molecules Article New Deposit of Mordenite–Clinoptilolite in the Eastern Region of Cuba: Uses as Pozzolans Jorge Luis Costafreda * and Domingo Alfonso Martín Escuela Técnica Superior de Ingenieros de Minas y Energía, Universidad Politécnica de Madrid, C/Ríos Rosas, 21, 28003 Madrid, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-609642209 Abstract: This work describes the newly discovered zeolites in the eastern region of Cuba. In the researched area, there have been no previous studies of natural zeolite exploration. Therefore, the results shown here are new. The main object of this research is to analyse five samples of zeolites and demonstrate their pozzolanic capacity and the possibility of their usage in the industrial manufacturing of pozzolanic cements. The study of the samples was performed by X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy (SEM). A chemical analysis (CAQ) to determine the quality of the samples as pozzolans was performed, by determining the total SiO2, reactive SiO2, total CaO, reactive CaO, Al2O3, MgO and the insoluble residue (I.R.). Lastly, an eight-day pozzolanicity analysis (PA) was carried out to determine the pozzolanic reactivity of the samples. The results obtained by XRD, XRF and SEM established that the researched zeolite samples have two main zeolitic phases: mordenite and clinoptilolite. Altered volcanic glass, quartz and smectite (montmorillonite) are the secondary phases. The results of the chemical quality analysis (CAQ) showed that the samples contain a considerable amount of reactive SiO and reactive CaO, as 2 well as a low content of insoluble residue, which reinforces their properties as pozzolans. The results Citation: Costafreda, J.L.; Martín, of the pozzolanicity analysis (PA) concluded that the analysed samples actively react with Ca(OH)2 D.A. New Deposit of after eight days. Based on all the results mentioned above, it is established that both mordenite and Mordenite–Clinoptilolite in the clinoptilolite behave like pozzolans and can be recommended for the manufacture of pozzolanic Eastern Region of Cuba: Uses as cements, which have more effective properties than Portland cement, in terms of physical, chemical Pozzolans. Molecules 2021, 26, 4676. and mechanical strength, low heat of hydration, resistance to sulphates, low CO2 emissions to the https://doi.org/10.3390/ atmosphere and negligible impacts on the environment. molecules26154676 Academic Editor: T. Jean Daou Keywords: mordenite–clinoptilolite; smectite; altered volcanic glass; pozzolanic reactivity Received: 13 July 2021 Accepted: 30 July 2021 Published: 2 August 2021 1. Introduction Several magmatic cycles formed the volcanic arc of Cuba during the Cretaceous. The Publisher’s Note: MDPI stays neutral erosive process created large deposits of volcano-sedimentary materials in the deposition with regard to jurisdictional claims in basins; many of these materials were enriched with volcanic glass that was gradually altered published maps and institutional affil- by meteoric waters and hydrothermal processes, forming large deposits of zeolites in many iations. places in Cuba. This paper presents the results of the research on the characterisation of new indications of natural zeolites of the eastern part of the Republic of Cuba, in the north-west of Holguín (Figure1). Knowledge of zeolites in Eastern Cuba is extensive, although not all deposits have been definitively discovered or studied. Zeolites represent Copyright: © 2021 by the authors. an important link to the economy of this country, so, based on this fact, the following is a Licensee MDPI, Basel, Switzerland. brief description of the use of zeolites in Cuba. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Molecules 2021, 26, 4676. https://doi.org/10.3390/molecules26154676 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 4676 2 of 15 Figure 1. Location of the area of study. Zeolites were first described in the province of Holguín in 1971 by geologists of the Institute of Geology and Paleontology (IGP) of La Habana (Cuba) [1,2]. Subsequently, a group of Cuban and Hungarian geologists carried out geological reconnaissance work on the locality of Loma Blanca, in San Andrés [3]. Researchers from the University of Moa and the Polytechnic University of Madrid also conducted a surveys for zeolites in the Loma Blanca deposit [4–6]. The result of their research allowed the opening of a mine, which is at present being operated. Currently, zeolites are widely used in various fields of the Cuban economy. The clinoptilolite is used to promote growth in plants and is a very valuable fertiliser [7]. It is used as a molecular sieve and as a means of filtering [4]. Cuban zeolites are used as food additives in animal diets and in the treatment and prevention of certain illnesses in poultry farms [8]. It is in great demand in construction as a pozzolanic material that offers good physical–chemical properties to cement [9]. These zeolites have been successfully characterised for the