Synergistic gel formation in geopolymers of superior mechanical strength synthesized with volcanic ash and slag

The present work studies gel evolution and microstructure of geopolymers synthesized with volcanic ash (VA) and blast furnace slag (BFS). The synthesis parameters such as BFS proportions on geopolymer formation were investigated. Gel evolution and microstructure of the geopolymers were studied by FTIR, X-ray diffraction (XRD), 29Si NMR spectroscopy and scanning electron microscopy measurements. Silicate gels (N–S–H) were mainly formed in VA-based geopolymers of low compressive strength (14.07 MPa). While with VA and BFS each account for 50%, VA-BFS–based geopolymers possessed a compressive strength of 55.6 MPa, as well as the homogeneous C–(A)–S–H and N–A–S–H gels were formed. The C–(A)–S–H and N–A–S–H gels show synergistic effects on the mechanical property of the geopolymers. This work provides a clue for the synthesis of geopolymers with superior mechanical properties in areas of architecture. Detailed characterization gel evolution and microstructure of geopolymers synthesized with volcanic ash (VA) and blast furnace slag (BFS) were studied. Silicate gels (N–S–H) were mainly formed in VA-based geopolymers of low compressive strength (14.07 MPa). When VA and BFS each account for 50%, VA-BFS–based geopolymers possessed a compressive strength of 55.6 MPa, as well as the homogeneous C–(A)–S–H and N–A–S–H gels formed. Synthesis protocol for VA-BFS–based geopolymers.


Introduction
Geopolymer is a kind of inorganic polymer gel material with three-dimensional network structure (Davidovits 1989). Geopolymerization refers to the dissolution, hydration, gel formation and condensation of aluminosilicate and other raw materials in alkaline solution. Due to the special polymerization network of geopolymers, they exhibit highly desirable engineering, durability properties (Zhang et al. 2017), environmental cleaning capabilities and solid waste heavy metal contamination treatment capabilities ). In addition, geopolymers are a prospective concrete to replace the functions of the cement mortar, as well as concrete of low carbon-dioxide (CO 2 ) footprint (Miller et al. 2018;Abdulkareem et al. 2021). In the last decade, industrial wastes such as fly ash (FA), blast furnace slag (BFS) and solid waste incineration fly ashes (MSWI) have been extensively exploited as raw materials in the preparation of geopolymers (Lemougna et al. 2018;Provis, 2018;Huang et al. 2019Huang et al. , 2020. Geopolymers are mainly viewed as a subset of alkali activated materials (AAMs) of low calcium content (Provis and Van Deventer 2013). However, for some industrial waste, e.g. slag, calcium is inevitably contained in the raw materials (Tripathy et al. 2020). Therefore, based on the nature of their cementitious components (CaO-SiO 2 -Al 2 O 3 system), geopolymers of low calcium and high calcium gels are usually classified (Nath and Sarker 2017;Nuaklong et al. 2018).
