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Article

Preliminary Reactivity Test for Precursors of Alkali-Activated Materials

by
Sâmara França
1,
Leila Nóbrega Sousa
2,
Marcos Vinicio de Moura Solar Silva
3,
Paulo Henrique Ribeiro Borges
1 and
Augusto Cesar da Silva Bezerra
4,*
1
Department of Civil Engineering, Federal Center for Technological Education of Minas Gerais, Belo Horizonte 30421-169, Brazil
2
Department of Materials Engineering, Federal Center for Technological Education of Minas Gerais, Belo Horizonte 30421-169, Brazil
3
Energy Company of Minas Gerais, Belo Horizonte 30190-131, Brazil
4
Department of Transport Engineering, Federal Center for Technological Education of Minas Gerais, Belo Horizonte 30421-169, Brazil
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(3), 693; https://doi.org/10.3390/buildings13030693
Submission received: 2 February 2023 / Revised: 20 February 2023 / Accepted: 1 March 2023 / Published: 6 March 2023
(This article belongs to the Special Issue Materials Engineering in Sustainable Buildings)
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Versions Notes

Abstract

:
Alkali-activated materials (AAMs) result from the dissolution process and polycondensation of precursors in high pH solutions. This material is considered alternative cement with similar properties and lower environmental impact than Portland cement. However, AAMs are subjected to the same standardization applied to cement-based materials since no formal methods exist to characterize this material and/or the precursor reactivity. Therefore, this work aims to develop a method to characterize the reactivity of the main precursors used to produce AAMs. Hence, the precursors were assessed in two steps after chemical, physical, and mineralogical characterization. The first step evaluated the crystallinity change of the material after the acid attack by mixing 1 g of each material in 100 mL of 1% HF solution for 6 h at ambient temperature. The crystallinity change was evaluated by comparing the X-ray diffraction of the materials before and after the acid attack. The second step involved evaluating the formation of geopolymerization products in the pastes of studied precursors through FTIR test. The pastes were produced with Na2SiO3 and NaOH as activators. After 28 days of curing, the pastes were submitted to a FTIR test for structural analysis. This method was tested evaluating the reactivity of traditional precursors for alkali activation (i.e., silica fume (SF), blast furnace slag (BFS), and metakaolin (MK)), in addition sugarcane bagasse ash mechanically treated (SCBAM) and sugarcane bagasse ash mechanically and heat treated (SCBAMH) since SCBA is a promising precursor for alkali activation. Considering the crystallinity change of precursors (step 01), the formation of geopolymerization products (step 02), and the chemical composition of precursors (preliminary characterization), it could be concluded that: (i) surface area is not relevant to materials with small particle size (<23 µm); (ii) amorphous area is only relevant if the material exhibits the optimal chemical composition; and (iii) the chemical composition is a crucial parameter for alkali activation. In addition, the potential precursors for alkali activation should have a significant amorphous halo and a SiO2/Al2O3 ratio of 2 to 5. Also, it could be concluded that SF and SCBAMH do not exhibit adequate reactivity while BFS, MK, and SCBAM can be classified as reactive precursors.

