1. Introduction
Volcanic tuffs are natural building materials that have been used since historical times because of their softness, ease of processing, and low thermal conductivity [
1,
2]. Their popularity follows from their common occurrence and a large variety of colours, textures, compositions, and grain sizes [
3].
Volcanic tuffs, the natural pozzolans, are the supplementary cementitious materials applied in the environment-friendly concrete industry [
4]. Depending on the characteristics, volcanic tuffs are considered as replacements for Portland cement at different replacement levels. As it has been shown, the addition of volcanic tuff can increase the compressive and flexural strength of mortar [
5,
6] as well as improve the resistance of concrete to sulphates and alkali-silica reactions [
7]. Zeolite-rich tuffs, which contain a large amount of reactive SiO
2 and Al
2O
3, are able to increase the flexural and compressive strength of concrete blocks in the late hardening phase [
8]. According to the paper of Ababneh and Matalhah [
9], the quantities of volcanic tuffs available in Jordan are high and estimated at 800 million tons. The chemical composition rich in SiO
2, Al
2O
3, and Fe
2O
3 qualifies these materials as supplementary cementitious materials. Jordanian volcanic tuffs were classified as natural pozzolans for use in concrete production. At a 10% replacement level, the volcanic tuffs produced compressive strengths comparable to those obtained when Portland cement was used alone [
9]. Moreover, the alkali-silica expansion was reduced when the replacement level increased. The SiO
2 content is positively correlated with the pozzolan activity of the additives, which was confirmed in the research on cement prepared based on Turkish tuff [
10].
Volcanic tuffs are also used in road construction. The pyroclastic volcanic aggregates, due to high porosity and ability to fragmentation, can be applied for use as permeable subbases and subgrades [
11]. Jordanian volcanic tuff was investigated as an addition to pavement construction, and the results showed the improvement of material properties, like strength, compaction, weathering, soundness, and resistance to abrasion [
12]. In addition, zeolitic tuff can be considered as a partial substitute for lime filler in the warm asphalt mix technology and does not adversely affect the mastic properties of asphalt [
13]. However, some investigations report that using the Abakaliki pyroclastic rock aggregates for road construction in a hot climate can exhibit long-term poor field performance even though the material parameters of aggregates meet the recommended limits [
14].
Rock materials are characterized by remarkably diverse properties, but the adsorption properties are especially significant in the removal of heavy metals and organic compounds [
15,
16,
17]. Locally available mined rocks are often tested for use in removing pollutants from water or wastewater [
18,
19,
20,
21,
22,
23]. The practical use of rocks for environmental engineering applications is determined by their hardness, mechanical resistance, density, grain morphology, and structural properties but mainly by the mineral composition. Mineral materials widely used in environmental technologies are clay materials and zeolites. Bentonite clay consists of smectite, which is a commonly used material in geosynthetic clay liner [
24], a passive technique of prevention the environment from pollution. Another solutions is the filled permeable reactive barrier, e.g., zeolite, which is proposed for protecting the environment against leachate from old landfills or runoff water from urbanized areas [
25,
26].
The clay minerals are applied for the removal of heavy metals from water due to their packet structure and the presence of exchangeable cations [
27]. Furthermore, the modified clays are effective adsorbents of inorganic anions, pharmaceuticals, and herbicides and are materials of bactericidal properties [
28]. The often-used natural origin mineral ion exchangers are zeolites and glauconites. The crystal lattice of both aluminosilicates and phyllosilicates contains excessive negative charge compensated by exchangeable cations. The cation exchange capacity of zeolites and glauconites may vary by origin and is equal to 64–229 meq/100 g [
29] and 11–35 meq/100 g [
30,
31,
32], respectively. As a result, zeolites and glauconites can be used as adsorbents of heavy metals and radionuclides [
29,
30,
33,
34,
35]. A feature of glauconite often used in water treatment by filtration in active techniques is the occurrence in the form of granular glauconite sands.
Volcanic tuffs are a type of pyroclastic rock formed from a material that is released during a volcanic eruption. These are fallout or flow deposits consisting of ash and dust compacted and cemented into rock. Tufts have diverse mineralogy [
36,
37] since it depends on the type of magma (basaltic, andesitic, rhyolitic). Additionally, pyroclasts can transform into zeolite, montmorillonite, or kaolinite during diagenetic processes, which further increases the diversity of their mineral composition [
38]. The tuffs also contain various cementing components, silica and carbonate minerals, clay minerals, and iron oxides [
38]. The mineral diversity of tuffs is of decisive importance in terms of their engineering applications.
