The raw ZVB was extracted from an outcrop located in the south of the Mendoza province (Argentina). It is light green to whitish and has a small grain size. The mineralized sector is ~200 m long and 6 to 10 m wide, with the base covered and the top exposed or partially covered by different materials.

Petrographic studies were done on thin sections under parallel and crossed nicols (//N, XN) using an Olympus B2-UMA trinocular petrographic microscope. Morphological studies were performed with a LEO EVO 40-XVP SEM on gold-coated samples working at 10 kV. Additional studies were carried out on carbon-coated fractured surfaces and polished thin sections (abrasive size up to 1 μm) with a Carl Zeiss high resolution FE (Field Emission)-Σigma SEM-EDS microscope, working at 8 and 15 kV for images and microanalyses, respectively. Secondary electron (SE) images and backscattered electron (BSE) images were obtained. XRD analyses were performed on Rigaku D-Max III-C, with Cu-Kα_1,2 (λ=1.541840 Å) radiation filtered with a graphite monochromator operating at 35 kV and 15 mA. Patterns were recorded on ground whole rock (in agate mortar), from 3 to 60° 2θ (0.02° 2θ step and a counting time of 2 s per step).

Due to the irregular mineralization of the rock, the zeolite content was estimated by the following staining technique. One sample of the altered sector was cut and polished, obtaining a slab measuring 30 × 25 mm and 10 mm in thickness. It was immersed in 0.01 N methylene blue solution for two minutes. After this treatment, photomicrographs were taken (30x magnification) under a stereomicroscope covering the total surface of the slab. Finally, the percentage of the stained area (zeolites are colored due to their CEC) was determined for each image with ImageJ software (25), and the average value was calculated for the whole slab.

A representative ZVB sample was processed in a laboratory jaw crusher to obtain particles smaller than 4.75 mm. Two kilograms of the crushed sample were dried at room temperature and ground in a laboratory ball mill until less than 12% was retained on 45 mm sieve. After grinding, the particle size distribution (PSD) and its parameters (d90, d50 and d10) were obtained by a laser diffraction granulometer (Malvern Mastersize 2000) with dry dispersant medium. The material retained on 45 μm sieve by the wet method (26), the relative density (27) and the Blaine specific surface area (28) were also determined.

The chemical composition of the ground ZVB was determined by inductively coupled plasma emission spectrometry (ICP-ES) in ACME Laboratories, Canada (LF200 procedure). Additionally, it was characterized by simultaneous differential scanning calorimetry and thermogravimetric analysis (DSC-TGA) and Fourier transform infrared spectroscopy (FT-IR). DSC-TGA was performed with a TA Q600 analyzer under inert atmosphere at a heating rate of 10 °C/min from room temperature up to 1000 °C. The FT-IR spectrum was recorded on a Nicolet 520 FT-IR spectrometer using 1 mg pressed pellets of the raw material dispersed in 100 mg KBr. For each spectrum, 100 scans were recorded in the 400–4000 cm⁻¹ spectral range with a resolution of 4 cm⁻¹ in transmittance mode.

The CEC and exchangeable cations (in solution) were determined in LANAQUI laboratory (Argentina). The sample was digested using nitric acid (29). Saturation with ammonium acetate was used to obtain the CEC. The chemical determinations were carried out with an Inductive Coupling Plasma Atomic Emission Spectrometer (ICP-AES), Shimadzu 9000 (30).

The ZVB-performance was assessed on blended cement containing an ordinary portland cement (OPC) with 25% by mass replacement as required by the IRAM 1654 standard for pozzolan (31). The chemical composition of OPC determined by X-ray fluorescence (XRF) is given in Table 1, and its mineralogical composition reported by the producer was C₃S = 57.8%, C₂S = 13.7%, C₃A = 3.0%, C₄AF = 14.2%, gypsum = 5.0% and limestone = 4.5%. The strength class of OPC was 40 (standard compressive strength > 40 MPa at 28 days).

The minislump test (32) was used to determine the flow and the flow loss on the cement paste. After standard paste mixing, the minicone placed on a glass plate was filled with the mixture and carefully lifted to allow the paste to flow on the glass. After 30 s, two orthogonal spread diameters of the pad were measured and the average was calculated. The paste remaining in the bowl was remixed every 15 min to avoid bleeding, and the minislump was measured at 30, 60, and 120 min.

The heat and cumulative heat release rate during cement hydration was measured in a conduction calorimeter operating under isothermal conditions at 20 °C for 48 h. The paste was prepared using 20 g of cementitious material in a small plastic bag carefully homogenized. The bag and mixing water were placed into the calorimeter until thermal stabilization, and then the cement was mixed with 10 g of water by hand for 30 s, with a water-to-cementitious materials ratio (w/cm) of 0.50 by mass. The bag was sealed, carefully placed in the calorimetric cup, and the measurement was started immediately.

For the Frattini test (33), 15 g of OPC plus 5 g of ground ZVB were mixed with 100 ml of boiled distilled water in plastic containers. Sealed containers were stored for 2, 7, 28 and 90 days at 40 °C. At test time, samples were vacuum filtered through paper, and the filtrate was analyzed for [OH⁻] by titration against dilute HCl with methyl orange indicator and for [Ca²⁺] by pH adjustment to 13, followed by titration with 0.025 M EDTA solution using murexide indicator. For this test, the mineral addition is considered active when the [CaO] and [OH⁻] determined in the supernatant solution are located below the solubility isotherm of Ca(OH)₂ in alkaline solution at 40 °C.

