A New Study on the Eastern Flank of the Loma Blanca Deposit (Cuba) to Establish the Mineralogical, Chemical, and Pozzolanic Properties of Zeolitised Tuffs

Abstract

The geological nature of the territory of the Republic of Cuba has favoured the formation of large and varied deposits of volcanic tuffs enriched by various species of zeolites. Today, new zeolite deposits continue to be discovered in the country. This work aims to present the results of a study carried out in an unexplored area that is located approximately 1.2 km east of the Loma Blanca deposit, outside the mining operation limits. To carry out this research and to establish a qualitative comparison between both sample populations, four samples were taken from the study area, and another four were taken from the Loma Blanca deposit. The characterisation of the samples was performed by XRD, SEM, and XRF. The pozzolan quality was determined by the pozzolanicity test (PT) and quality chemical analysis (QCA). Finally, a study of the mechanical strength (MST) was performed at 7, 28, and 90 days, using mortar specimens made with PC/ZT: 75–25% and PC/ZT: 70–30%, respectively. The results of the studies using XRD, SEM, and XRF indicated that both groups of samples had a similar complex mineralogical composition, consisting mainly of mordenite and clinoptilolite accompanied by secondary phases such as quartz and amorphous materials in the form of altered glass. The pozzolanicity test showed that both the samples from the study area and those from the Loma Blanca deposit behaved like typical pozzolans, which is a trend that can be seen in the high values of mechanical strength to compression up to 72 MPa for the PC/ZT: 75–25% formulation and 66 MPa for the PC/ZT: 70–30%. The results obtained establish that the zeolite varieties detected in the study area are similar to those of the Loma Blanca deposit, which could have a positive impact on the increase in current reserves, especially for manufacturing pozzolanic cements with properties that contribute to the preservation of the environment.

1. Introduction

In the geological make-up of Cuba, the lithologies of the volcanic arc of the Cretaceous, Paleocene, and Eocene ages stand out, as well as those of the ophiolitic complex, which favours the formation of zeolitised tuff deposits, due to weathering and volcanic glass alterations contained in these formations [1,2,3]. The work of Brito and Coutín [4] has shone a light on the geology, characterisation, and application of the natural zeolites of Cuba, and they have focused their research on 10 main deposits. However, Costafreda et al. [5] have gone further by extending their research on natural zeolites to the entire environment of Central and South America, publishing very encouraging results. Iturralde [6] and Orozco and Rizo [7] made a detailed description of the geological environments of the formation of the zeolitised tuffs of Cuba, which makes up the basis of the current knowledge of these deposits in that country. From the point of view of the application of zeolitised tuffs in Cuba, it is worth saying that they have been widely mentioned, as is seen in the works of Pérez-Pérez et al. [8], referring to the use of natural Cuban zeolite in the treatment of wastewater from pig farms and establishing that the addition of natural zeolite has an efficiency of more than 70%, despite the high variability in the chemical composition of the wastewater. On a different matter, Reyes et al. [9] have demonstrated the efficiency of zeolite from the Loma Blanca deposit in the treatment of liquid waste generated by the ferro–nickeliferous ore processing plant in Moa (Holguín, Cuba). Furthermore, Llanes and Castro [10] formulated a new food diet for fish farms that consists of replacing traditional food with natural zeolite, achieving a survival rate in species close to 90% as well as a notable economic effect. Rumbo et al. [11] have used zeolites from the Loma Blanca deposit for the separation of mixtures of H₂O/H₂SO₄ and H₂O/C₂H₅OH. From the results obtained, they simulated a process of absorption using pressure oscillation to dehydrate the ethanol when the pressure was raised, and a constant temperature was maintained in the range of 100–200 °C. Cervini-Silva et al. [12] focused their studies on the biomedical applications of zeolites from the San Andres deposit, specifically in the monitoring of the antiphlogistic effect from a model of murine inflammation. For this, they used 12-0-tetradecanoylphorbol-12 acetate (TPA) as an inflammatory agent and the activity of the enzyme myeloperoxidase (MPO), which is an indicator of the degree of migration of neutrophils. In this way, these researchers managed to demonstrate the degree of the quantitative adsorption of histamine, reductions in oedema formation, and the inalterability of the neutrophil migration process.