Zeolitic tuff (from the region of Barsana, Maramures, Romania) containing clinoptillolite (zeolite), smectites (clay minerals), and seladonite (mica group) was able to remove Pb
2+ and Zn
2+ by ion exchange [
39]. What is worth emphasizing is that the research was conducted with the use of wastewater from a slag granulation basin in a metallurgical factory. Volcanic tuff rich in zeolite also has been assessed as a natural material helpful in removing Pb
2+ from water in a slightly acidic environment [
40]. Other studies have confirmed that zeolitic tuff is a potential low-cost adsorbent for removal of Cr
6+, Fe
2+, Cu
2+, Zn
2+, and Pb
2+ from pharmaceutical wastewater [
41]. The Jordanian volcanic tuff, rich in phillipsite (zeolite) and hematite (ferric oxide), effectively removed phosphates from the water [
42]. The literature reports also the results of research on Ukrainian volcanic tuff mainly consisted of saponite (clay material), an adsorbent of heavy metals, such as Mn
2+, Ni
2+, and Pb
2+ [
43,
44]. The adsorption occurs quickly in a slightly acidic environment and at a low temperature of 10 °C, the typical conditions for groundwater treatment. Additionally, the adsorption capacity of Ukrainian tuff from the Ivanodolynsky quarry (Rivne region) is twice that of basalt and zeolite [
44,
45].
The territory of the western part of Ukraine is rich in deposits of volcanic tuffs, which make up a huge Babin formation of the Volyn series of the Lower Vendian. They make up a whole province of aluminosilicate and other raw materials with possible unique adsorption or ion exchange properties. In the territory of Ukraine, tuffs form a volcanic-clastic cover with an area of approximately 200,000 km
2 and an average thickness of 100 m [
13]. Their estimated resources are over billion tons, which shows the importance of tuffs as a potential raw material for the region. This prompts to conduct exploratory work within the deposits and search for potential applications of Ukrainian tuffs, e.g., in the field of construction and environmental technologies.
The most well-known and studied are the Ukrainian Tashkivske and Varvarivske volcanic tuff deposits, which have reserves of about 60 million tons [
13]. The correlation between the chemical and mineralogical compositions of volcanic tuffs from Varvarivske quarry and their depth position was analysed. It was established that the dominant mineral is saponite, the trioctahedral smectite. The saponite stratum of the Varvarivske deposit is two-layered: interval 20.3–35.8 m of saponite horizon (50–70% of saponite); 35.8–68.9 m of analcime-saponite horizon (saponite 40–60% and analcime 20–35%) [
46]. According to chemical analysis, these deposits contain a significant amount of petrogenic Fe
2O
3 (12.30–12.72%) [
46]. The mineralogical composition of a volcanic tuff sample taken from a depth of 47.5 m from Ivanodolynsky quarry (Rivne region, Ukraine) was mainly the saponite at 56%, quartz at 22%, and Fe
2O
3 at 17% [
44]. Tsymbalyuk [
47] reported the chemical composition of basalt tuff taken from the Politske–2 quarry (Rivne region, Ukraine) as follows: SiO
2, 67.44%; Al
2O
3, 12.82%; Fe
2O
3, 10.14%; and TiO
2, 1.75%. Unfortunately, there are no data concerning the stratigraphic position of sample occurrence and its mineralogical composition.
Geologic and geochemical information of volcanic tuffs and associated minerals from the Rivne region in Ukraine is essential to determine those characteristics that affect the use in environmental and technical applications. Industrial use of tuffs from different locations will vary due to differences in physical and chemical properties depending on the origin. The aim of this work is to characterize the physical, mechanical, chemical, and mineralogical properties of volcanic tuffs from two wells located in the Khmelnytsky region (Ukraine). Based on these results and literature reports, a theoretical analysis of tuff use in engineering applications, such as construction and environmental technologies, was carried out. The purpose of these considerations is to guide further research on the economic use of Ukraine’s resources.
3. Results
The investigated physical and mechanical parameters suggest that the differences between the petrographic varieties of tuffs are insignificant. The results of physical and mechanical analyses of the studied volcanic tuffs are presented in
Table 2. The maximum moisture content for core R–1 for point R–1(47.2) is 7.4%, and the minimum is 4.2% for point R–1 (18.0). The maximum moisture content for core R–2 for point R–2(60.8) is 9.9%, and the minimum value is 3.3% for point R–2(86.2). The obtained range of average values of natural density was 1.92–2.66 g/cm
3 for all tuff samples, and the range of dry density values was 1.79–2.45 g/cm
3. The numerical values of these physical and mechanical characteristics of tuffs indicate that R–1(18.0), R–1(27.6), and R–2(19.6) have lower values than R–1(47.2), R–2(46.6), R–2(60.8), R–2(76), and R–2(86.2).
Table 2 shows that there are significant differences in the uniaxial compressive strength in the studied tuff samples. The highest values of this parameter correspond to R–1(63.2), R–2(60.8), and R–2(76.0) and the lowest to R–2(19.6), R–1(18.0), R–1(27.6), and R–2(86.2). This phenomenon can be explained by the mineral changes taking place as these rocks are formed. The content of water or porosity decreases in the rocks as a result of the changes typical for minerals in the deep. The analysis of the test results allows to draw a conclusion that it is not only the quantity but also the type of silica mineral phases that affects the geotechnical parameters of the rocks studied.