1. Introduction

Civil construction is directly linked with the economy of countries and social growth. The consumption of buildings material in developing countries is more expressive owing to the highest demands for civil construction infrastructure. It is estimated that the cement consumption in the world is about 450 kg per capita, and therefore cement is the most demanded building material globally [1]. This consumption tends to increase since it is estimated that the world population in 2050 should be 9.8 billion, and 68% of this total should live in urban areas. In 2020, the population increased to 7.8 billion, with nearly 55% living in urban areas [2].
At the same time, the cement industry exhibits a wide impact on the environment. It is estimated that the production of clinker, a major cement component, releases into the atmosphere around 0.84 kg of CO2 for each kilogram of clinker produced [3]. The cement industry is responsible for 6–7% of CO2 emissions around the world owing to anthropogenic actions [4] and contributing to 5–8% of greenhouse gases [5,6,7,8,9]. In this scenario, researchers for alternative types of cement are increasing around the world and could be divided into two groups: (i) supplementary cementitious materials; and (ii) alkali-activated materials.
Supplementary cementitious materials (SCM) partially replace the clinker to produce sustainable cement [10,11,12]. These materials should be pozzolanic, that is, react with calcium hydroxide present in the hydration of clinker and form hydrated calcium silicate. The hydrated calcium silicate is the major product of cement’s hydration, which is responsible for strength and material durability. Different methods are used to classify a material as pozzolanic: (i) chemical analyses [13,14]; (ii) indirect pozzolanic activity index [15,16]; and (iii) direct pozzolanic activity index such as the Chapelle test [17,18] and Luxán test [19].
Alkali-activated materials (AAMs) result from the dissolution and polycondensation in high pH media of materials rich in silicon, aluminum, and calcium contents [20]. Thus, residues and by-products from industry or agriculture, which are often landfilled, can be used as raw materials [21,22,23]. For this reason, the production of AAM has great potential to release less CO2 when compared to Portland cement [24]. However, despite the AAMs are already accepted and applied for industrial production in some countries, there is still a lack of formal methods to characterize their reactivity. This is a result of the complexity of the factors involved in the synthesis of AAMs, such as the variety of chemical composition of the precursors, types of precursors, combinations of activators, the molarity of activators, and the conditions of curing [25,26,27,28].
Regarding the activators, some points are already consolidated in the literature, such as the optimal molarity ratio to produce AAMs using sodium hydroxide as an activator [29], and the role of sodium silicate in the reaction. It is known that the greater the increase of the concentration of NaOH, the faster the precursors dissolve in the medium. However, if the concentration of OH- is too high (around 12 M), the polycondensation phase might be delayed in geopolymerization reactions [30,31]. Besides that, according to Palomo et al. [32], the presence of soluble silica in the silicate contributes to cross-links between the chains formed in the reactions, which improves the properties of alkaline binders owing to a more compact structure. However, the higher concentrations of soluble silica tend to delay the reactions due to a reduction in pH and an increase in the viscosity of the mixture [33].
Regarding the reactivity of the precursors, it is known that three main factors must be observed: (i) chemical composition; (ii) surface area; and (iii) crystallinity [34]. It is known that greater reactivity is achieved through the highest silica and alumina contents, high surface area, and greater amorphous phase content. Previous works already evaluated the reactivity of some precursors used in geopolymers [35,36,37,38,39]. However, these works focused on only one type of precursor, and none of them assesses the reactivity of sugarcane bagasse ash. In addition, these reactivity evaluation methods use NaOH or HF for the chemical attack to dissolve the vitreous phase of the materials. Nonetheless, as mentioned before, the vitreous phase is only one of many factors that guarantee the efficient reactivity of alkaline precursors.
Therefore, this work aims to develop a method that can characterize the reactivity of the many precursors used for the production of activated alkali materials. The method created could be used to determine the viability of the precursor for the alkaline activation through tests of simple execution and interpretation.
In this method, in addition to evaluating the amorphous phases of the precursors already exhibits in the literature [35,36,37,38], pastes were produced with commonly used precursors (blast furnace slag, silica fume, and metakaolin) and NaOH and NaOH + Na2SiO3 as activators for structural analysis. This proposed new step was performed by FTIR test to investigate the formation of geopolymeric bonds in these pastes. The other novelty of this method is the evaluation of the reactivity of sugarcane bagasse ash (SBCA). The SBCA has already been studied as a pozzolanic material due to its chemical composition, high potential for clinker substitution, and availability [40,41,42,43,44,45,46,47]. However, the research on SCBA as a precursor for alkaline binders is incipient.

2. Materials and Methods

2.1. Materials

Blast furnace slag was supplied by Supermix company. The silica fume was supplied by Tecnosil company. The metakaolin was supplied by Metacaulim do Brasil. Sodium hydroxide (97% purity, Êxodo Científica Fina Indústria e Comércio LTDA) and sodium silicate (14.98% Na2O, 32.85% SiO2 and 52.17% H2O, Sulfal Química Limitada) were used as activators. Hydrofluoric acid (37–51% concentration) was used to assess the crystalline phases in the precursors.
The SCBA was submitted to a beneficiation process to evaluate its influence on reactivity [4,48]. The ashes samples were separated into categories according to the treatment applied, namely: (i) sugarcane bagasse ash mechanically treated (SCBAM) and (ii) sugar cane bagasse ash mechanically and heat-treated (SCBAMH). Then, the SCBAM was ground in a ball mill for 2 h at 90 rpm speed with 90 steel spheres of 30 cm in diameter. The SCBAMH was calcined in a muffle oven at 700 °C for 3 h before grinding.

