Characterization of different types of silica-based materials

Waste glasses based on silicon dioxide (silica) belong to one of the most demanded secondary raw materials. Besides the glass industry, with strict requirements for source materials, silica-based granular materials from waste glasses are considered suitable for use as fillers in alkali-activated cement-based composites. However, due to variations of ground waste glass powder composition and properties a comprehensive characterization is often needed. This study investigated commercially available silica-based powders by scanning electron microscopy, X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy and Fourier transform infrared Raman spectroscopy, zeta-potential and pH measurements, etc. The differences found in the non-silica impurities composition and structure of SiO2 particles (amorphous vs crystalline) were considered as determinative factors that will affect particles’ interaction with water and cement binder. The observed results provide a fundamental background and will contribute to a better understanding and explanation of the silica-based secondary raw materials interaction reactions in concrete or mortar.


Introduction
The glass manufacturing industry products based on silicon dioxide, such as flat glass panels, glass containers, automotive glass, glassware, optical elements, fiberglass, etc., are the background objects of contemporary everyday life.Withal, it needs to be mentioned that silicon dioxide-based glasses, e.g.soda-lime glass, borosilicate glass, aluminosilicate glass, quartz glass, etc., are non-biodegradable and therefore can cause significant environmental problems when landfilled as waste [1][2][3].Fortunately, the glass recycling technology allows to retain the main glass properties using the waste glass as a secondary raw material and saving natural resources [1][2][3][4].However, the requirements for the quality of input waste glass material prohibit its complete reuse for new glass manufacturing [1][2][3][4].Besides the separation of waste glass from organic contaminants, plastic, metallic impurities, ceramic, or stone fragments there are requirements for the glass itself.Indeed, the composition of silicon dioxide-based glass significantly modifies the properties of different types of glass and therefore affects the required recycling procedures [1,2].For instance, the aforementioned durable borosilicate and aluminosilicate glasses have higher melting temperatures and lower thermal expansion coefficients than more common soda-lime glass and therefore would cause inhomogeneities in the remelted substance [1,2].If consider the differently colored soda-lime glass packaging, e.g.bottles from beverages, it requires at least 70% of the same colored waste glass for green glass manufacturing, at least 80% for brown glass, and 94-98% for clear glass manufacturing [1].Such strict requirements for waste glass separation forced the search for other ways of waste glass utilization [1,3,4].
Apart from the glassware industry, the one of promising directions for waste glass utilization is the use of ground waste glass as a sand replacement in cement-based composites (concrete, mortar, etc.) [4][5][6][7], which is expected to reduce carbon dioxide emission.Indeed, the construction and building industry is one of the largest consumers of natural sand.Therefore, any alternative secondary raw material that can replace sand will be beneficial for sustainable urban development strategy and reduction of environmental burden [6,7].Unfortunately, even for silica-based powders from waste glasses usage in cement-based products exist certain limitations.Indeed, the waste glass-based sand replacement in concrete on the one hand should react with a cement binder and form a calcium-silicate-hydrate gel (the substance responsible for concrete strength) and on the other hand should not negatively affect the alkalisilica-reactivity (ASR) of concrete that can cause serious expansion and cracking [8,9].To prevent concrete failure, it is necessary to alter the reactivity of recycled silica-based waste particles and/or improve the bonding between them and cement paste.It was shown that such additives as metakaolin, fly ash, or silica fumes are able to reduce high ASR and improve concrete properties [8,9].However, some researchers indicate that the use of waste glass powder as partial sand replacement in concrete has a positive pozzolanic reaction and can reduce the negative effect of ASR on concrete as well [10].
Conclusively, knowledge about properties of ground waste glass powder will be able to help us predicting its interaction with alkali-activated cement binders and mitigating risks of concrete failure by powder modification or using of proper additives.Therefore, in this study characterization of selected silica-based materials (powders) suitable for use in cement-based composites by microscopy and spectroscopy methods was performed and discussed.

Materials and methods
The following two industrially available silica-based micro-granular materials (powders) were investigated in this study.The first one, marked as "SiO2 type 1", is a micro-milled silica sand, with stated SiO2 content above 99 % (Sklopísek Střeleč, a.s.).The second, marked as "SiO2 type 2", is a glass powder made by grinding (milling) of ordinary, colorful (translucent, green, and brown) waste glass shards (Refaglass Trade, s.r.o.).The laser diffractometry shows that the selected materials have comparable median particle sizes, which was 59.5±1.4 µm for SiO2 type 1 and 36.0 ±0.2 µm for SiO2 type 2. Industrially used SiO2 type 1 and type 2 materials were compared with the reference of SiO2 white quartz powder (Sigma-Aldrich) with the stated size of microcrystals in a range of 0.5-10 m.