manufacturing and development of nanostructured materials [10]. The pozzolanic cements made with mordenite–clinoptilolite possess mechanical strength superior to 50 MPa for up to 28 days [11]. The mordenite–clinoptilolite from the Loma Blanca deposit is used in pharmaceutical products due to its properties of irreversible acquisition of histamine under both acidic (pH 3.5) and neutral (pH 7) conditions [12]. It is used in the processes of lead absorption and in the decontamination of waters [13]. Other research indicates that these zeolites are efficient in catalytic processes for their thermodynamic and thermophysical properties [14]. Buenaño et al. [15] carried out a study of cation exchange with zeolite from the Tasajeras deposit (Cuba) as an alternative to manage acidic water, using a breakthrough curve methodology. Cháves et al. [16] have used zeolites from the Loma Blanca deposit as a cation exchanger to capture rare-earth elements in the debris of depleted metal deposits. This zeolite (clinoptilolite) has a high cation exchange capacity, which provoked an increase in the capture of rare-earth elements until the breaking point was reached. Cuban zeolite was widely used in urban agriculture during the world socialist block collapse [6], where it was used as a substrate in zeoponics for the provision of greens and vegetables to the populace. These zeolites are effective in the formulation of medicines such as petrolatum and squalene for the treatment of mycosis [17]. In the latest research, clinoptilolite was used from the Tasajeras deposit in the development of reactive permeable walls to extract inorganic contaminants from subterranean water [18]. The study by Cerri et al. [19] demonstrated that mordenite–clinoptilolite from the Loma Blanca deposit, Holguín, in its sodium form (M-Na) prepared by cation exchange, is able Molecules 2021, 26, 4676 3 of 15 to inhibit the growth of Helicobacter Pylori bacteria. At the present moment, the deposit of Loma Blanca (San Andrés) is being mined. It is a concession of the Geological Unit of San Andrés, a subsidiary of the Geomining Company of the Orient (Santiago de Cuba). The greater part of the production of zeolite (mordenite-clinoptilolite) is exported to Central America and South America. The national production of zeolite from the Republic of Cuba amounted to 53,000 tons in 2020 [20]. The other part of this plant’s production, together with the plant deposit of Tasajeras, in Villa Clara (central Cuba), is used in various national industries. The present work had two main phases. In the first phase, a mineralogical, petrological and chemical study of zeolites found in an unexplored area in the eastern region of Cuba was carried out; in the second phase, their technological qualities were determined in order to establish their properties as pozzolans. The studies carried out in both phases considered three fundamental questions: the geological nature of the deposit, the pozzolanic properties of these zeolites and the possibility of their use as a raw material in the manufacture of pozzolanic cements. 2. Geological Setting The area of study is within the Loma Blanca Formation from the Lower Cretaceous Aptian to the Upper Cretaceous Campanian [3]. It is divided into two geological units, the Normal Series of volcanogenic–sedimentary composition, formed during the subduction of the oceanic plate with the arc formation of volcanic islands, and a Final Series, during the tectonic destruction of the volcanic arc accompanied by sintectonic magmatism with a formation of subvolcanic bodies [21]. The materials are tuff, clays and zeolites of medium and acidic composition, which are sedimented in deep or relatively deep pelagic waters. In the upper part, acidic and sedimentary rocks were formed. The marl and limestones are mixed with sediments of volcanic origin of an andesitic and dacitic composition, with the presence of radiolarians [3,22,23]. 3. Results and Discussion 3.1. X-ray Diffraction (XRD) The results obtained by XRD established that the researched zeolite samples have two main zeolitic phases, mordenite and clinoptilolite, which is evident in the X-ray diffraction patterns presented in Figure2. Altered volcanic glass, quartz and smectite (montmorillonite) are the secondary phases. According to Figure2, the samples have a high concentration of zeolitic ore. The peak of maximum intensity (Imax = 100%) of the mordenite is located at 2θ = 26.65 (Figure2) (the term 2 θ is equivalent to the position ◦2 Theta). Other peaks stand out at the angular positions of 2θ from 9.75 to 35.68. The remaining peaks are the lowest in intensity. In clinoptilolite, the peak of greater intensity (Imax = 100%) is located at the angular position 2θ = 9.88 and is overlapping with a mordenite peak. A second peak of clinoptilolite of appreciable intensity is seen in 2θ = 11.19, with an intensity equal to 40%. Other peaks with less intensity appear at angular positions 13.06 to 32.01, with intensities varying between 14% and 48.2%.