2.2. Methods

The chemical composition of blast furnace slag (BFS), metakaolin (MK), silica fume (SF), and sugarcane bagasse ashes (SCBAM and SCBAMH) were determined by X-ray fluorescence analysis (XRF) using Philips/Panalytical spectrometer and are listed in Table 1. The particle size distribution in the material was measured through laser granulometry, using a Cilas 1090 Laser Particle Size Analyzer and the results are shown in Table 1.
The BFS shows CaO as the major compound owing to its process of production [49]. SF is a by-product of silicon and ferrosilicon alloy production industries and presents silica in the range of 85–95% [50]. MK exhibits SiO2 and Al2O3 as their major compounds [51]. Regarding the ashes, the SCBAM and SCBAMH are composed mainly of SiO2, Al2O3, and Fe2O3. The heat treatment leads to an increase in SiO2 and a decrease in Fe2O3 content. This behavior has been reported before [4,40].
SiO2 and Al2O3 are essential oxides for the alkaline activation of low calcium precursors. At the same time, CaO and SiO2 are the main oxides for alkaline activation of high calcium precursors. According to previous studies [34,37,52,53]. SiO2/Al2O3 reactivity is fundamental for the reaction degree, the composition and structure of the N-A-S-H gel formed, and consequently the strength and durability of the low-calcium binder. Recent studies have been using this ratio from 2 to 4 to assess the reactivity of precursors [54,55,56,57,58,59]. In addition, precursors with high calcium content must have a CaO/SiO2 ratio between 0.9 and 1.2 for efficient activation [60,61,62,63,64]. Considering this range, only BFS and MK would be considered appropriate precursors for alkaline activation.
Regarding the particle size test, all of the samples presented a particle size smaller than 0.5 mm which significantly increases the performance of AAMs [65]. In addition, the ashes had the smallest DM among the materials studied, and it could be indicative of high reactivity since smaller particle size is related to higher surface area inducing higher reactivity in the material [66,67,68].
Materials crystal phases and mineralogical composition were analyzed by XRD. The X-ray diffraction was performed with a Shimadzu XRD-7000 diffractometer operating with Cu K-alpha radiation (40 kV/30 mA), with 2θ from 5° to 100° at a step of 2°/min. The peaks were identified using the software Match! 3 (Version 3.7.0.124) with the Crystallography Open Database (COD) revision no. 211633. The XRD patterns of SF, BFS, MK, SCBAM and SCBAMH are shown in Figure 1. BFS (Figure 1e) and SF (Figure 1c) presented a wide hump at around 30° and 23° [69], respectively, which is related to the presence of amorphous phases. In the BFS (Figure 1e), the hump is also attributed to calcium silicate oxide; and it can be attributed to the SiO2/Al2O3 ratio up to 2.2 [70]. This amorphous feature present in BFS and SF is related to the reactivity in those materials. At the same time, the MK spectra (Figure 1b) exhibit intense peaks of quartz (COD 96-101-1160) that imply high silica content [51]. In addition, kaolinite (COD 96-900-9235) and muscovite (COD 96-900-6330) is also observed in MK diffraction. This high content of crystalline phases could indicate a reduction in its reactivity. The diffraction of SCBAM (Figure 1d) exhibits an angle between 10° and 35° (2θ) [41,71]. Those angles are related to silica in three forms: (i) crystalline quartz; (ii) cristobalite; and (iii) amorphous silica [43,72]. On the other hand, the amorphous hump could also be related to the presence of carbon content. The cristobalite phase in SCBAM describes the phase changing of amorphous silica during uncontrolled burning while the quartz phase is associated with the sand attached to the SCBA during harvesting [42,73]. Besides that, the amorphous content of SCBAMH (Figure 1a) decreased [40], while the quartz peak intensity increased with increasing temperature.
The analysis of the particle morphology of each material was performed using SEM and is presented in Figure 2. The scanning electron microscopy was performed on a Hitachi TM3000 scanning electron microscope under a low vacuum with a backscattered electron detector and electron acceleration voltage of 15 kV.
The BFS micrograph (Figure 2a) shows that the slag particles are granular and with sharp edges. BFS also exhibits a diversity of particle size and shape; the smaller particles cover the surface of the larger ones. In addition, the material has a compact appearance [74]. At the same time, the SF micrograph (Figure 2b) shows that the shape of the silica particle is spherical and can form clusters when stacked together [51]. In the MK micrograph (Figure 2c), the particles presented a granular structure; and it is possible to observe that the smallest particles agglomerate on top of larger ones. The presence of unreacted fiber is evident in the micrograph of SCBAM (Figure 2d). These fibers represent unburned material consisting basically of carbon, which does not contribute to the geopolymerization reactions. In addition, the presence of particles with different shapes can also be noticed. Spherical particles are mainly associated with Si and O and some other minor components such as MgO and Al2O3. Compacted and prismatic particles could also be associated with the presence of Si and O elements [75]. The presence of powdery material over the surface of the particles is also observed. As expected, there is no presence of fibers in the SCBAMH micrographs (Figure 2e). This indicates that the thermal activation could eliminate the unburned residue. The reduction of the particles could also be observed due to mechanical activation. The remaining particles are compacted and covered with a finer material on their surface.

