate the solid material in the reaction and balancing the chemical species formed in the mold of the silico-aluminous gel [4]. Dissolution is the stage at which the Si-O-Si, Al-O-Al and Al-O-Si bonds are broken. The above-mentioned chemical bonds exist in the source material of solid aluminosilicate (fly ash (FA), used as a raw material). The release of silicate and aluminium elements occurs into the liquid phase, most likely in the form of monomers [3,5]. The oligomerization process consists in the formation of oligomers, which are molecules that form the 3D networks of the geopolymer binders. These elements are considered the main binding units that produce the geopolymerization process [3,6]. When polycondensation occurs, during the stage of oligomerization, the coagulated structures of the geopolymer are formed by the dissolution of the monomers. During this process, several different structures are formed: tetramers, dimers, trimmers, and larger molecules of the polymeric covalent bond [3], called oligomers. Several oligomers that form the geopolymer binder are Poly (siloxane) Si-O-Si-O, Si-O-Al-O, and poly (silate-disiloxo) Si-O-Al-O-Si-O-Si-O [5]. According to Koleżyński et al., 2018 [7], 2 or more basic (primary) oligomers built from atoms of Si, Al, O, Na and H, can connect by the means of 2, 3 or more oxygen bridges, forming complex oligomers; the geopolymer structural models made can reach about 200 atoms, having different Si:Al ratios: 4, 5, 6 and 10 [7]. Geopolymerization is the stage at which a rearrangement and binding of oligomers takes place, leading to the formation of a three-dimensional aluminosilicate network which further generates the formation of the geopolymer binder [3]. A structural geopolymer model may include more than 800 atoms with a Si:Al ratio of 2.81 [7]. In general, the chemical formula of a geopolymer is Mn[-(SiO2)Z-AlO2]nwH2O, where M is a cation of Na, K, Ca, or Li; n is the degree of polycondensation; w represents the number of water molecules in the system and z is 1, 2, 3, or a number much greater than 3 [8]. Therefore, it is currently known that a parallel cannot be drawn between the mechanism for strengthening the grout that underlies the understanding of the structure of cement stone and concrete and the mechanism of geopolymer formation that underlies the understanding of the structure of the so-called “green concrete”. If in the case of cement paste the hardening occurs through hydration—hydrolysis reactions of calcium-based compounds, silicates, and aluminates, in the case of geopolymer paste, the calcium oxide component does not have a significant contribution, the main components of the raw material really valuable in this case being the suppliers of Si and Al.
Literature has reported numerous studies in which the geopolymerization mechanism process was assessed using different analytical and instrumental techniques [5,9,10]. FTIR analysis of the geopolymer binder, at different time intervals and different curing methods, is considered an optimal tool to identify the gelling mechanism of the geopolymer binder. Several researchers [5,9,10,11] have studied the geopolymerization process using different raw materials in the alkali-activation process and different curing stages. The formation of the geopolymer gel in two stages was initially suggested by Fernandez-Jiménez et al. [12]. Results have shown that the geopolymerization process began with the formation of an aluminum-rich gel because of the dissolution of the weaker Al-O bond, when compared to the Si-O bonds. This mainly happened because the concentration of Si was higher than Al and its leaching began later due to the more stable Si-O connection. The Si-O bonds present in the second phase were substantially higher, which led to the geopolymerization. In terms of FTIR analysis, studies have shown that the geopolymerization process begins with the initial occurrence of a peak at 1003 cm−1, which represents the formation of the aluminum-rich gel (gel A) [10]. Afterwards, the gel is converted into a gel rich in Si (gel B) represented by the appearance of a peak at 1018 cm−1. The current mechanism was found consistent with the modelling study of the geopolymerization process assessed in the current study. Similar results were provided by other studies in the literature, using more precisely techniques such as ATR-FTIR (attenuated total reflection) and FTIR techniques [9,10].
To study the chemical bonds of Si, Al, O, H and alkaline cations, as important parameters of geopolymers, Fourier Transformation Infrared Spectroscopy (FTIR) were used to assess the formation and microstructure of geopolymer gels. FTIR is a non-destructive method used for the characterization of materials’ microstructure by measuring the infrared spectrum of absorption of a solid, gas or liquid in the range of 4000−400 cm−1 [13]. Generally, most of the bonds occur in FITR models in wide bands or in the form of hangers [13]. FTIR bands regarding Si-O-T bonds (T is Si or Al) show an asymmetrical vibration band in the range 900–1300 cm−1 [14,15,16,17,18,19]. This is usually attributed to the main strip of geopolymer gels. The main band can be attributed to the raw materials used in the production of the geopolymer binder, since they are also rich in Si and Al, with wave numbers close to 1000 cm−1. Analyzing the adjustment of the peaks and the deconvolution of the main peaks provided valuable information about the nature and extent of the link in geopolymers [14,15,16,17]. Studies have shown that the wave number of the main band in geopolymers is lower than the one characterizing the raw materials [9,10,14,15,16,17]. Therefore, the main bands of raw materials and geopolymers are always compared