Through the calculation with partial charge model (PCM), it is demonstrated that the dissolution and hydrolysis of low calcium aluminosilicate materials (e.g. metakaolin) can yield Al(OH) 4 − , [SiO 2 (OH) 2 ] 2− and [SiO(OH) 3 ] − , of which the schematic diagram is as follows (1)-(4) (Glukhovsky 1994;Weng and Sagoe-Crentsil 2007). Subsequently, the hydrolysis reaction begins when Responsible Editor: George Z. Kyzas Alkaline activation of high calcium systems is consistent with that of low calcium-based alkali-activated systems, but the structural development of geopolymers is a highly heterogeneous reaction process. The differences in the high calcium systems are dependent on the calcium source and alkalinity Li et al. the OH − ions in the alkaline activator break the Si-O-Al and Si-O-Si bonds (Garcia-Lodeiro et al. 2015). The ions redistribute their electronic density around the Si and Al atoms, weakening the Si-O-Si and Si-O-Al bonds and making them more liable to rupture, with reaction sketches of (5)-(6) (Garcia-Lodeiro et al. 2013. Then, nucleophilic substitution reactions take place in the condensation process (Glasser and Lachowski 1980;Glasser and Harvey 1984;Weng et al. 2005). When the OH groups from [SiO 2 (OH) 2 ] 2− , [SiO(OH) 3 ] − and the Al ions in Al(OH) 4 − are interconnected by mutual attraction, the two OH groups form an intermediate oligomer by releasing a H 2 O molecule, the reaction process being shown in the diagram below (Kamseu et al. 2021). Depending on the type of polymeric bond formation, the main alumino-silicate stabilized network structures in 3D are: (Rahmiati et al. 2014). The positive charge of the metal ions balances the negative charge of the intermediate oligomer to form an amorphous gels known as basic alkaline aluminosilicate hydrate (Mn-(-SiO 2 )z-(-AlO 2 )n·wH 2 O), denoted by N(C)-A-S-H gels (Garcia-Lodeiro et al. 2015;Palomo et al. 2015;Walkley et al. 2016). (1) 2022). At pH > 12, Ca 2+ is involved in the [SiO 2 (OH) 2 ] 2− , [SiO(OH) 3 ] − and Al(OH) 4 − formation, resulting in C-(A)-S-H or (N, C)-A-S-H gels. And it is accompanied by secondary reaction products, such as calcium aluminate hydrate (AFm) type phase, hydrotalcite and zeolite (Yip et al. 2005;Temuujin et al. 2009). However, as the reactions advance, the alkaline cations are taken up into the structure, and OH − is consumed during hydrolysis; thus, the pH of the system decreases, and the formation of C-(A)-S-H gels becomes complicated. Furthermore, in a system with both C-(A)-S-H and N-A-S-H gels, there is lack of discussions about the orientation, polymerization and condensation between these gels.
Volcanic ash (VA) is the fragments of pulverized rocks, minerals and volcanic glass with particles less than 2 mm in diameter (Rose and Durant 2009). Volcanic ash dumps have adverse effects on human health and aviation, such as long-term silicosis hazard and jet engine damage (Horwell and Baxter 2006;Farquharson et al. 2022). In China, VA is mostly found in remote areas (e.g. Xin jiang and Xi zang province) and might be exploited to as industrial shortage concrete materials or additives for remote urban construction (Pan et al. 2021). It is noteworthy that volcanic ash is a potential and abundant raw material for building application. However, although VA endures an atomization process of hot molten magma, it seldom possesses enough intrinsic reactivity for the geopolymerization process, which is related to the chemical composition, fineness of particles, etc. (Djobo et al. 2017;Lemougna et al. 2018). Some studies reported the processes employed to promote the reactivity of VA for geopolymer synthesis, such as calcination, mechanical activation (Bondar et al. 2011;Song et al. 2021). Usually, reactive components are added for the synthesis of VA-based geopolymers. For example, Djobo et al. reported that volcanic ash-based geopolymers was of 27 MPa in 28 days of curing when volcanic ash was replaced by 4% of BFS (Djobo and Stephan 2021). Lemougna et al. reported that 50% BFS substituted for low activity volcanic ash and that the geopolymer reached an optimum compressive strength of 85 MPa after 28 days of curing at 25 °C. And only 10 wt% of slag was enough to reduce its initial setting time from more than 7 days to 6.7 h (Lemougna et al. 2020). Therefore, due to the higher reactivity, high calcium oxide content and more glass phase, BFS can supply sufficient oligomers to form densified C-(A)-S-H gel structures in geopolymers (Jeong et al. 2016). Furthermore, it is extensively used as a raw material or as an additive component in the preparation of VAbased geopolymers. Obviously, the substitution of BFS to VA facilitates the formation of high calcium geopolymer systems of both and C-(A)-S-H and N-A-S-H gels.