2.2.1. Acid Attack—Method

After preliminary characterizations, 1 g of each material was mixed in 100 mL of 1% HF solution. The mixture was stirred for 6 h at ambient temperature. After this time, the mixture was filtered in, and the residue was rinsed with deionized water until the filter reached a pH value of 7. Subsequently, the retained material was dried for 1 h in an oven at 100 °C, and its mass was recorded. After mass measurement, part of the material was reserved for further XRD testing to determine changes in the crystalline phases of the precursors. The other part of the material was subjected to calcination by thermogravimetric analyzer equipment STA7000 series of the Hitachi brand, with the aluminum crucible, flow of nitrogen gas at 60 mL/min. The sample was exposed from 25 °C to 600 °C, with an increment rate of 40 °C/min to verify the chemical constitution of the retained material.

2.2.2. Structural Analysis—Method

Pastes with the studied precursors were produced: the pastes 01 were produced exclusively with NaOH and precursor/activator ratio = 0.7, and the pastes 02 were produced with Na2SiO3 (SS) and NaOH (SH) at the SS/SH = 2.5 ratio and the same precursor/activator ratio as the paste 01. The concentration of NaOH solution used in the two pastes was 8 mol/L and the molar ratios of the oxides constituting the activators are shown in Table 2. After 28 days of curing, the pastes were submitted to a FTIR test in the Shimadzu IR-Prestige 21 equipment for structural analysis.