with each other in terms of this parameter, in order to assess the formation of geopolymer gels [14,15]. The main band moves to a lower wave number when the geopolymer is formed and this change can be seen in the FTIR analysis with result changes from 1054, 1080 or 1100 cm−1 in raw materials at a wave number less than 1000 cm−1 [14,20,21]. In terms of geopolymer binder production, another band can be formed around 840–900 cm−1 and it can be assigned to the formation of the Si-OH link. This confirms the formation of the geopolymer gel by its appearance, being observed only in geopolymers, but not in raw materials [14,22,23,24]. The FTIR peaks commonly seen in geopolymers include analyzed bands with a range between 1440 cm−1 (which represent sodium bicarbonate), around 3500 and 1600 cm−1 (for water) and O-H bonds. During the geopolymerization process, the sodium bicarbonate is formed when Na+ and atmospheric CO2 react. This occurs when the geopolymer samples are exposed to air [12,24,25]. Subsequently, water is removed through the pores or absorbed by the surface. The decrease in water strips phenomenon can be used to observe the hardening mechanism of geopolymer samples [12,24]. Results obtained in the literature regarding the geopolymerization process, analyzed using FTIR analysis, are presented in Table 1. Although results in the literature are consistent, there are several contradictions regarding the interpretation of FTIR bands. Symmetrical Al-O bonds were assigned to several values [14,23,24]. Reports from the literature, in agreement with the fact that the appearance of the bands centered around 870 cm−1, can be used to indicate the formation of geopolymer gels [23,24].
The Circular Economy, an integral part of the concept of sustainable development, is based on a series of principles that can be summarized in the 4Rs (Reduce, Reuse, Recycle, Recover) [35,36,37,38]. However, it should be noted that during the evolution of this concept there has been a correlation with several other principles that underpin European good practice, one of which is found in international environmental law, the Precautionary Principle (PP) [35,36,37,38,39]. At present, it is not possible to talk about the successful implementation of the Circular Economy without considering contextual links with guiding principles of international environmental law [40,41]. Moreover, the implementation of the Circular Economy cannot be achieved in a single sector of activity and, moreover, this approach would not be successful because if in one sector of activity the product is waste or an industrial by-product, in another sector the same product may be a valuable and under-exploited raw material. An example of this is blast furnace slag, a useful raw material in the development of geopolymer mixtures, but currently seen as a huge polluting waste dump. In the case of Romania, a specific case that supports this example is that of the SOMETRA non-ferrous metal plant, Copsa Mică, Sibiu County, which, at the time of sale, had a clause in the contract requiring the buyer to “green the slag waste dump”. This contractual obligation concerned “the greening of the current landfill by waterproofing the slopes, covering it with a layer of clay, covering it with a layer of fertile soil and fixing it with a vegetable carpet” [42,43,44,45,46], followed by “the construction of a new ecological landfill on a new site with waterproofing of the substrate, a drainage and collection system for the rainwater percolating down the ramp, a neutralization and dewatering station for the resulting water and its discharge into a surface watercourse” [42,43,44,45,46]. According to Mangau et al. [35], respecting the principle of prevention would have required a risk analysis of the complex situation in this case, since, in addition to the main waste, blast furnace slag, there are several other atmospheric, soil and/or water pollutants, the entire ecosystem being affected, and there is also a major risk factor in terms of the health of the population [42,43,44,45,46]. When considering slag properties, it is of interest to analyze its chemical composition to assess the possibility of using this waste as a raw material in the production of alkali-activated geopolymer materials, namely the so-called “green concrete”.
Similarly, fly ash, waste from thermal power plants, with its specific chemical composition, induces several environmental problems and storage costs [47,48,49,50,51,52,53,54,55]. Its contamination with various elements such as arsenic, barium, beryllium, boron, cadmium, chromium, thallium, selenium, molybdenum and mercury or traces of heavy metals, make it difficult to store it safely to eliminate the risk of contamination of soil or groundwater. Research in recent years has also focused on evaluating the possibilities of recycling this waste by using it in geopolymer composites [47,48,49,50,51,52,53,54,55]. In Romania there are several thermal power plants, most of which are currently equipped with electrostatic precipitators that have the capacity to filter the fly ash removed with the flue gases (e.g., Govora Power Plant, Vâlcea County, Romania) [47]. This, once again, supports the possibility and the need to identify more and more possibilities to consider this product as a raw material and not as waste.
Therefore, also considering the specifics of the local problem and the chemical characteristics of the Romanian raw materials, the aim of this research is to analyze the importance of the chemical composition of the raw materials used in the preparation of alkali-activated geopolymers, on the polymerization reaction products, on the microstructure and on the physical—mechanical performance of the binder.