This work is structured in two distinct parts. The first consists of a detailed chemical and mineral characterisation of the samples used, and the second consists of determining the technological quality of the samples to demonstrate and establish the pozzolanic character of the samples investigated. Both stages include as a common factor the comparison of two representative samples, on the one hand, from the Loma Blanca deposit, and on the other, from the new study area, which is located on the eastern flank of the deposit. The proposed study can significantly increase knowledge of the chemical, mineral, and technological properties of the zeolitised tuffs that lie outside the limits of the current deposit and can additionally provide sufficient data to expand this deposit towards its easternmost flank.

2. Materials and Methods

2.1. Materials

Four samples were taken in the study area, and another four were sampled within the boundaries of the Loma Blanca deposit, currently in operation and located approximately 1.2 km west of the study area (Figure 1). In each case, samples were taken at the outcrops using the litho–geochemical method of outcrops. Each sample weighed the equivalent of 30 kg. From a petrological point of view, the samples consisted of strongly zeolitised tuffs of various colours, mainly clear, and they were hard, coherent, and partially tectonised (

Table 1. List of analysed samples in this research.

Sample	Petrological Description	Location
SAEF 1-01	Zeolitised tuffs	Eastern flank of the Loma Blanca deposit (New study area)
SAEF-02
SAEF-03
SAEF-04
SACZ 2-01	Zeolitised tuffs	Central part of the Loma Blanca deposit (Mining operations area)
SACZ-02
SACZ-03
SACZ-04

¹ SAEF-01 to 04: Eastern flank of the Loma Blanca deposit. ² SACZ1-01 to 04: Central zone of the Loma Blanca deposit.

). All samples were cleaned, crushed, and ground up in two phases. Then, they were sieved to obtain a granulometric fraction size of the equivalent of 63 microns as specified in the Standard UNE-EN 933-1:2012 [13].

This research used Type 1 Portland cement, class 52.5 R, the properties of which are specified in Standard UNE-EN 197-1:2000 [15]. Standardised sand, coded NORMANSAND, was chosen as a fine aggregate for the preparation of mortar specimens. Distilled water was used throughout the mortar preparation process.

2.2. Methods

2.2.1. X-ray Diffraction (XRD)

In this research, a study of the mineralogical phases of the samples was carried out using X-ray Diffraction (XRD). The equipment consisted of a diffractometer Rigaku Miniflex-600 (600 w) (Tokyo, Japan). The equipment had a goniometer with a Cu cathode X-ray tube and, in addition to a graphite monochromator, a standard scintillation counter as well as a rotary loader with the capacity to analyse up to 6 samples. In the analysis of the samples, 100–240 v was used, with a frequency of 50–60 Hz. The tests were controlled by a computer attached to the diffractometer, which was in the installed PDXL software. The crystalline phases were detected in an angle range of 2θ = 4–60°. The voltage that was used was 40 kV, and the current was 15 mA. To perform the test, 500 mg of the sample was ground up and sieved to 74 µm, Then, the test samples were prepared in standard moulds for analysis.

2.2.2. Scanning Electron Microscopy (SEM)

In this study, a Hitachi S-570 scanning electron microscope (Tokyo, Japan) was used in the investigation of the selected samples. The main objective of this study was to establish the morphological properties of the mineralogical phases contained in the samples. The microscope belonged to the Centralized Laboratory of the Escuela Técnica Superior de Ingenieros de Minas y Energía of the Universidad Politécnica de Madrid. The equipment consisted of the following: a Kevex-1728 analyser, a Polaron BIORAD, a power supply for evaporation, and a Polaron SEM coating system. The resolution of the microscope reached 3.5 nm. It had two software brands, Winshell and Printerface, to process the data and to obtain images, respectively. The samples were previously ground up to 0.2–0.5 cm. Then, they were put on a graphitised tape and were placed in the sample holder. Lastly, the samples were covered with a layer of vacuum graphite.

2.2.3. X-ray Fluorescence (XRF)

X-ray Fluorescence (XRF) was used in this research for the quantitative determination of the chemical composition of the researched samples. A Philips WDXRF spectrometer (PW 1404) (Eindhoven, The Netherlands) with a collimator was used to correct the divergence of X-rays. The tests were performed with a radiation intensity of 10–100 kV. The equipment also had a monochromator that guaranteed the isolation of the measured radiation as well as maintaining adequate wavelength. For the test, the samples were ground up in an automatic agate mortar and were sieved up toa mesh size of 200. Then, 6–8 g of sample was mixed with 1.5 mL of elbaite and was dried at room temperature for 5 min. After this, the 5 cm diameter test pill was prepared with the help of the Herzog press. Finally, the pill was placed inside the spectrometer for analysis.