3. Results

3.1. Acid Attack—Results

The analysis of materials’ mineralogical composition before and after the acid bath (Figure 3) were performed by software Match! 3 with the Crystallography Open Database (COD) revision no. 211633. According to Figure 3, the vitreous phases of precursors decrease after the acid bath. This occurs since the acid attacks these phases. The sugarcane ashes present a larger decrease of vitreous phases, which could indicate that sugarcane ashes have a high content of vitreous phases. Besides ashes, MK presents a significant decrease of theses phases. The crystallinity of SF and BFS did not change significantly. This suggests that in materials that exhibit a higher vitreous phase, the acid attack reduces the intensity of amorphous halos and maintains the material’s low crystallinity degree, as can be seen in Figure 3a,b.
With regards to peaks intensity, as can be seen also at Figure 3, the intensity of all peaks usually decreases after the acid bath. However, SCBAM exhibits higher peaks after 1% HF solution. This could be associated with a reduction of vitreous phases which leads to an increase in peak intensity. Other precursors presented a reduction of the intensity of both crystalline phases and vitreous phases.
Regarding the mineralogical composition of precursors, no intense variation was observed after the acid bath, as expected for BFS and SF. The peaks of diffractograms were identified using the software Match! 3 with the Crystallography Open Database (COD) revision no. 211633 or according to literature. The diffractogram of BFS after acid bath exhibits a peak around 45° that was attributed to aluminum calcium fluoride (COD 96-100-0301) [76]. SF shows a peak around 60° assigned to silicon fluoride after an acid attack [77]. SBCAM and SCBAMH XRD spectrum exhibit quartz (COD 96-901-3322) as the main crystalline phase before and after an acid attack. MK also exhibits the same crystalline phase, muscovite (COD 96-900-6330), and quartz, before and after the acid bath.
According to the thermogravimetric analysis of retained material (Figure 4a–c), SF, SCBAM, and SCBAMH exhibit similar mass loss of around 4% after an acid bath. This can be attributed to the chemical composition not changing after a 1% HF solution. Peaks of quartz exhibited on the diffraction spectrum of these materials which is a stable structure could explain this behavior [78]. SCBAM shows one stage of mass loss from 25–400 °C assigned to the removal of physically and/or chemically adsorbed water or organic matter [4,79]. SCBAMH also exhibits mass degradation until 450 °C related to free water evaporation and volatilization of organic matter. In addition, it is possible to observe a weight loss in around 500 °C attributed to the crystallization of metastable cristobalite in SCBAMH. This crystallization occurs at lower temperatures due to the metal impurities present in the ash [78,80]. SF exhibits a mass loss up to 200 °C which is associated with the release of free water, and others in the range of 400–800° that is attributed to the dehydration of the silanol groups [81].
Besides that, MK also exhibits a low mass loss value at about 6% (Figure 4d). This also suggests that the presence of stable crystalline phases in these materials, such as quartz and kaolinite, reduces the mass loss degree. However, MK shows a higher loss of mass that SF, SCBAM and SCBAMH as a result of the dihydroxylation of kaolinite that occurs around 450–600 °C [82,83]. At the same time, BFS exhibits weight loss of 12% in the range of 25–300 °C, assigned to the release of physically or chemically adsorbed water. Usually, the removal of hydroxyl groups depends on their chemical bonding with metal ions. Non-coordinating hydroxyl groups or physically adsorbed water molecules are removed at a lower temperature [84].