2. Materials and Methods

2.1. Materials

Raw materials from Romania were used to produce the alkali-activated geopolymer binder. The raw material used as the main geopolymerization material was a low-calcium fly ash, obtained from the Rovinari Power Plant, Gorj County, Romania.
In order to assess to possibility of using other waste/by-products in the production of geopolymer materials, fly ash was partially substituted with marble dust (MD), obtained from the Rușchița marble deposit, Caraș-Severin County, Romania and blast furnace slag (BFS), obtained from the ArcelorMittal Steel Factory, Galați County, Romania. The waste samples were sieved to obtain a maximum particle size of 0.063 mm. The chemical composition of the raw materials used in the mixtures was established by X-ray fluorescence analysis (XRF analysis) (Table 2).

2.2. Methods

The alkaline activator used in the production of the geopolymer samples was prepared using 8M sodium hydroxide solution (NaOH) and aqueous solution of sodium silicate (Na2SiO3) with a concentration of 35%–40%. The mass ratio between the two solutions was Na2SiO3/NaOH = 2.5. The NaOH solution was prepared in the laboratory by dissolving 99% purity NaOH pearls in distilled water until the desired molar concentration was obtained (e.g., 320 g of NaOH pearls were dissolved in water, for one liter of solution to obtain 8M NaOH solution) and the Na2SiO3 was conditioned at 23 ± 2 °C. The alkaline activator solution was prepared 24 h prior mixing. The mix-design ratio for the alkali-activated geopolymer binder samples is presented in Table 3.
After casting into 40 × 40 × 160 mm molds, with a corresponding 10 min vibration, control sample C and samples P1–P6 were subjected to heat treatment (holding at 70 °C for 24 h), in order to study the effect of the fly ash substitution on the mechanical properties of the material.
Subsequently, samples with the partial substitution of fly ash with marble dust and blast furnace slag were also produced (P7–P12) but were not subjected to heat treatment. The samples were stored in laboratory conditions to study the effect of heat treatment on the mechanical properties of the material. The demolding of these samples was achieved only 48 h after casting because the lack of heat treatment caused a delay in the geopolymerization process.
After demolding, the geopolymer samples were stored in laboratory conditions at the temperature T = 23 ± 2 °C and relative humidity RH = 60 ± 5% until mechanical strength tests were conducted (7 days).
The density of the geopolymer samples was measured by weighing the samples and relating them to their volume. Initially, the density of the samples was measured at the end of the mixing, according to EN 12350-6:2019 [56]. After demolding (24 h for samples subjected to heat treatment and 48 h for samples stored in laboratory conditions), density was measured by weighing the samples and relating them to their volume (according to EN 12390-7:2019) [57]. Before conducting the mechanical strength tests (7 days), apparent density was also measured for each sample.
To obtain results regarding the mechanical performances of the geopolymer binder, a minimum of three samples were tested to determinate the average value of the assessed parameters. Tests were performed at the age of 7 days. The flexural strength of the alkali-activated geopolymer paste samples was determined by adopting the three-point bending (3PB) test, according to EN 196-1:2016 [58], the standard method for evaluation of mechanical performances of OPC paste and standard type mortars. Using the half prismatic test specimens resulting from the three-point bending test (3PB), compressive strength of the samples was determined according to the same standard on 40 × 40 mm samples [58].
Subsequently, geopolymer samples were subjected to infrared analysis using the Fourier Transformed Method (FTIR) to assess their microstructure. Laboratory tests by the FTIR method were performed using a Jasco FT/IR-6200 Fourier Transform Infrared Spectrometer apparatus (JASCO, Tokyo, Japan). Subsequently, a microscopic analysis was performed using a Leica DNC2900 optical stereomicroscope (Leica, Wetzlar, Germany), to assess possible changes in terms of pore size and distribution in the geopolymer matrix, as well as its homogeneity.
The analysis and interpretation of the results were performed both from the point of view of the values recorded for each parameter separately, as well as the increase/decrease compared to the control sample for alkali-activated geopolymer samples subjected to heat treatment. Discussions were also made based on comparing the results obtained on samples subjected to heat treatment and the ones conditioned in laboratory conditions to assess the influence of this parameters on the mechanical properties of the material.

3. Results and Discussions

3.1. Influence of Fly Ash Substitution with Marble Dust on Alkali-Activated Geopolymer Samples Subjected to Heat Treatment

The effect of fly ash substitution on alkali-activated geopolymer samples subjected to heat treatment was evaluated on samples P1–P3. The amount of fly ash substituted by marble dust was 10%, 25% and 50%. The results in terms of apparent density, flexural strength and compressive strength are graphically presented in Figure 1. All results obtained are compared to the control sample, produced using only fly ash as a raw material in the production of the alkali-activated geopolymer binder.
When analyzing the geopolymer samples subjected to heat treatment, as the quantity of fly ash (FA) was substituted with marble dust (MD), the following are observed:
-
the fresh-state density increases by 6.3% to 15.8%.
-
the density of the geopolymer paste immediately after completing the 24 h heat treatment increases by 6.8% to 18.4%.
-
the mechanical properties of the geopolymer paste at 7 days are reduced by up to 27.8% in the case of flexural strength values, and by a minimum 11.3% and a maximum 66.7% in the case of compressive strength.