However, the effect of synergistic gel formation on the mechanical properties of VA-based geopolymers has not yet been studied. Thus, in the present work, we attempted to synthesize VA and BFS-based geopolymers at various BFS additions, so as to elucidate the combination of N-A-S-H and C-(A)-S-H gels. Furthermore, the effects of the alkali activators Na 2 SiO 3 and NaOH species and water content on the formation of gel microstructures were analyzed using structural and mechanical characterization tools to explore the optimum synthesis conditions.  Sodium metasilicate nonahydrate (Na 2 SiO 3 •9H 2 O, AR, 98%) and sodium hydroxide (NaOH, AR, 96%) were supplied by Sigma-Aldrich in China, which was used as alkaline activator in the preparation of geopolymers. The water used for the experiments was deionized water. Table 2 gave the regime of the preparation of the VA-based geopolymers. The BFS was increased from 0 to 60%, the alkaline activator was combination of 0.5 mol Na 2 SiO 3 and NaOH, the dosage of water was increased from 4 to 8 mol. A total of 220 g of raw materials were used for the synthesis. First, diverse proportions of alkaline activator were dissolved in a certain molar amount of water. The well-mixed VA and BFS were added to alkaline activator, agitated for 5 min, and the slurry was then poured into 30 mm × 30 mm × 30 mm cubic steel mold. Then, steel mold was vibrated on the vibrating platform until liberating all

Characterizations
The compressive strength was tested on 7-day-cured geopolymer specimens using a DNS100 universal testing machine. The displacement rate of 0.3 mm/min was selected. For each geopolymer, at least three replicate samples each geopolymer were tested, and the average value was used by the three specimen measurements, and the standard deviation was calculated. By using the X-ray diffraction (XRD, DY1602 Empyrean, China) technology, geopolymers were ground less than 100 mm to produce specimens for mineralogy analysis. Fourier transform infrared spectroscopy (FTIR, Nicolet is50, America), with dried potassium bromide (KBr), was used as blank background and was utilized to identify the chemical linkages in geopolymers. The products of the geopolymerization reaction were characterized using solid-state nuclear magnetic resonance spectrometer (NMR, AVANCE III 500, Switzerland) to analyze their intrinsic structural features. The 29 Si NMR spectra was collected using a solid-state NMR spectrometer operating at 500 MHz (B 0 = 11.7 T), generating a Larmor frequency of 99.362 MHz for 29 Si spectra. Tetramethylsilane (TMS) as an external referred to criterions for 29Si chemical shift. The 29 Si studies were carried out utilizing ZrO 2 rotors with a diameter of 7 mm, a single pulse MAS probe head, a rotating rate of vR = 5 kHz and a recycle delay of 30 s, with scans ranging from 108 to 2000. The microscopic morphology and elemental mapping of geopolymer specimens were examined with the tungsten filament scanning electron microscope (SEM, Quanta 250, China) equipped with energy dispersive X-ray spectroscopy (EDS). Figure 3 shows the compressive strength of the VA-based geopolymers synthesized as a function of the BFS proportion. As increasing the BFS from 0 to 50%, compressive strength of the geopolymers increased from 14.07 to 55.6 MPa. Then, it decreased to 46.05 MPa as increasing the BFS proportions to 60%. Without BFS addition (No. 1), low contents of Al 2 O 3 and CaO are involved in the geopolymerizaton process, resulting in low contents of gel formation and low compressive strength. As increasing the BFS proportions, more N-A-S-H and C-(A)-S-H gels are formed; thus, a geopolymer with high compressive strength (> 50 MPa) is synthesized. However, at high BFS proportions, a virtually impermeable layer of calcium-deficient hydrated aluminosilicate is formed on the surface of the slag particles, which hinders the alkaline activation reactions (Myers et al. 2013). In addition, alumina species in excess can delay the dissolution of silica