3.2. Structural Analysis—Results

The structural analysis in pastes was carried out by FTIR test. According to Figure 5, the SF pastes did not exhibit a significant presence of hydrated products associated with O-H and OH bonds, which are attributed to bands around 3400 and 1700 cm−1, respectively [85,86,87]. The absence of these bands could be associated with inefficient alkali activation of this precursor. In addition, there is a band at about 1500 cm−1 that is associated with the O-C-O bond and possible carbonation of activated samples [86,88,89]. The intensity of the characteristic Si-O-T band is reduced due to activation, and the wavenumber is shifted to lower values due to the silicon bond change through alkaline solutions addition. The Si-O bond assigned to the band around 800 cm−1 also reduced the intensity through activation [90]. In addition, bands at around 600 and 400 cm−1 associated with the Si-O-Si bond also changed [91,92]. There is a reduction of the band around 400 cm−1 and an appearance of the band near to 600 cm−1 which could indicate a structural change in the silicon bond.
FTIR spectrum of both SF activated pastes is quite similar, except for the intensity of the characteristic band of Si-O-T bond, which is more intense for the paste activated with sodium silicate and sodium hydroxide. This may be associated with the major presence of silicon oxide in this paste.
The presence of hydrated products is related to the presence of O-H bond at around 3400 cm−1 in activated pastes, as observed for MK-pastes (Figure 6). In addition, there is the presence of OH bond assigned to the band around 1700 cm−1 in these pastes which is also associated with the formation of hydrated products. The intensity of these bands could be attributed to the effective activation of the precursor. In addition, there is also the presence of the Si-O-T bond in activated pastes, and it shifted to lower values in the precursor. This change in wavenumber is attributed to the introduction of aluminum on silicon bond for N-A-S-H formation which is a characteristic product of polymerization of low calcium precursor [93,94].
The band assigned to the Si-O bond, which is larger in the precursor, is attributed to 800 cm−1 wavenumber and exhibits a reduction of its intensity through alkaline activation. This change occurs due to the dissolution of precursors and the reorganization of silicon bonds. In addition, there is a reduction of the band near 400 cm−1 associated with the Si-O-Si bond which could indicate structural changes in the silicon bond.
FTIR spectrum of MK activated pastes is quite similar, except by intensity and wavenumber of a characteristic band of Si-O-T bond. The band is stronger for sodium hydroxide activation and exhibits lower values for sodium silicate activated paste. The lower wavenumber value is associated with the aluminum presence in paste structure which favors the formation of many crosslink bonds on products resulting from geopolymerization.
Figure 7 shows the FTIR spectrum of SCBAM and SCBAMH activated pastes. In the SCBAMH pastes, only the paste activated with sodium hydroxide exhibits the hydrated products, however, the bands associated with these products are not significant. In addition, the bands assigned to C-O-C, Si-O-T, Si-O, and Si-O-Si bonds exhibit similar behavior to MK. However, at the FTIR spectrum of the pure precursor, it is possible to observe a band around 700 cm−1 associated with the Al-O bond which decreases the intensity after activation due to the precursor dissolution [95,96].
The absorption band for BFS-pastes (Figure 8) also exhibits similar behavior to MK-pastes. However, the characteristic band assigned to the Si-O-T bond exhibits a typical shoulder of C-(N)-A-S-H product resulting from high calcium precursor activation. It is also worth noting that the bands are stronger on BFS activation and there is a band at about 1500 cm−1 associated with O-C-O bonds and possible carbonation of activated samples. Another difference between the precursors is the absence of the band around 800 cm−1 assigned to the Si-O bond, and the presence of the band around 700 cm−1 associated with the Al-O bond in the precursor which becomes lower in the alkaline activation. This change occurs as a result of precursor dissolution and reorganization from aluminum bonds.
SCBAM-pastes (Figure 9) exhibits the bands assigned to hydrated products for both types of activators. The bands assigned to C-O-C, Si-O-T, Si-O, and Si-O-Si bonds exhibit similar behavior to MK. However, the characteristic band around 800 cm−1 associated with the Si-O bond exhibits an increase of intensity through activation, which is only observed for SCBAM-pastes. This band could be assigned to the formation of N-A-S-H in the paste samples. The presence of the band related to hydrated products could indicate that alkaline activation is effective for SCBAM.
Table 3 summarizes the wavenumber of the most important bands analyzed in the present work. According to the FTIR spectra (Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9), a change in wavenumber may have occurred due to a structural change in precursors due to alkaline activation. However, there is no significant change for SCBAMH and SF, which indicates that alkaline activation was not effective for these precursors.
Although SF shows a higher vitreous phase and smaller particle size, the alkaline activation was not effective due to the absence of aluminum in its chemical composition. The aluminum presence is essential for N-A-S-H formation on low calcium precursors. SCBAMH also exhibits lower aluminum content (SiO2/Al2O3 = 11.74), and higher crystallinity which also impairs its activation. Whereas, SCBAM shows chemical composition (SiO2/Al2O3 = 4.77), significant amorphous halo, and particle size (DM 17.27 µm) suitable for alkali activation, despite its high loss on ignition. Therefore, SCBAM is a viable precursor for effective alkali activation. MK and BFS are already classic precursors for alkali activation and meet all requirements. According to stages proposed, two parameters could be attributed to effective alkali-activation: (i) significant amorphous halo and (ii) SiO2/Al2O3 proportion from 2 to 5.