3.2. Influence of Fly Ash Substitution with Blast Furnace Slag on Alkali-Activated Geopolymer Samples Subjected to Heat Treatment

The effect of fly ash substitution on alkali-activated geopolymer samples subjected to heat treatment was evaluated on samples P4–P6. The amount of fly ash substituted by blast furnace slag was 10, 25 and 50%. The results in terms of apparent density, flexural strength and compressive strength are graphically presented in Figure 2. All results obtained are compared to the control sample, produced using only fly ash as a raw material in the production of the alkali-activated geopolymer binder.
When analyzing the geopolymer samples subjected to heat treatment, as the quantity of fly ash (FA) was substituted with blast furnace slag (BFS), the following are observed:
-
casting density increases by 2.5% to 16.5%.
-
the density of the geopolymer paste immediately after going through the 24 h of heat treatment, increases by 5.4% to 16.3%.
-
the density of the geopolymer paste at 7 days after mixing increases by 7.4% to 33.8%.
-
the mechanical properties of the geopolymer paste. at 7 days after casting, are reduced by minimum 4% and maximum 78.3% in terms of flexural strength and by minimum 21% and maximum 64.3% in case of compressive strength.
By analyzing Figure 1 and Figure 2, it can be stated that by increasing the amount of the fly ash substitution with 10%, 25% and 50% of the total amount of dry material (fly ash + substitution), the physico-mechanical properties of the material do not vary proportionally with the percentage of the substitution. Thus, when analyzing the apparent density of the mixtures, they increase, as the amount of marble dust (MD) or blast furnace slag (BFS) increases in the mixture, by a maximum of 11%.
Compared to the same situation (mixtures P1 and P4), the mechanical properties are strongly influenced by the increase in the amount of the raw activating material, less if the substitution is the marble dust and more if the substitute is the blast furnace slag. When assessing marble dust as substitution, a decrease of a maximum 18% is observed in terms of flexural strength and 48% in terms of compressive strength. When using blast furnace slag as substitution, a decrease of a maximum 77% is observed in terms of flexural strength and 55% in terms of compressive strength.

3.3. Influence of Fly Ash Substitution with Marble Dust on Alkali-Activated Geopolymer Samples Subjected to Laboratory Conditions

The effect of fly ash substitution on alkali-activated geopolymer samples subjected laboratory conditions was evaluated on samples P7–P9. The amount of fly ash substituted by marble dust was 10, 25 and 50%. The results in terms of apparent density, flexural strength and compressive strength are graphically presented in Figure 3.
When analyzing the geopolymer samples subjected to laboratory conditions, as the quantity of fly ash (FA) was substituted with marble dust (MD), the following are observed:
-
in terms of density, it can be seen that it remains constant within the analysis limits for each sample.
-
in terms of the percentage increase in substitution, the density increases as the amount of thermal power plant ash is substituted with marble dust.
-
flexural strength increases from 2.49 N/mm2 for samples produced with a 10% fly ash substitution to 3.25 N/mm2 for samples with a 50% substitution.
-
compressive strength decreases from 23.40 N/mm2 for samples produced with a 10% fly ash substitution to 10.80 N/mm2 for samples with a 50% substitution.

3.4. Influence of Fly Ash Substitution with Blast Furnace Slag on Alkali-Activated Geopolymer Samples Subjected to Laboratory Conditions

The effect of fly ash substitution on alkali-activated geopolymer samples subjected laboratory conditions was evaluated on samples P10–P12. The amount of fly ash substituted by blast furnace was 10, 25 and 50%. The results in terms of apparent density, flexural strength and compressive strength are graphically presented in Figure 4.
When analyzing the geopolymer samples subjected to laboratory conditions, as the quantity of fly ash (FA) was substituted with blast furnace slag (BFS), the following are observed:
-
in terms of density, it remains constant within the analysis limits for each sample.
-
in terms of the percentage increase in substitution, the density increases as the amount of thermal power plant ash is substituted with marble dust.
-
flexural strength decreases from 3.73 N/mm2 for samples produced with 10% fly ash substitution to 1.39 N/mm2 for samples with 50% substitution.
-
compressive strength decreases as the amount of fly ash is substituted. Particularly it can be seen that for a 25% substitution of FA with BFS a slight increase in compressive strength was obtained.