in the first stage of geopolymerization reaction, which in turn forms more oligomers and affects the hardening of the geopolymers adversely (Sagoe-Crentsil and De Silva 2015; Bayiha et al. 2019). Experiment was carried out using 0.5 mol Na 2 SiO 3 as alkali activator with the most suitable water content of 6 mol. Detailed experimental results are shown in SI (Figs. 1, 2, 3 and 4). Therefore, the optimal BFS proportion of 50% was found in the synthesis of VA-based geopolymers. Figure 4 shows the XRD spectra of the VA-BFS-based geopolymers. The hematite (Fe 2 O 3 ) and calcium silicate (Ca 8 Si 5 O 13 ) feathers of the VA and BFS, respectively ( Fig. 1) are disappeared, which implies the geopolymerization process. The hump of the VA precursor is centered at approximately 22° 2θ (Fig. 1), and a broad feature centered at 24°-30° is observed for the VA-BFS-based geopolymers, indicating the formation of N-A-S-H and C-(A)-S-H gels (García-Lodeiro et al., 2010;Zhao et al., 2019). As increasing the BFS proportions, the hump is shifted to higher angles. Although it is generally accepted that the XRD hump of N-A-S-H and C-(A)-S-H gels is centered ambiguously among 24-30° Walkley et al. 2016), some studies reported that the N-A-S-H and C-(A)-S-H gels are centered at 26° and 30° (Garcia-Lodeiro et al., 2011; Perez-Cortes and Escalante-Garcia, 2020), respectively. Therefore, the right shift of the hump indicates the formation of C-(A)-S-H gels. Figure 5 shows the FTIR spectra of VA-BFS-based geopolymers. The absorption peaks observed in the 3700-3200 cm −1 range correspond to O-H stretching vibrations bonds, while the bands seen at wavenumbers 1636-1656 cm −1 are assigned to H-O-H bending vibrations, which match the water lattice and hydroxyl group (OH − ) within the geopolymeric gel structures (Lemougna et al. 2020). The antisymmetric stretching and symmetric stretching of CH 3 are represented by the spectral bands at 2933 and 2849 cm −1 , whereas bending vibration of C-H is due to the band at 1553 cm −1 (Mansur et al. 2008;Tian et al. 2021), which might be due to the residual coal in the BFS. The band at 1433 cm −1 is linked to the stretching vibrations of the O-C-O bonds in CO 3 2− , which indicates the presence of calcium carbonate (CaCO 3 ) formed by carbonation reaction during the sample preparation (Ekolu 2016). to this band, which is relatively wide (García-Lodeiro et al. 2008). As increasing the BFS proportion, the band of VA-BFS-based geopolymers shifts toward lower wave numbers, which can be interpreted as related to a wider extent of aluminum polymerization into the silicate framework (Garcia-Lodeiro et al. 2011). The shoulder of around 779 cm −1 is assumed to be the symmetric stretching of Al-O-Si bonds induced by joining SiO 4 and AlO 4 group connections, as previously reported in synthesized geopolymer gels (Fernández-Jiménez and Palomo 2005). Nuclear magnetic resolution (NMR) spectra through the absorption of radio frequency radiation by atomic nuclei, which produces the so-called NMR phenomenon, is one of the most powerful tools for molecular structure analysis and has been used as an effective method for the analysis of amorphous gels. The insufficiency in the spectral resolution for silicon in these geopolymers was surmounted by using Gaussian peak deconvolution to distinguish and quantify Q n (mAl) species (0 ≤m ≤ n ≤ 4, m, n = integer) (Duxson et al., 2005;Lee and Stebbins, 1999;Tian et al., 2020a, b). For aluminosilicates, 29 Si chemical shifts are relative to the quantity of Al atoms in the subneighborhood, and Q n (mAl) is commonly employed to represent the aluminum-oxygen tetrahedra at the connection with the silicon-oxygen tetrahedra. n is the same as the silicate representation, while m denotes the amount of tetrahedral silicon atoms replaced by tetrahedral aluminum. The peaks at −108 ppm, −99, −93, −89, −84 correspond to the presence of Q 4 (0Al), Q 4 (1Al), Q 4 (2Al), Q 4 (3Al), Q 4 (4Al) in the binder N-A-S-H, respectively (Garcia-Lodeiro