2.2.4. Quality Chemical Analysis (QCA)

The bimodal chemical analysis (quantitative–qualitative) was carried out to establish the pozzolan quality of the samples, following the prescription of the Standard UNE-EN 196-2-2014 [16]. This test makes it possible to determine qualitative and quantitative compounds, the quality of which establishes the character of the samples as natural pozzolans and ensures their possible use in manufacturing the clinker of pozzolanic cements. The main compounds that were determined in this study were as follows: total SiO₂ (TS), SiO₂ reactive (RS), total CaO (TC), CaO reactive (RC), MgO, Al₂O₃, Fe₂O₃, and insoluble residue (IR).

2.2.5. Chemical Analysis of Pozzolanicity (PT) (8 and 15 Days)

The chemical analysis of pozzolanicity at 8 and 15 days was carried out in accordance with the guidelines in the Standard UNE-UN 196-5:2006 [17] to evaluate and establish the pozzolanic properties of the analysed samples. In essence, pozzolanicity is the ability of certain materials, both natural and artificial or a combination of both, to react with Ca(OH)₂ in solution if they are finely ground up. In this case, the test was performed with two different mixtures: PC/ZT: 75–25% and PC/ZT: 70–30%, at 8 and 15 days. The evaluation of pozzolanic properties considered the calculation of the concentration of hydroxyl ions and the concentration of calcium ions, expressed as calcium oxide. The result of the calculation of the above concentrations shows that the samples had a pozzolanic characterisation below the isothermal solubility curve at 40 °C.

2.2.6. Mechanical Strength at 7, 28, and 90 Days

This test was carried out to determine the mechanical strength to the compression of mixed specimens made of a standardised combination of portland cement (PC) and zeolitised tuffs (ZT), cured over three periods: 7, 28 and 90 days. The steps followed in this test were rigorously performed in accordance with the guidelines of the Standard UNE-EN 196-1:2005 [18]. The percentages of substitution of PC by ZT in the formulation of the mortar specimens were PC/ZT: 75–25% and PC/ZT: 70–30%, respectively.

Table 2. Dosage project of mixed mortars and reference mortars specifying the proportions in all cases.

Components of Mortar Specimens
Zeolitised Tuff (g)	Portland Cement (g)	Normalised Sand (g)	Distilled Water (g)
Formulation for Portland cement (PC)/zeolitised tuff (ZT) (75–25%)
125	375	1350	225
Formulation for Portland cement (PC)/zeolitised tuff (ZT) (70–30%)
150	350	1350	225
Formulation for the reference mortar specimen (RMS)
-	450	1350	225

provides details on the dosages of the different components of the specimens studied in this work.

3. Results and Discussion

3.1. X-ray Diffraction (XRD)

Figure 2a,b show the results of the analysis of mineral phases using XRD. According to the results obtained, the samples were made of mordenite and clinoptilolite in their main phases, and mordenite was the most abundant. In the secondary phases, amorphous materials appeared in the form of volcanic glass, as well as scarce quartz grains. It is noteworthy that, in general, practically all the samples that were studied were formed by the same mineral phases, both those from the central part of the Loma Blanca deposit (Figure 2a) and those from the study area (Figure 2b).

The works of Pentelényi and Garcés [1] commented on the zeolitisation process and how it developed in the eastern region of Cuba, especially the main varieties of mordenite and clinoptilolite, which was confirmed in this work. This process is also described in the research conducted by Kozák and Rózsa [2], who confirmed the presence of the above-mentioned mineral varieties. The works carried out by Costafreda and Martin [19] also report the presence of mordenite and clinoptilolite formed from the alteration of volcanic glass. The above may consolidate the hypothesis that the Loma Blanca deposit could continue towards its eastern flank, which positively affected the increase in zeolite reserves.