3.5. Influence of Heat Treatment on the Mechanical Properties of Alkali Activated Geopolymer Samples Produces Using Marble Dust as Fly Ash Substitution

The effect heat treatment on the mechanical properties of the samples using marble dust as substitution and was studied to assess the differences in mechanical strength. The results in terms of flexural strength and compressive strength are graphically presented in Figure 5.
By analyzing Figure 5a, it can be seen that the flexural strength of the samples subjected to laboratory condition decreases when the amount of marble dust in the mixture increases from 10% to 25%. In the case of using marble dust as fly substitution up to 50% the flexural strength increases for the samples which were subjected to laboratory conditions.
In terms of compressive strength (Figure 5b), it can be seen that the values obtained for samples subjected to laboratory conditions are lower than the ones subjected to heat-treatment. These results are in accordance with the literature stating that heat treatment not only decreases the demolding time, but also generates a more powerful geopolymerization process, thus, resulting in better mechanical properties of the material [47,48,49,50,51,52,53,54].

3.6. Influence of Heat Treatment on the Mechanical Properties of Alkali Activated Geopolymer Samples Produces Using Blast Furnace Slag as Fly Ash Substitution

By analyzing Figure 6a, it can be observed that, as in the case of the samples produced using marble dust as fly ash substitution, in terms of flexural strength, samples present different variations. For samples with 10 and 50% blast furnace slag an increase in flexural strength is observed for samples subjected to laboratory conditions. For samples with 25% blast furnace slag substitution, a decrease in flexural strength was observed.
The compressive strength results obtained for samples produced using blast furnace slag as fly ash substitution show the same behavior: the compressive strength of samples subjected to heat-treatment are higher than the ones subjected to laboratory conditions (Figure 6b).

3.7. Influence of Geopolymer Binder Chemical Composition on the Mechanical Properties

The influence of the geopolymer binder chemical composition on the mechanical properties was established by reporting the above experimental results to the percentage content of SiO2, Al2O3, Fe2O3 and CaO. The mass percentage of the elements was calculated cumulatively for the dry mixture of raw materials. By analyzing Figure 7, it can be said that the high CaO content is harmful to the physico-mechanical performances of the binder. Unlike the strengthening mechanism of the cement paste in which the CaO content is particularly important, in the case of the strengthening mechanism of the alkali-activated geopolymer a high percentage of CaO is harmful for the geopolymerization process. It is possible that a high CaO content will contribute to the reduction of the curing time of the alkali-activated geopolymer paste in the absence of heat treatment, but in terms of mechanical properties, the damaging effect far exceeds the benefit of reducing the curing time. Therefore, to obtain flexural and compressive strength as high as possible, the chemical analysis of the raw materials is particularly important, being able to identify even the possibilities of substituting fly ash with other materials, if they contribute efficiently through their own intake of SiO2, Al2O3 and Fe2O3, the main participants in the geopolymerization mechanism.

3.8. FTIR Analysis of the Alkali-Activated Geopolymer Samples

The FTIR analysis of the alkali-activated geopolymer samples produced using marble dust or blast furnace slag as fly substitutions are shown in Figure 8. The IR band of increased intensity, centered at 1026 cm−1 corresponds to deformation vibrations of the Si-O-Si or Si-O-Al angles in the alumino-silicate gel and is responsible for the formation of amorphous aluminosilicate gel in binary systems [32]. The IR band centered at 1443 cm−1 comes from elongation vibrations of the C-O bond in the carbonate ion. The IR band located at 3451 cm−1 is attributed to symmetrical and antisymmetric elongation vibrations of the H-O bond in water molecules.
Figure 8a shows the 10% and 50% substitutions of fly ash with marble dust (samples P7 and P8). In both cases the position of the IR band centered at 1026 cm−1 moves towards smaller numbers, reaching 1014 cm−1. This displacement reflects the Si-O/Al-O bond formation of the aluminosilicate network characteristic of an aluminosilicate gel from binary systems with amorphous structure. The intensity of the IR bands decreases over the entire range between 400 and 4000 cm−1 by partially substituting fly ash with marble dust up to 50%. The intensity of the IR bands centered at 475 and 1100 cm−1 decreases in intensity, which suggests that the content of Al-O or Si-O bonds from different SiO2 crystalline phases in quartz and Al2O3-SiO2 in mullite decreases.
For sample P7 (10% marble dust, subjected to laboratory conditions), the intensity of the centered band at 1400 cm−1 decreases which indicates a decrease in the number of Al-O bonds. The higher the marble dust content (up to 50%), a trend of displacement of the band located at 1014 cm−1 towards smaller wave numbers and a decrease in water content can be observed. This evolution highlights the formation of an aluminosilicate gel with a disordered structure, and by substituting the flying ash with marble dust, materials with a lower intake of crystalline phases SiO2 or SiO2-Al2O3 are obtained.
Figure 8b shows the IR spectra of the samples produced using 10% and 50% blast furnace slag as fly ash substitutions. By subjecting the samples to heat treatment (samples P4 and P6), the intensity of the IR bands increases. The IR bands centered at 1014 and 1400 cm−1 move towards higher wavelengths (1022 and 1425 cm−1). The intensity of the IR band centered at 1100 cm−1 also increases. These structural developments indicate that the number of deformation vibrations of the Si-O-Al, Si-O-Si bonds and al-O elongation vibrations increases and as a result the degree of crystallinity of the gel increases. For samples subjected to laboratory conditions, the geopolymerization degree increases and the P12 sample indicates a lower absorption of water.
The vibrations of the bound water molecules, recorded in 3438, 2934/2928 and 1654 cm−1 are attributed to stretching (–OH) and bending (H–O–H), respectively [40,41,42]. These results can be seen for both types of geopolymer binder in Figure 8a (samples produced using marble dust as fly ash substitution) and in Figure 8b (samples produced using blast furnace slag as fly ash substitution).
The IR bands due to asymmetrical elongation vibrations of the Al-O (1400 cm−1) and O-H (1654 and 3438 cm−1) bonds are shifted to higher wavelengths with the partial substitution of the fly ash. According to the literature, when analyzing the samples produced only using fly ash, an indicator strip of sodium bicarbonate formation is recorded at 1443 cm−1. The bond analyzed in correlation with the movement trend of the samples prepared with marble dust or blast furnace slag (Figure 8a,b), could be moved below the limit indicated in the literature (~1440–1453 cm−1) [12,39], at wavelengths 1437 and 1425 cm−1. In the case of the P4 sample, the sodium bicarbonate content is lower than for the P6 sample.