et al., 2010;Tian et al., 2020a, b;Wan et al., 2020). N-A-S-H is a three-dimensional amorphous gel structure (Q 4 ) composed of tetrahedral Al and tetrahedral Si entangled by a high degree of cross-linking. The peaks at −77 ppm, −87.8 ppm and −96.4 ppm in C-(A)-S-H gels are ascribed to Q 1 , Q 2 and Q 3 (Singh et al. 2005;Bernal et al. 2013). Q 2 was thought to be the intermediate chain in C-(A)-S-H gels, and the resonance at −82 ppm was attributed to Q 2 (1Al) when Si is replaced with Al (Richardson 1999). With a resonance peak approximately of 104 ppm, Q 3 (R) emerged when the H in OH is replaced by alkali metal ion (Ca 2+ or Na + ) (Le Saout et al. 2011). Peaks with values greater than 110 ppm were ascribed to a variety of microcrystalline zeolite silicon (CS) (Fernandez-Jimenez and Palomo 2003;Olejniczak et al. 2005). Figure 6 gives the 29 Si NMR spectra of the VA-BFS-based geopolymers. Each geopolymer exhibits a broad resonance spanning from δ iso = − 65 to − 125 ppm and centered approximately at δ iso = − 95 ppm, with an inconsistent line shape of the distribution of δ iso across each sample. In this study, the principal products were switched from N-S-H gels to N-A-S-H and C-(A)-S-H gels when BFS replacement in VA ranged from 0 to 50%. In the pure VA-based geopolymers, the main component is silicate.

Results and discussion
Therefore, the main products are silicate (N-S-H) gels, where the percentages of Q 4 , Q 3 and Q 2 are 29.21%, 49.06% and 14.91%, respectively. When the BFS is from 10 to 50%, the percentage of [SiO 4 ] coordination in C-(A)-S-H (Q 0 , Q 1 , Q 2 (1Al), Q 2 and Q 3 ) was observed at 12.51, 21.05, 31.96, 34.41 and 36.26%, respectively. The percentage of [SiO 4 ] coordination in N-A-S-H gels (Q 4 (mAl)) was 63.24, 57.8, 56.04, 51.3 and 53.35%, respectively. The percentage of Q 3 (R) was 13.98, 11.47, 11.04, 9.56 and 7.91% respectively. The microcrystalline CS decreased from 6.82 to 2.49%. It can be concluded that the percentage of gel products increases significantly with increase of BFS content at constant total mass, indicating more N-A-S-H and C-(A)-S-H gel formation. The higher NMR spectrum shifts to higher frequencies, which may be due to the extent of crosslinking between silicate monomers, and the geopolymer acquires better mechanical properties. Meanwhile, microcrystalline CS is generally not detected by XRD (Fig. 4) due to the small size of the grains, and their encapsulation in the gel values over 110 ppm was assigned to distinct crystal silicon. Figure 7 shows the SEM images of VA-BFS-based geopolymers. As the BFS increases from 0 to 50%, the geopolymers exhibit a highly homogeneous and dense structure in which pores become smaller, cracks are reduced and trace amounts of unreacted particles are present. It indicates that a large amount of C-(A)-S-H and N-A-S-H gels were formed. In the alkaline activation system, BFS enhances the activity of reactants, so the unreacted particles decrease and the content of the main product gels increase. Figure 8 shows the SEM images with EDS elemental mapping of Si, Ca, Al, O and Na of VA-BFS-based geopolymers by partially enlarging the SEM images of 100% VA and 50% BFS in Fig. 7 to 1000 × . EDS element analysis shows that the 100% VA-based geopolymers consist mainly of Si, O and Na, indicating the formation of N-S-H gels. Fifty percent BFS of the geopolymer structures are composed of Si, O, Na, Al, Ca and K according to EDS analysis, which proved that N-A-S-H and C-(A)-S-H gels were formed in the system. It can be seen that the distribution of Si, O, Al and Na elements is relatively uniform, and Ca is locally distributed with no obvious gel separators, which indicates that N-A-S-H and C-(A)-S-H gels synergistically exist. By comparing the compressive strength and microstructure of the geopolymer mortar with 100% VA and 50% BFS, the more homogeneous structure and the superior mechanical strength of 50% BFS geopolymers are attributed to the synergistic effect of N-A-S-H and C-(A)-S-H gels.