3.2. Scanning Electron Microscopy (SEM)

Figure 3a–h provide several microphotographs obtained from the SEM study. Figure 3a–d represent the morphological characteristics of the samples taken in the study area on the eastern flank of the Loma Blanca deposit, whereas Figure 3e–h show the samples from the central area of the aforementioned deposit. According to these results, the two main varieties of zeolites detected were mordenite and clinoptilolite as well as amorphous materials and quartz, which were also detected by XRD (Figure 2a,b).

According to Figure 3a–h, it seems evident that the zeolitic species represented by mordenite were comparatively more abundant than clinoptilolite [1,2], and that they were formed as a result of a previous process of the devitrification of volcanic glass with the subsequent manifestation of zeolitisation [19,20,21]. Figure 3d,h depict this fact, where some mordenite and clinoptilolite crystals grow at the expense of volcanic glass.

3.3. X-ray Fluorescence (XRF)

Table 3. Results of the X-ray fluorescence analysis obtained by studying the samples.

Sample	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	Na2O	LOI *	Total
SAEF 1-01	65.59	13.10	1.44	4.33	0.41	2.28	0.36	12.15	99.66
SAEF-02	65.53	13.25	1.47	4.25	0.29	2.27	0.53	13.07	100.00
SAEF-03	65.62	13.17	1.50	4.29	0.30	2.35	0.40	11.19	98.82
SAEF-04	65.01	13.24	1.42	4.35	0.63	2.37	0.38	11.13	98.53
SACZ 2-01	65.43	13.12	1.82	4.31	0.38	2.22	0.47	12.57	99.82
SACZ-02	65.42	13.11	1.91	4.35	0.35	2.21	0.45	12.00	99.80
SACZ-03	65.41	13.14	1.93	4.40	0.37	2.24	0.48	11.86	99.83
SACZ-04	65.42	13.16	1.15	4.23	0.60	2.31	0.51	10.99	98.37

¹ SAEF-01 to 04: Eastern flank of the Loma Blanca deposit (New study area); ² SACZ1-01 to 04: Central zone of the Loma Blanca deposit. * LOI: Loss on Ignition.

shows the results of the chemical analysis performed on the samples. Note how the contents of SiO₂ and Al₂O₃ vary within very close intervals, both for samples from the study area and for those from the Loma Blanca deposit. An almost similar fact was found in the values of the remaining compounds, such as Fe₂O₃, CaO, MgO, K₂O, and Na₂O. A somewhat more evident difference is seen in the values obtained by calculating the loss on ignition (LOI), which fluctuate between 10.99–12.57% for samples from the study area and between 11.13–13.07% for those from the Loma Blanca deposit.

According to the data sets obtained in both study areas, it seems that the new researched area is a geological continuation of the Loma Blanca deposit to the east, which is evident in the similarity of its chemical make-up. Pentelényi and Garcés [1] have previously established that the Loma Blanca deposit should extend uninterruptedly to the southeast from its current position. Costafreda and Martín [19], in their research, which took place at a point further south of the study area, calculated the SiO₂ and Al₂O₃ contents, as well as the LOI of SiO₂ and Al₂O₃, and they had an LOI of 64.20–65.02%, 11.30–13.25%, and 12.11–14.53%, respectively, which seem to have a certain similarity with the results that were obtained in this work.

3.4. Quality Chemical Analysis (QCA)

Table 4. Chemical characterisation to determine the pozzolanic quality of the researched samples.

Sample	Total SiO2 (%)	Reactive SiO2 (%)	Total CaO (%)	Reactive CaO (%)	Al2O3 (%)	MgO (%)	Fe2O3 (%)	SO3 (%)	I.R. (%)	SiO2/(CaO + MgO) (%)	SiO2 + Al2O3 + Fe2O3 (%)
SAEF 1-01	65.17	64.45	3.14	1.57	11.40	1.43	1.36	0.04	4.5	14.26	77.93
SAEF-02	67.29	65.33	2.25	1.27	11.33	1.11	1.29	0.01	4.05	20.02	79.91
SAEF-03	65.13	63.10	3.21	1.32	11.15	1.04	1.14	0.03	4.12	15.32	77.42
SAEF-04	65.77	62.31	2.50	1.18	11.73	1.16	1.45	0.02	3.71	17.96	78.95
SACZ 2-01	68.59	66.19	2.19	1.10	11.11	0.77	1.37	0.03	3.5	23.17	81.07
SACZ-02	67.79	65.20	2.33	1.20	11.37	1.01	1.25	0.03	4.8	20.29	80.41
SACZ-03	65.19	63.00	3.10	1.63	11.09	1.13	1.23	0.02	4.0	15.41	77.51
SACZ-04	65.75	62.13	2.63	1.30	11.69	1.10	1.40	0.02	3.62	17.62	78.84