4. Discussion

Lancellotti et al. [38] studied the reactivity of bottom ash. After a chemical attack with 100 mL of 8 mol/L NaOH solution and stirred constantly for 5 h in a flask bathed at 80 °C, it concluded that the ashes were not adequate to obtain a geopolymer since its Si/Al mass ratio were below the value of 3. Kuenzel et al. [36] studied the reactivity of metakaolin through 1% (mass) HF solution for 20 h at ambient temperature and concluded that HF removes the amorphous phase in MK associated with the intensity background between 20 and 30 degrees 2θ, with the remaining sample consisting of crystalline phases present as impurities and the Si/Al mass ratio after acid attacks is equal to 2.85. Fernández-Jimenez et al. [37] studied the reactivity of fly ashes through 100 mL of 1% HF stirred for 6 h at ambient temperature and concluded that when the ashes are treated with 1% HF, most of the halo associated with its glassy constituent disappears. At the same time, these diffractograms evidence an increase in the intensity of the peaks associated with the crystalline phases. The precursors evaluated in this studied (SF, MK, BFS, SCBAM and SCBAMH) exhibits the same behavior of work of Fernández-Jimenez et al. [37].
Regarding the effect of reactivity of precursors in the mechanical and durability properties of the resulting products, previous studies concluded that the reactivity of precursors is an important parameter to a good performance of geopolymer. The higher reactivity of precursors led to a higher geopolymerization rate resulting in development of a dense and strong matrix explaining the high mechanical and durability performances [97]. At the same time, the surface area was not a fundamental parameter for determining the reactivity of the precursors in this work, since all precursors have reduced particle size. However, previous works [22,98,99,100] concluded that the finer particles of the precursors exhibit a faster geopolymerization and result in more formation of activation products, when there is a higher difference in particle size between the precursors.

5. Conclusions

Non-complex execution and easy-interpretation tests were proposed to help in the preliminary characterization of the reactivity of SCBA, BFS, MK, and SF as precursors for alkali-activated materials. According to these tests, the following conclusions can be drawn:
  • The surface area is not relevant when the materials exhibit particles size medium smaller than 23 µm.
  • The amorphous area is only relevant if the material exhibits the optimal chemical composition.
  • The chemical composition is a crucial parameter of alkali activation.
  • Potential precursors for effective alkali activation should exhibit significant amorphous halo and SiO2/Al2O3 proportion from 2 to 5.
  • Silica fume is not viable as a single-precursor for alkali-activation due to the absence of aluminum on its chemical composition.
  • Sugarcane bagasse ash mechanically and heat-treated is not a suitable precursor for alkali-activation due to its crystalline character and lower aluminum content.
  • Sugarcane bagasse ash mechanically-treated, blast furnace slag, and metakaolin are viable precursors for alkali-activation.
Future research on determining the maximum acceptable crystallinity for viable precursors is highly recommended.

Author Contributions

Conceptualization, S.F. and A.C.d.S.B.; Methodology, S.F. and A.C.d.S.B.; Software, M.V.d.M.S.S.; Validation, A.C.d.S.B.; Formal Analysis, S.F. and A.C.d.S.B.; Investigation, S.F.; Resources, A.C.d.S.B., M.V.d.M.S.S. and P.H.R.B.; Data Curation, S.F.; Writing—S.F. and L.N.S.; Writing—Review & Edit, S.F., L.N.S. and A.C.d.S.B.; Visualization, S.F. and A.C.d.S.B.; Supervision, A.C.d.S.B. and P.H.R.B.; Project Administration, A.C.d.S.B., M.V.d.M.S.S. and P.H.R.B.; Funding Acquisition, A.C.d.S.B., M.V.d.M.S.S. and P.H.R.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Energy Company of Minas Gerais (CEMIG) and the National Electric Energy Agency (ANEEL) for funding through the PD ANEEL CEMIG GT616 project, the Minas Gerais State Research Foundation (FAPEMIG) [grant number APQ-01425-22], and National Council for Scientific and Technological Development (CNPq) [grant number PQ 315653/2020-5].

Data Availability Statement

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

Acknowledgments

The authors would like to thank Energy Company of Minas Gerais (CEMIG) and the National Electric Energy Agency (ANEEL) for funding through the PD ANEEL CEMIG GT616 project. The Minas Gerais Research Support Foundation (FAPEMIG) for their assistance with the infrastructure and National Council for Scientific and Technological Development (CNPq) for the research productivity incentive grant.

Conflicts of Interest

The authors declare no competing financial interest or personal relationship that could appear to influence the work reported in this paper.