The composition of the main elements, determined by XRF analysis of the selected samples, is shown in
Table 3 and
Table 4 and in
Figure 4. Chemical analyses of volcanic tuff have shown that oxygen, silicon, aluminium, iron, and magnesium are the base elements of the raw samples. All samples show that the ranges of the percentage composition of the main oxides have the following values: SiO
2, 41.65–53.27%; Al
2O
3, 11.98–13.60%; Fe
2O
3, 11.09–14.90%; and MgO, 3.06–9.86%. In terms of chemical composition, the samples of the studied volcanic tuffs are close to a number of volcanic rocks of the Tashkovske and Varvarivske deposits [
46].
The performed analyses of the XRD allowed us to obtain diffractograms of all samples of the studied volcanic tuffs, which are shown in
Figure 5 and
Figure 6, and all identified minerals are compiled in
Table 5 and
Table 6. The analysis of the obtained diffractograms showed that the studied volcanic tuffs consist of several phases. It can be seen from the obtained diffraction patterns that the intensity of the reflections of each mineral phase of the tuff sample depended on its amount in the sample under study and the degree of crystallization. The degree of crystallization of the minerals was estimated by analysing the obtained reflections for each sample. That is, the higher the diffraction peak and, at the same time, the narrower the half-width, the fewer or no amorphous minerals were present in the tuff samples. All diffraction patterns show that the crystallinity of the samples is variable, and the smallest can be found in those from the surface.
Based on the results of the analysis of the chemical and mineral composition, two horizons were distinguished in the studied volcanic tuff deposits. The visualization is presented in
Figure 7. The upper horizon is represented by the core samples R–1(18.0), R–1(27.6), and R–2(19.6). This horizon of volcanic tuffs is polyphasic and contains seven minerals, the percentage of which is greater than 2%. Chlorite was identified as the predominant mineral (30–35%). This was also evidenced by the high content of MgO in the samples. The accompanying clay mineral was kaolinite. In addition, core samples of the upper horizon contained pyroxene, quartz, hematite, and small amounts of calcite and anatase. The lower horizon of the tuffs can be assigned to the following core samples: R–1(47.2), R–1(63.2), R–2(46.6), R–2(60.8), and R–2(76.0). Intense
X-ray reflection in the range from 26–28° in 2θ corresponded to the presence of analcime, which belongs to the group of zeolites. It was the main mineral present in about 40–62%. Other identified minerals are quartz, hematite, calcite, and residual anatase. Sample R–2(86.2) had the strongest
X-ray reflection (at about 27° in 2θ). This indicated that the dominant mineral phase was quartz (about 70%). The composition of this sample was so different that it was not included in the lower horizon of the tuffs.
5. Conclusions
In the volcanic tuffs coming from two boreholes in Khmelnytsky region (Ukraine), layers that differed significantly in visual characteristics were identified. All layers were characterized by visible brown-grey and red-brown colour, low compressive strength (4.34–11.13 MPa), and high water absorption of about 30%. The oxygen, silicon, aluminium, iron, and magnesium were the base elements of the raw samples. and the percentage composition of the main oxides was: SiO2, 41.65–53.27%; Al2O3, 11.98–13.60%; Fe2O3, 11.09–14.90%; and MgO, 3.06–9.86%. Well-crystallized mineral phases have been identified.
Based on the results of the analysis of the chemical and mineral composition, two polyphasic horizons were distinguished in the studied volcanic tuff deposits. The upper horizon contained chlorite (30–35%), quartz (15–20%), kaolinite (16–18%), pyroxene (10–11%), hematite (8.5–10%), calcite (8–10%), and small amounts of anatase (2–2.5%). In the lower horizon, analcime (40–62%), quartz (15–35%), hematite (15–18.5%), calcite (5–10%), and residual anatase (1.5–2%) were found.
Given the high value of water absorption value, the tuffs cannot be an effective supplementary cementitious materials (SCMs). The distinctive colour also makes it impossible to use them as an SCMs despite the significant content of analcime in the lower horizon. The colour of the tuffs suggests that they could be used as an economically viable fine aggregates for the exterior design of engineering objects.
Based on the obtained results and literature analysis, we concluded that the two potentially expedient horizons can have an engineering and economic effect on environmental technologies. The peculiarity of the upper and lower horizons is that they contain minerals that have good sorption properties to heavy metals. The special value is the upper horizon with minerals (chlorite and pyroxene) containing iron (Fe2+) in the crystal structure. It is a potential line of research to check whether volcanic tuffs from Khmelnytsky region (Ukraine) can be effective in technologies for reducing the toxicity of Cr6+, U4+, and halogenated organic matter. Thus, volcanic tuffs of the upper horizon can become an alternative to expensive metallic iron (Fe0).
The chemical and mineralogical characteristics of the second horizon made it possible to predict the sorption properties of tuffs due to the presence of analcime and hematite. Analcime appears to be a moderate ion exchanger with potential application in Pb2+, Cu2+, and Cd2+ ions removal. Efficient water treatment can be expected with ions with a small hydration radius. The significant content of hematite in tuffs guides the recognition of potential application for the removal of As5+, As3+, Cr6+, Cr3+, U6+, Sb5+, and Se4+ oxyanions from the water.