¹ SAEF-01 to 04: Eastern flank of the Loma Blanca deposit (New study area). ² SACZ1-01 to 04: Central zone of the Loma Blanca deposit.

shows the data corresponding to the results of quality chemical analysis (QCA) carried out on the researched samples. In all cases, with respect to both the samples from the study area and those from the Loma Blanca deposit, the total SiO₂ contents exceeded 65%, whereas the total CaO ranged from 2.19–3.21%.

In the specific case of the study area, the values of reactive SiO₂ (RS) vary between 62.31–65.33%, which means that 96.6–99.3% of the total SiO₂ (TS) was able to react with Ca(OH)₂. Regarding the reactive CaO (RC), it was calculated that 98.11–99.02% behaved reactively. Comparing the values mentioned in the calculated samples from the Loma Blanca deposit (Table 4), it can be seen that 96.4–97.8% of the total SiO₂ (TS) behaved reactively, whereas 98.5–98.9% of the total CaO (TC) could react. According to the analysis, both calculations yielded very similar results, which allowed us to establish that the studied samples had marked pozzolanic characteristics. In the studies carried out by Costafreda et al. [20], the pozzolanic characteristics of the zeolitised tuffs of the Loma Blanca deposit were mentioned, which points to the results presented in this work.

Another aspect worth highlighting in this study is the behaviour of Al₂O₃ in both areas, which showed an obvious similarity; according to the Standard UNE-EN 196-2-2014 [16], the most efficient pozzolanic material must have Al₂O₃ and MgO of <16% and <5%, respectively, which, in this case, are comparatively low in relation to the aforementioned standard. A fact to consider is the insignificant values of SO₃ which are less than 4% [16]. The result of the calculation of the insoluble residue (IR) for the samples of both areas also reflects a close similarity; according to [16], the best quality pozzolans need to have an IR content of <5%. With all the above, it is established that the zeolitised tuffs that lie in the study area have the same pozzolanic behaviour as those of the Loma Blanca deposit.

Lastly, the calculations that were made to determine the relationship among the SiO₂, CaO, and the MgO show that this relationship is evidently significant, indicating a predominance of SiO₂ in both study areas. The Standard UNE-EN 196-2-2014 [16] states that this relation must always be >3.5%. This situation was mentioned in the works of Costafreda and Martín [19], in those of Pentelényi and Garcés [1], and in the works of Kozák and Rózsa [2] for proximal and distal areas to the study area.

3.5. Chemical Analysis of Pozzolanicity (PT) at 8 and 15 Days

The graph in Figure 4a,b shows the distribution of the samples under the isothermal solubility curve, as well their pozzolanic characteristics. Figure 4a shows the results obtained after 8 days, whereas Figure 4b shows the results at 15 days. As can be seen, all the samples that were studied at 8 days manifested strong pozzolanic behaviour, very close to each other. However, at 15 days, each sample highlighted its own individual trend, meaning that the SACZ-01 and SAEF-02 samples depicted the greatest pozzolanic activity, whereas those with the lowest reactivity were SAEF-04 and SACZ-02.

This research considers that the behaviour of the samples, both in a and b, were intimately related to their chemical, mineral, and morphological compositions. Note, for example, in Figure 3, the porous structure formed by mordenite and clinoptilolite; in addition to this, the cationic exchange capacity (CEC) of these zeolites should also be mentioned, which is crucial in their pozzolanic behaviour as pozzolans. The fact that the samples from the study area had pozzolanic behaviour like those of the central area of the Loma Blanca deposit is, without a doubt, one of the main objectives that was achieved in this work, since it establishes the continuity of the deposit to the east, as already mentioned in the previous subsections. Costafreda et al. [20] and Costafreda [21] detailed in their previous research the pozzolanic characteristics of the zeolite of the Loma Blanca deposit, which were demonstrated once again in this work. Works such as those of Montesano et al. [22], Uhlík et al. [23], and Mertensa et al. [24] established well the pozzolanic reactivity of zeolitised tuffs consisting mainly of mordenite and clinoptilolite.