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Figure 1. XDR spectrum of (a) SCBAMH, (b) MK, (c) SF, (d) SCBAM, and (e) BFS.
Figure 1. XDR spectrum of (a) SCBAMH, (b) MK, (c) SF, (d) SCBAM, and (e) BFS.
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Figure 2. Micrographs of (a) BFS, (b) SF, (c) MK, (d) SCBAM, and (e) SCBAMH.
Figure 2. Micrographs of (a) BFS, (b) SF, (c) MK, (d) SCBAM, and (e) SCBAMH.
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Figure 3. XRD test (a) SCBAMH, (b) MK, (c) SF, (d) SCBAM, and (e) BFS.
Figure 3. XRD test (a) SCBAMH, (b) MK, (c) SF, (d) SCBAM, and (e) BFS.
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Figure 4. Thermogravimetric analysis of (a) SF, (b)SCBAM, (c) SCBAMH, (d) MK, and (e) BFS.
Figure 4. Thermogravimetric analysis of (a) SF, (b)SCBAM, (c) SCBAMH, (d) MK, and (e) BFS.
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Figure 5. FTIR of SF and SF activated pastes.
Figure 5. FTIR of SF and SF activated pastes.
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Figure 6. MK and MK activated pastes.
Figure 6. MK and MK activated pastes.
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Figure 7. SCBAMH and SCBAMH activated pastes.
Figure 7. SCBAMH and SCBAMH activated pastes.
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Figure 8. BFS and BFS activated pastes.
Figure 8. BFS and BFS activated pastes.
Buildings 13 00693 g008
Buildings 13 00693 g009 550
Figure 9. SCBAM and SCBAM activated pastes.
Figure 9. SCBAM and SCBAM activated pastes.
Buildings 13 00693 g009
Table
Table 1. Chemical composition by XRF (wt%).
Table 1. Chemical composition by XRF (wt%).
BFSSFMKSCBAMSCBAMH
SiO232.0595.9462.0032.8973.53
Al2O314.330.2130.5511.7210.65
Fe2O31.270.122.5118.266.14
CaO39.460.350.042.902.39
MgO8.770.410.252.821.39
TiO20.530.011.455.900.95
K2O0.170.40.492.842.22
MnO0.630.020.010.250.13
P2O5<0.01<0.01<0.010.931.14
ZrO2-<0.010.060.040.04
SO31.43<0.01-<0.010.02
Na2O0.96<0.1<0.1<0.1<0.1
Cr2O30.11<0.010.020.270.20
Sum of pozzolanic oxides47.6596.2795.0662.8790.32
Loss on ignition (LOI)0.182.152.4420.680.98
SiO2/Al2O33.80776.663.454.7711.74
Particle size distribution
DM (μm)22.9221.9122.7917.2717.77
D10 (µm)1.755.123.082.562.20
D50 (µm)15.7820.5219.6413.3412.73
D90 (µm)55.1040.8646.8438.2641.68
Table
Table 2. Pastes compositions.
Table 2. Pastes compositions.
Precursor (g)Paste 01Paste 02
H2O/Na2OH2O/Na2OSiO2/Na2O
3010.4411.381.36
Table
Table 3. Summary of bands in the FTIR spectrum.
Table 3. Summary of bands in the FTIR spectrum.
Wavenumber (cm−1)Function BandReference
3400O-H[85,86,87]
1700OH[85,86,87]
1500C-O[86,88,89]
1000Si-O-T (T = Si or Al)[93,94]
900Si-O-T (T = Si or Al)[93,94]
800Al-O and Si-O[90]
600Al-O and Si-O[95,96]
500Si-O-Al and Si-O-Si[91,92]
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França, S.; Sousa, L.N.; Silva, M.V.d.M.S.; Borges, P.H.R.; Bezerra, A.C.d.S. Preliminary Reactivity Test for Precursors of Alkali-Activated Materials. Buildings 2023, 13, 693. https://doi.org/10.3390/buildings13030693

AMA Style

França S, Sousa LN, Silva MVdMS, Borges PHR, Bezerra ACdS. Preliminary Reactivity Test for Precursors of Alkali-Activated Materials. Buildings. 2023; 13(3):693. https://doi.org/10.3390/buildings13030693

Chicago/Turabian Style

França, Sâmara, Leila Nóbrega Sousa, Marcos Vinicio de Moura Solar Silva, Paulo Henrique Ribeiro Borges, and Augusto Cesar da Silva Bezerra. 2023. "Preliminary Reactivity Test for Precursors of Alkali-Activated Materials" Buildings 13, no. 3: 693. https://doi.org/10.3390/buildings13030693

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