Scanning electron microscopy
SiO2 powders were studied using a low voltage scanning electron microscope (SEM) MAIA3 (TESCAN, a.s.) employing a secondary electrons detector.The SiO2 powders were spread in a dense layer over sticky copper tape on a solid support.All tested samples were placed on a rotational table (carousel) of the SEM chamber that was evacuated to the pressure in order 10 -3 Pa.For comparing the size, shape, and surface morphology of SiO2 particles, the images were captured at different magnifications in the resolution and field modes of the microscope at acceleration voltage 10 kV.

X-ray photoelectron spectroscopy
The X-ray photoelectron spectroscopy (XPS) characterization of SiO2 powders was performed using the AxisSUPRA photoelectron spectrometer (Kratos Analytical) with base pressure of 10 -7 Pa, which enables collecting XPS signal from a 0.5 mm 2 small spot.Before measurements the SiO2 powders were spread over a sticky copper tape (similarly to SEM measurements) and evacuated.Monochromatized Al Kα with emission of 10 mA and charge compensation was used for all measurements.The survey spectra were collected at take-off angle 90° from 1200 to -5 eV with a passing energy of 80 eV.The signals from oxygen, carbon, and silicon were collected with a passing energy of 10 eV (for purposes of more detailed analysis).The signals from additional detected elements were collected with a passing energy of 80 eV (for purposes of quantitative analysis).
Measured XPS spectra were fitted using the KolXPD software (Kolibrik.net,s.r.o.).The Si 2p spectra were fitted by four G*L doublets with the splitting of 0.6 eV and the 2p3/2 to 2p1/2 ratio of 2. Shirley background was subtracted, data were normalized to the maximum intensity and offset was added for better clarity.The doublets correspond to Si 4+ , Si 3+ , Si 2+ and Si 1+ oxidation states (from higher to lower binding energy).Because of the use of charge neutralizing system, all XPS data were shifted to lower binding energies.All data were corrected so the Si 4+ component of Si 2p region would be located at 103.7 eV.The O 1s spectra were fitted by three to four peaks of Voigt profile.The peaks correspond to H2O, binding oxygen (BO) in a SiO2 (Si-O-Si bond), a mixed contribution of non-binding oxygen (NBO) in a Si-O-D+ (D+ as dopant) and hydroxyl groups (-OH), and an O-C-Si component.Shirley background was subtracted, data were normalized to the maximum intensity and offset was added for better clarity.

Fourier transform infrared spectroscopy
The infrared absorbance spectra of SiO2 powders were recorded using the dry-air ventilated Nicolet IS50 Fourier Transform Infrared (FTIR) spectrometer (Thermo Fischer Scientific Inc.) equipped with Attenuated Total Reflectance (ATR) and Raman accessories.Before analysis, the SiO2 powders were dried at 150°C in an evacuated chamber to remove adsorbed water.Then, the SiO2 powders (4 mg) were pressed into ⌀3 mm pellets and analyzed employing the ATR FTIR spectroscopy in a spectral range of 200-1800 cm -1 with 4 cm -1 spectral resolution using a solid-state beam-splitter, diamond prism, and cooled DTGS detector.Additionally, pellets were analyzed by the FTIR Raman spectroscopy in 70-3500 cm -1 spectral range with 2 cm -1 spectral resolution using YAG:Nd laser with excitation at 1064 nm and InGaAs photodiode as a detector.Measured infrared spectra were processed using OriginPro 2016 software (OriginLab Corporation) under the academic license.

Surface potential and pH measurements
The result of interaction of SiO2 powders with deionized (DI) water was studied via zeta (ζ) potential and pH measurements using Zetasizer Nano ZS -ZEN3600 equipped with multi-purpose titrator MPT-2 (Malvern Panalytical).The zeta potentials of highly diluted SiO2 suspensions were determined using Electrophoretic Light Scattering (ELS) mode which should be applicable for particles with sizes from 3.8 nm to 100 m.The analysis of SiO2 particles' surface properties was supported by measuring the surface potential of as-received SiO2 powders spread over a copper tape, using Scanning Kelvin Probe (SKP) System SKP5050 (KP Technology).

Scanning electron microscopy
The electron microscopy images of the SiO2 type 1, SiO2 type 2 and SiO2 reference powders are shown in figure 1.According to the SEM images, where the large, irregular pieces with sharp edges decorated by the small fragments can be seen (figure 1(a), 1(b)), the investigated industrially used SiO2 powders are represented by particles with a broad size range.For SiO2 type 1, the largest observed particles were around 90 µm, while for SiO2 type 2 the particles as large as around 50 µm were observed.No significant difference between the surfaces (flat with parallel "scratches" due to cracking) of those, the largest particles of both mentioned SiO2 powders, was observed (figure 1(d), 1(e)).The smallest particles for both analyzed SiO2 powders were in fact the sameirregular fractions in the range of tens of nanometers.The particles of SiO2 reference powder (figure 1(c), 1(f)) are smaller (found in a range of approximately 0.2-5 m) and with less sharp edges than investigated industrial SiO2 powders.