3.9. Microscopic Analysis of the Alkali-Activated Geopolymer Samples

The microscopic analysis (Figure 9) was performed using a Leica DNC2900 optical stereomicroscope, at 1× magnification, aiming at possible changes in terms of pore size and distribution in the composite matrix, as well as its homogeneity.
As the substitution of fly ash with marble dust (sample P1) and with blast furnace slag (sample P4) increases, the dimensions and/or number of pores in the matrix increases. This can be attributed to the fact that the samples were subjected to heat-treatment.
Comparing the two microscopic figures (P1 vs. P7, and P6 vs. P12) in the case samples P1 and P6 (subjected to heat treatment), the size and number of pores are larger than in the case of sample P7 and P12 subjected to laboratory conditions. These results are in correlation with the lower bulk densities at 7 days of age. It is estimated that the heat treatment facilitates the formation of the geopolymer gel through a more uniform and homogeneous process, hence a better compressive strength, but influences the structural microscopy.
In the case of geopolymer samples subjected to laboratory conditions, their structure is much denser and the pores are smaller and fewer.

4. Conclusions

The aim of the work was to analyze the influence of the raw material on the properties of mechanical strength and on the reaction of geopolymerization by microstructural analysis.
As fly ash (FA) is replaced by marble dust (MD) or blast furnace slag (BFS) compared to the control sample, it is noticed that, with the increase in the quantity of the raw material, the fresh state density of the geopolymer paste and the hardened state density after the heat treatment and at the age of 7 days increases. In terms of mechanical performances results obtained on geopolymer samples showed that the parameters decrease as the quantity of fly ash in the mixture is substituted from 10 to 50%.
Increasing the amount of the substitution by 10%, 25% and 50% of the total amount of dry material (fly ash + substitute), it is observed that it is not possible to identify a proportionality function among the measurable indicators for physico-mechanical properties modifying the preparation mixture. Thus, in the case of densities, they increase, as the amount of marble dust (MD) or blast furnace slag (BFS) increases in the mixture, by a maximum of 11% compared to the values recorded for the situation with the lowest substitution of the fly ash (substitution 10%).
Comparing the results obtained for the same mixtures, in the case of substitution of fly ash (FA) with marble dust (MD), with and without heat treatment, it is noticed that the lack of heat treatment will cause an increase in the apparent density of the matured material 7 days after casting, compared to the density of the material subjected to heat treatment, as well as a reduction of mechanical resistances, both for flexural and compressive strength by a maximum of 48%, variable, depending on the amount of the substituted fly ash.
Comparing the results obtained for the same mixtures, in the case of substitution of fly ash (FA) with blast furnace slag (BFS), with and without heat treatment, it is noticed that the lack of heat treatment will cause an increase in the apparent density of the matured material 7 days after casting, compared to the density of the material subjected to heat treatment and a reduction of the compressive strength, by over 50%–77%, depending on the amount of the substituted fly ash.
After analyzing the microscopic images taken at 1× magnification, it was found that geopolymer pastes contain less pores with the substitution of 10% fly ash with marble dust and blast furnace slag. Geopolymer samples produced without heat treatment (23 °C) compared to those made with heat treatment (70 °C) are denser. Although geopolymer pastes are much more compact, their mechanical strengths decrease with the substitution of fly ash and are smaller compared to those produced using heat treatment. Thus, it can be said that the dimensions and number of pores do not influence their mechanical performances in terms of flexural and compressive strength.
In conclusion, it can be said that heat treatment helps the geopolymerization process both from the point of view of the formation of reaction products and the homogeneity of the samples. On the other hand, although there is a possibility of substituting fly ash with various other Si and Al supplying material, it is particularly important that these raw materials are analyzed in terms of their intake in Si, Al, and Ca, so that the geopolymer product is characterized by an optimal Si/Al ratio for the possibility of obtaining satisfactory mechanical performances.
Further studies are in progress in order to evaluate the possible direction for the development of alkali-activated geopolymer concrete, by adding aggregates to the mixtures and finding possible applications of the material based on specific mechanical requirements.