3.6. Mechanical Strength Test (MST) at 7, 28, and 90 Days

Figure 5a,b show the results of the mechanical strength test (MST) at 7, 28, and 90 days, both for PC/ZT: 75–25% as well as for PC/ZT: 70–30% formulations.

According to what can be observed in Figure 5a, at 7 days of curing, both the specimens made with samples from the study area and those from the Loma Blanca deposit exhibited a certain degree of hydraulic reactivity with moderate gains in mechanical strength. This highlights the fact that both groups of samples had relatively similar behaviour, with small differences seen in the SAEF-02, SACZ-02, and SACZ-04 samples. In this period, the PCS-R specimen significantly exceeded the mechanical strength of the remaining analysed samples. Many authors point out that, in mortars prepared with specific proportions of pozzolanic materials, the increase in mechanical strength tends to be both slower and more effective than in cement mortars without these additions [25,26,27,28].

At 28 days of curing, the behaviour became more differentiated between the samples. The highest values were in SAEF-01 and SAEF-02 as well as in SACZ-01 and SACZ-04. It should be noted that, in this period, four samples exceeded the mechanical strength of the reference samples (PCS-R).

At 90 days of curing, all specimens had visibly increased their mechanical strength compared to that at 7 and 28 days. Those samples that surpassed the PCS-R at 28 days of age further increased their mechanical strength. According to Standard UNE-EN 196-1:2005 [15], all samples meet the limits of mechanical strength at each specific age of study.

In Figure 5b, the behaviour of the mechanical strength is similar to that which can be observed in Figure 5a, although it has notable variability in the values. All samples exhibited a clear tendency to gain mechanical strength, although in no case did they equal the PCS-R sample, a trend also observed in Figure 5a 7 days after curing.

At 28 days, all samples showed a notable increase in mechanical strength; however, the behaviour of SAEF-02 was remarkable, as it exceeded the PCS-R sample.

An increase in mechanical strength was obtained at 90 days of curing. However, no sample exceeded the value of the PCS-R. The behaviour of the mechanical strength of the specimens with formulations PC/ZT: 75–25% and PC/ZT: 70–30% establishes in this research that both dosages are adequate. This fact is supported by the conclusions reached by Presa et al. [29] in their research on zeolites of the mordenite variety, in which they achieved mechanical resistances of up to 43.05 MPa at 28 days of curing in specimens made with 25% natural mordenite.

The efficiency of zeolitised tuff mixtures with portland cement, from the point of view of durability and mechanical strength, has been extensively discussed by numerous authors, such as Stamatakis et al. [30], Dimitrios et al. [31], and Özen et al. [32].

Lastly, the results obtained through the mechanical strength test correspond directly to the properties that are described and discussed throughout this work, and they can even be expressed as a relationship of dependence regarding the chemical, mineral, morphological, and pozzolanic properties.

4. Conclusions

The chemical and mineral characterisation analyses of the samples established a clear similarity between the samples taken from the Loma Blanca deposit and those from the study area, and these characterisations are similar to the technical tests, such as the chemical quality analysis, the pozzolanicity test, and the mechanical strength test, which are the object of this research.

According to the results that were discussed, all the samples that were studied had a marked pozzolanic nature, a property that varies comparatively within very narrow limits.

The mineral and chemical composition of the samples seems to establish a link with the same formations that lie beyond the limits of the Loma Blanca deposit, which is in agreement with comparisons made by previous work conducted by other researchers.

In relation to the results of mechanical strength to compression, what is especially highlighted is the fact that the partial replacement of portland cement by zeolitised tuff in mortars was effective in both substitutions of 25% and 30%, and the differences in results are very few. As mentioned before, the Loma Blanca deposit is currently in operation and could continue towards its eastern flank, and this positively affects the increase in zeolite reserves. It should be considered that the use of this zeolite in the clinker of pozzolanic cements can have a positive implication with respect to their impact on the environment and the low emission of CO₂ into the atmosphere. Finally, in line with the above, the results of this research could have strong implications on how to mitigate climate change through the use of sustainable natural raw materials, such as zeolitised tuffs.