X-ray photoelectron spectroscopy
The wide scan XPS survey spectra on analyzed SiO2 powders are shown in figure 2. In the case of SiO2 type 1 powder (micro-milled silica sand), signals originating from oxygen, carbon, silicon, and sodium were detected.In the case of SiO2 type 2 powder (ground waste glass), signals from oxygen, carbon, silicon, sodium, calcium, magnesium, nitrogen, and potassium were detected.In the spectrum of reference (chemically clean from manufacturing) SiO2 powder, signals originating from oxygen, carbon and silicon were found.The atomic concentrations of elements in studied materials derived from the XPS measurements were compared in table 1.From these results we can conclude that the SiO2 type 1 is chemically almost as clean as the SiO2 reference with a similar O/Si ratio and slightly lower amount of C.Here it should be noted that observed carbon may originate from samples, since is typically found on open-air stored materials [11], but also from the used substrates.In contrast, the SiO2 type 2 contains a lot of different non-silica impurities which generally originate from additives in the waste glass material and relates to glass manufacturing or recycling process [5][6][7]12].Apart from higher amount of carbon, the detected elements in SiO2 type 2 are comparable to additives in the soda-lime translucent (clear) glass reported in literature [5][6][7].However, such a variety of elements that can be in different chemical states (which is also affected by recycling process) complicates the interpretation of the results.For instance, sodium and especially lime ions may form a bridging element in the Si-O glass chains [13].Nevertheless, these observations can be taken as generally positive for the potential use of SiO2 type 2 in cement-based composites since no unwanted elements, for instance chlorine, were detected.Next, the more detailed analysis of Si 2p and O 1s regions (figure 3) shows that in the case of SiO2 type 1, most of the Si 2p signal originates from the SiO2 where silicon is present in the 4+ oxidation state.In the case of SiO2 type 2, Si 2p region consisted only of a single broad doublet originating from the SiO2.No other Si oxidation states, than related to SiO2, were observed in this case.In contrast, the SiO2 reference contains quite a large amount of silicon suboxides (SiOx), especially in a Si 3+ state.
The binding oxygen, non-binding oxygen, traces of water and the presence of hydroxyl groups on the SiO2 particles' surface were deduced from the analysis of the O 1s region.In the case of the SiO2 type 2, the Na and Ca dopants (figure 2 or table 1) are bonded to the oxygen in the Si vicinity (NBO contribution), while silicon is in Si 4+ state [13].Besides, the strong signal from water or hydroxyl groups was deduced for this powder.In the case of SiO2 type 1, most of the signal in O 1s region is related to binding oxygen with weak traces of water and hydroxyl groups.The lowest water content on the surface of particles was deduced from the analysis of SiO2 reference O 1s region.On the other hand, in this case presence of non-binding oxygen, hydroxyl groups and O-C-Si component on surface was deduced.
It has to be noted, that Si 2p and O 1s peaks originating from the SiO2 type 1 powder were narrower than the same peaks of SiO2 reference and SiO2 type 2 (figure 3) clearly indicating better-formed crystalline structure of the SiO2 type 1 sample compared to other two [11,14].The wide peaks of the  SiO2 type 2 and SiO2 reference in XPS spectra indicate rather the amorphous structure of these materials that can be affected by contaminants and hydroxyl groups.

Fourier transform infrared spectroscopy
The ATR FTIR spectra of analyzed SiO2 materials are shown in figure 4(a).As can be seen, the IR absorptance spectra of reference SiO2 and the SiO2 type 1 in the Far-UV -Mid-IR range are very similar.The absorbance bands found in the spectra were attributed to Si-O-Si (in some cases noted just as Si-O) vibrations: asymmetrical bending at 463 cm -1 , symmetrical stretching at 788 cm -1 , and asymmetrical stretching at 1082 cm -1 [11,[15][16][17].These narrow bands observed in case of SiO2 reference and SiO2 type 1 correspond to the spectra of a crystalline form of SiO2, which agrees with the results of XPS analysis only for SiO2 type 1.In case of the SiO2 reference, the high concentration of silicon suboxides in the sample and consisting of the Si-O-Si chains with irregular bonding of Si atoms to their oxygen neighbors broaden the Si 2p and O 1s peaks (figure 3) which indicate deteriorating of the crystalline quality of the SiO2 reference powder near the surface.In contrast to previous, in the spectrum of SiO2 type 2 the broad bands related to the mentioned vibrations clearly indicate the amorphous character of the investigated material.
The FTIR Raman measurements (figure 4(b)) confirm the crystalline structure of SiO2 type 1 and the SiO2 reference by a clearly seen narrow peak near 464 cm -1 , characteristic of quartz together with other peaks ensemble [15,18].In agreement with the XPS and ATR FTIR spectroscopy, no peaks were found in the amorphous SiO2 type 2 spectrum.Here it needs to be mentioned, that in contrast to surface sensitive XPS analysis the infrared spectroscopy method also gives information about the bulk of investigated material (especially important for powder) which in our case allows better results interpretation.According to the Raman spectroscopy, SiO2 type 1 and SiO2 type 2, in contrast to SiO2 reference, even after drying still contain the water (characteristic hydroxyl groups) on the surface of particles.These findings correspond to higher signal from water traces deduced from the XPS analysis of the O 1s peak (figure 3).Since the IR Raman is quite sensitive to the OH groups these observations can indicate the high hygroscopic properties of investigated industrially used silica-based powders, especially in case of ground waste glass [12].