Author Contributions

Conceptualization, B.A.I. and A.-V.L.; methodology, B.A.I., A.-M.B., A.-V.L., S.R., T.G. and C.F.; validation, B.A.I., S.R. and T.G.; formal analysis, B.A.I., A.-M.B. and A.-V.L.; investigation, B.A.I., A.-M.B., A.-V.L., S.R., T.G. and C.F.; data curation, B.A.I., A.-V.L., S.R. and C.F.; writing—original draft preparation, B.A.I. and A.-V.L.; writing—review and editing, A.-M.B. and A.-V.L.; visualization, B.A.I., A.-M.B., A.-V.L., S.R., T.G. and C.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This paper was supported by the Program Advanced research on the development of eco-innovative solutions, composite materials, technologies and services, in the concept of a circular economy and increased quality of life, for a sustainable digitized infrastructure in a built and urban environment resilient to climate change and disasters, “ECODIGICONS”, Program code: PN 23 35 05 01: “Innovative sustainable solutions to implement emerging technologies with cross-cutting impact on local industries and the environment and to facilitate technology transfer through the development of advanced, eco-smart composite materials in the context of sustainable development of the built environment”, financed by the Romanian Government.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 00403 g001 550
Figure 1. Physico-mechanical performances of the alkali-activated geopolymer binder using MD as FA substitution subjected to heat treatment: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Figure 1. Physico-mechanical performances of the alkali-activated geopolymer binder using MD as FA substitution subjected to heat treatment: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Coatings 13 00403 g001
Coatings 13 00403 g002a 550Coatings 13 00403 g002b 550
Figure 2. Physico-mechanical performances of the alkali-activated geopolymer binder using BFS as FA substitution subjected to heat treatment: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Figure 2. Physico-mechanical performances of the alkali-activated geopolymer binder using BFS as FA substitution subjected to heat treatment: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Coatings 13 00403 g002aCoatings 13 00403 g002b
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Figure 3. Physico-mechanical performances of the alkali-activated geopolymer binder using MD as FA substitution subjected laboratory conditions: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Figure 3. Physico-mechanical performances of the alkali-activated geopolymer binder using MD as FA substitution subjected laboratory conditions: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Coatings 13 00403 g003
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Figure 4. Physico-mechanical performances of the alkali-activated geopolymer binder using BFS as FA substitution subjected laboratory conditions: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Figure 4. Physico-mechanical performances of the alkali-activated geopolymer binder using BFS as FA substitution subjected laboratory conditions: (a) Apparent density; (b) Flexural strength and (c) Compressive strength.
Coatings 13 00403 g004aCoatings 13 00403 g004b
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Figure 5. Influence of heat treatment on the mechanical properties of alkali activated geopolymer samples produces using marble dust as fly ash substitution: (a) Flexural strength and (b) Compressive strength.
Figure 5. Influence of heat treatment on the mechanical properties of alkali activated geopolymer samples produces using marble dust as fly ash substitution: (a) Flexural strength and (b) Compressive strength.
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Figure 6. Influence of heat treatment on the mechanical properties of alkali activated geopolymer samples produces using blast furnace slag as fly ash substitution: (a) Flexural strength and (b) Compressive strength.
Figure 6. Influence of heat treatment on the mechanical properties of alkali activated geopolymer samples produces using blast furnace slag as fly ash substitution: (a) Flexural strength and (b) Compressive strength.
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Figure 7. Geopolymer binder chemical composition for different mixtures (wt.%).
Figure 7. Geopolymer binder chemical composition for different mixtures (wt.%).