In this alkaline activation high calcium system, the superior mechanical properties and more homogeneous dense structure of the geopolymers were attributed to the synergistic effect of N-A-S-H and C-(A)-S-H gels. In the high calcium system, Ca 2+ and Na + mainly exist as used to balance the negative charge caused by Al(III) substitution of Si(IV). Ca 2+ (r = 1.00 Å) and Na + (r = 1.02 Å) are similar in charge and size, so they do not change the 3D structure of the reaction products (Garcia-Lodeiro et al., 2010;Myers et al., 2013). However, the properties of the reaction products are slightly different, and the smaller ionic radius and a higher number of protons with Ca 2+ made it easier to combine with negative charges and form more gels. According to previous studies, it was shown that in high-calcium (Ca/Si > 0.60) systems, the formation of C-A-S-H takes place, in which it is believed that Na + is absorbed in the interlayer and on the surface of C-A-S-H, while partially replacing Ca 2+ ions to compensate for the negative charge caused by aluminum substitution, leading to the formation of crosslinks between C- is Na + or Ca 2+ . In the present study, 29 Si NMR (Fig. 6) indicates the presence of N-A-S-H and C-(A)-S-H gels in the system, while EDS (Fig. 8) demonstrates the existence of Na + only regions, as well as Na + and Ca 2+ overlapping regions. This indicates the co-existence of N-A-S-H and C-(A)-S-H in this alkaliactivated high calcium system, forming cross-linked and non-cross-linked gels of tightly mixed products. The crosslinked synergistic effect of N-A-S-H and C-(A)-S-H gels can help to connect the gaps between the hydrated phase and its particles, which is responsible for a homogeneous structure and superior mechanical properties obtained by the geopolymers. It is noteworthy that the best synergy and the highest content of N-A-S-H and C-(A)-S-H gels were observed in 50% VA and 50% BFS geopolymers. This is highly consistent with the compressive strength (Fig. 3) and SEM (Fig. 7) results. Furthermore, the review of the literatures showed that geopolymers can reduce CO 2 emissions from the construction industry. The recycling of such VA and BFS in geopolymer can provide desired strength, environmental benefit by reducing the environmental impacts and minimizing the waste disposal problem and conform to the concept of green and sustainable development.
Conclusion 1) In this work, it was found that the proportions of BFS significantly affected the compressive strength of VA-BFS-based geopolymers. With increasing the proportions of BFS, the compressive strength continues to increase and reaches a maximum of 55.6 MPa when adding 50% BFS, which is four times as that of geopolymers without BFS. 2) With increasing the proportions of BFS, N-A-S-H and C-(A)-S-H gel content continued to increase, reaching a maximum of 89.61% when adding 50% BFS, which was 13.9% higher than that of geopolymers without BFS.
3) The synergistic effect gels are of great significance and facilitate the development of high performance geopolymers, and the N-A-S-H and C-(A)-S-H gels achieve the best synergistic effect at 50% VA and BFS, leading to dense homogeneous structure and superior mechanical strength of VA-BFS-based geopolymers.
Author contribution YZ: Data curation, methodology, writing-review and editing. FR: conceptualization, supervision, writing-reviewing and editing, corresponding author. XT: conceptualization, supervision, writing-review and editing. SL: data curation, visualization, writing-original draft. All the authors provided critical feedback and helped shape the research, analysis and manuscript.
Funding Project supported by the National Natural Science Foundation of China (Project No. 51974093) and the Minjiang Scholar Talent Foundation of Fujian Province (Grant No. GXRC-20067).
Data availability Data are available from the authors upon request.

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The authors declare no competing interests.