Surface potential and pH measurements
The measured pH and ζ-potential values for SiO2 powders' suspensions in DI water are summarized in table 2. It can be seen that the SiO2 type 1 dispersed in DI water forms a suspension with neutral pH 6.5±0.5 similar to the suspension of SiO2 reference with pH 6.7±0.5 and water itself with pH 6.6±0.5.Contrariwise, SiO2 type 2 dispersed in DI water forms a suspension with a more alkaline pH of 9.8±0.5.These observations agree with the surface-sensitive XPS measurements, where for SiO2 type 2 the alkali-forming Na and Ca elements [5][6][7] were detected.Therefore, it should be considered that SiO2 type 2 used with alkali-activated cement-based composite materials can result in a more alkaline pore solution than the use of sand or SiO2 type 1 [7,10].The negative ζ-potentials in DI water with a median value from -26 to -30 mV were observed for all analyzed SiO2 powders (table 2).This means that all analyzed powders can be well dispersed in water.However, it also needs to keep in mind the broad size distribution of particles in powders (figure 1) that may affect this parameter (as well as pH change).For instance, the Dynamic Light Scattering mode for particle size analysis in the current case was not applicable due to too large particles that generally should sedimentate faster.
Finally, it should be mentioned that for as-received SiO2 powders spread over a copper tape, it was possible to measure surface potential and identify the potential difference (which for flat glass slices generally is not reproducible).The SiO2 type 2 demonstrates about 440±10 mV lower surface potential than SiO2 type 1.It is believed that it becomes possible due to water and OH groups (detected by XPS and FTIR Raman) on the particles' surface and/or possible particles' surface defects.

Conclusions
Commercially available silica-based powders were investigated by microscopy and spectroscopy analytic techniques in this study.In particular, the characterization of SiO2 powders by scanning electron microscopy, X-ray photoelectron spectroscopy, ATR FTIR spectroscopy, FTIR Raman spectroscopy, ζpotential, and pH measurements was performed.The SiO2 type 1 and SiO2 type 2 powders have similar appearances, and both are composed of particles with sizes from tens of nano-to tens of micrometers.Nevertheless, significant differences were found in the chemical composition and structure of industrially available SiO2 particles and SiO2 reference.
It was found that the SiO2 type 1 powder (micro-milled silica) is chemically almost as clean as reference white quartz sand with particles that have a crystalline nature in a bulk, clearly seen from both FTIR (ATR and Raman) measurements.On the other hand, the XPS analysis reveals a lot of Si suboxides on the surface of SiO2 reference indicating amorphization of particles surface.In contrast, the SiO2 type 2 powder (ground waste glass), contains a lot of non-silica impurities related to the glass manufacturing (or recycling) process with particles that have an amorphous structure.Both investigated industrial SiO2 powders demonstrate negative ζ-potentials (median value from -26 to -30 mV) in water, i.e. can be well dispersed.However, the pH measurements show that powders' interaction with water is different.The SiO2 type 1 forms a neutral suspension with pH 6.5±0.5 similar to water itself and reference SiO2.Contrariwise, SiO2 type 2 forms a more alkaline suspension with pH 9.8±0.5, attributed to the presence of sodium and other alkali-forming impurities in SiO2 type 2 powder.These findings provide a fundamental background that will contribute to a better understanding and explanation of the interaction reactions of selected powders in alkali-activated cement-based composite materials as well as will be helpful in the particles' surface modification.

Figure 3 .
Figure 3.Comparison of the normalized Si 2p and O 1s peaks from XPS measurements.

Table 1 .
Atomic concentration of chemical elements derived from the XPS survey spectra.

Table 2 .
Measured pH and ζ-potentials in deionized water.