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Figure 8. FTIR spectra of geopolymer pastes made with: (a) 10% and 50% fly ash substitution with marble dust; (b) 10% and 50% fly ash substitution with blast furnace slag.
Figure 8. FTIR spectra of geopolymer pastes made with: (a) 10% and 50% fly ash substitution with marble dust; (b) 10% and 50% fly ash substitution with blast furnace slag.
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Figure 9. Microscopic images of geopolymer pastes with different substitutions, made at 1× magnification for mixtures: (a) C; (b) P1; (c) P7; (d) P6 and (e) P12.
Figure 9. Microscopic images of geopolymer pastes with different substitutions, made at 1× magnification for mixtures: (a) C; (b) P1; (c) P7; (d) P6 and (e) P12.
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Table
Table 1. Assignment of IR bands from the geopolymer structure according to the specialized literature.
Table 1. Assignment of IR bands from the geopolymer structure according to the specialized literature.
BondWave Number (cm−1)Ref.
Symmetric and antisymmetric O-H bond elongation vibrations in water
H-O-H deformation vibrations in water
Antisymmetric O-H bond elongation vibrations in water		~3500 and ~1600[12,24,25]
3445[26]
3400–3650[27]
1650[28]
1640[9]
Elongation vibrations of the C-O bond in the carbonate ion~1440–1453[12,25]
Antisymmetric elongation vibrations of the Al-O bond~1400[19]
Vibrations of antisymmetric deformation of angles Si-O-Al1180[17]
990[29]
Antisymmetric deformation vibrations of the Angles Si-O-Si (from quartz and mulit)1100[11]
Si-O-T angle deformation vibrations (T=Si or Al from silicate alumino gel)1025–1091[30]
Antisymmetric deformation vibrations of angles Si-O-T (T=Si or Al)900–1300[14,15,16,17,18,19]
Symmetrical elongation vibrations T-O-Si (T=Si or Al)1020[31]
Si-O/Al-O of the aluminosilicate network reflecting the formation of amorphous aluminosilicate gel in binary systems1015[32]
Si-OH bond elongation vibrations840–900[14,22,23,24]
Elongation vibrations of the Si-O link~800–810[10,24]
Al-O bond elongation vibrations in AlO4750–900[10,19,25,29]
680[14]
Vibration deformation Si-O-Al700[25,33]
Al-O bond elongation vibrations667[34]
Si-O bond elongation vibrations575[28]
530[26]
Si-O-Al angle deformation vibrations569[25,33]
Si-O-T deformation vibrations (T=Si or Al) bending540–555[33]
Elongation vibrations of the T-O bond (T=Si or Al)475[24,29]
O-Si-O deformation vibrations454[12]
Table
Table 2. Raw materials’ chemical composition.
Table 2. Raw materials’ chemical composition.
OxidesFly Ash (FA)
(wt.%)		Marble Dust (MD)
(wt.%)		Blast Furnace Slag (BFS)
(wt.%)
SiO246.940.2830.20
Al2O323.831.3710.05
Fe2O310.080.1714.70
CaO10.7254.6337.40
MgO2.630.434.05
SO30.45--
Na2O0.62-0.20
K2O1.65-0.38
P2O50.25--
TiO20.92-<0.52
Cr2O30.02-<0.05
Mn2O30.06-2.15
ZnO0.02--
SrO0.03--
CO2-42.65-
L.O.I. *2.110.37-
SiO2 + Al2O370.771.6540.25
* Loss on Ignition.
Table
Table 3. Alkali-activated geopolymer binder mix design.
Table 3. Alkali-activated geopolymer binder mix design.
MaterialMixtureNaOH
(M)		FA
(wt.%)		MD or BFS
(wt.%)		Na2SiO3/NaOHAA/
(FA + MD/BFS)
FAC8100-2.50.78
Samples subjected to heat treatment (70 °C for 24 h)
FA + MDP189010
P27525
P35050
FA + BFSP489010
P57525
P65050
Samples subjected to laboratory conditions
FA + MDP789010
P87525
P95050
FA + BFSP1089010
P117525
P125050
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MDPI and ACS Style

Ionescu, B.A.; Barbu, A.-M.; Lăzărescu, A.-V.; Rada, S.; Gabor, T.; Florean, C. The Influence of Substitution of Fly Ash with Marble Dust or Blast Furnace Slag on the Properties of the Alkali-Activated Geopolymer Paste. Coatings 2023, 13, 403. https://doi.org/10.3390/coatings13020403

AMA Style

Ionescu BA, Barbu A-M, Lăzărescu A-V, Rada S, Gabor T, Florean C. The Influence of Substitution of Fly Ash with Marble Dust or Blast Furnace Slag on the Properties of the Alkali-Activated Geopolymer Paste. Coatings. 2023; 13(2):403. https://doi.org/10.3390/coatings13020403

Chicago/Turabian Style

Ionescu, Brăduţ Alexandru, Alexandra-Marina Barbu, Adrian-Victor Lăzărescu, Simona Rada, Timea Gabor, and Carmen Florean. 2023. "The Influence of Substitution of Fly Ash with Marble Dust or Blast Furnace Slag on the Properties of the Alkali-Activated Geopolymer Paste" Coatings 13, no. 2: 403. https://doi.org/10.3390/coatings13020403

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