The strength activity index (SAI) was determined as the ratio between the compressive strength of the OPC+ZVB and that of the OPC mortar curing at room temperature (34). The compressive strength was determined on prims (40 × 40 × 160 mm) made with standard EN 196-1 (35) mortar (w/cm = 0.5 and cement:sand = 1:3). Prior to casting, the flow test was carried out according to ASTM C1437 (36). Specimens were cured for 24 h in the molds in fog room, and thereafter demolded and submerged in tap water at 20 ± 1 °C until the test ages. At 2, 7, 28 and 90 days, the compressive strength was determined on three specimens (six values) using an INSTRON 33R4485 test machine. The mean value, the dispersion and the SAI were calculated. According to EN 450-1 standard (34), fly ash behaves as pozzolan when the SAI is higher than 0.75 at 28 days and higher than 0.85 at 90 days.

At 90 days, a slice of standard mortar specimen containing blended cement was studied by SEM (see LEO EVO equipment in section 3.1) operating at 20 kV. A carbon-coated polished thin section was prepared (abrasive size up to 1 μm) and studied by BSE images and mapping mode (Si, Ca, Al, K, Na, Fe, Mg, S) to analyze the components of the cement paste, and line profile mode (Si, Al, Ca, Na, K) to determine the chemical variation in the interfacial transition zone (ITZ).

The hydration compounds were determined on pastes made with w/cm of 0.5 and cured in sealed plastic bags at 20 °C for 2, 7, 28 and 90 days. At test time, the paste was crushed, immersed in isopropyl alcohol to stop the hydration, and the non-evaporable water (Wn) was determined by using the procedure proposed by Powers (37). Other paste fragments were carefully ground in an agate mortar immersed in isopropyl alcohol and immediately measured by XRD analysis using the same equipment mentioned above.

Petrographic studies show that ZVB is heterogeneous, with vitreous sectors slightly zeolitized in the upper part of the outcrop and other sectors strongly altered to zeolite, sometimes with good crystallinity (Figure 1a), which predominates in the lower sector of the outcrop. In the former, fluidal structures (with tubular microvesicles) and perlitic-spherulitic texture can be recognized (Figure 1b). Generally, more altered sectors are associated with zones of breccias of high permeability (Figure 1c). The heterogeneous reddish to brown coloration of some sectors is attributed to the presence of iron oxides/hydroxides. A great percentage of the vitreous material shows anisotropy (partially crystalline), although mineral phases cannot be easily identified by this methodology. In these sectors, some fibrous zeolites (~50 μm long) with radial growth are recognized, although zones with very fine grain size (< 10 μm) predominate (Figure 1d). As accessory phases (< 1%) K-feldspar, plagioclase, clay minerals and prismatic zeolite were identified.

SEM observation shows zeolite-rich sectors over volcanic glass. Also, well-developed fibrous zeolite (Figure 2a) is scarce and mainly associated with high permeability sectors (cracks and cavities), while zeolite of small grain size (≤ 100 nm) and with no obvious habit (Figure 2b) is the most distributed phase replacing the vitreous material. The texture is mottled (Figure 2c) with relicts of volcanic glass (light gray) in a fine-grained mass of mordenite (dark gray) (Figure 2d). In some sectors, veins of K-feldspar (white) are also observed crosscutting the rock texture. Table 2 lists the EDS analysis composition of the volcanic glass and the zeolite (spots 1 and 2 in Figure 2d, respectively).

XRD analysis (Figure 3) indicated that the main zeolite identified was mordenite (ICSD 00-029-1257) with main reflections at 9.06 Å, 4.0 Å and 3.48 Å. Additionally, plagioclase and K-feldspar were identified as accessory phases. A hump of the background was observed between 20 and 30° 2θ, indicating the presence of amorphous material (relict volcanic glass). In some samples, opal-CT (SiO₂ phase) was also detected with minor amounts of clinoptilolite, beidellite-montmorillonite and Ti- or Fe-oxides, in agreement with other authors (38).

The zeolite content in the ZVB is variable, with 25% on average and a standard deviation of 6.6%, although some sectors in the rock are almost totally zeolitized (~80%). More permeable sectors such as cavities and cracks show intense blue staining, while the rest of the matrix is blue to light blue (Figure 4). The non-zeolitized vitreous clasts are not stained. The content of vitreous phase determined by this method assuming a small influence of other crystalline phases (K-feldspars, plagioclase, Opal-CT and oxides) ranges between 70% and 75%.

The physical characteristics of the ground ZVB are summarized in Table 3 and compared to those of OPC. The PSD curves for ZVB and OPC are presented in Figure 5, and their granulometric parameters are reported in Table 3. A grinding time of 120 min was necessary so that less than 12% ZVB was retained on 45 μm sieve. The PSD curve of ZVB has unimodal distribution with a high volume of particle size at 21.4 μm and two humps at 5.4 μm and 0.9 μm. Compared with OPC, the PSD is thinner. The specific surface of ZVB determined by the Blaine method was larger than that of the OPC, and the density value was lower.

Table 2 also reports the chemical composition of the ground ZVB. The sum of SiO₂+Al₂O₃+Fe₂O₃ (80.1%) is greater than 70% and the SO₃ is below the detection limit of the equipment, allowing the chemical requirements of ASTM C618 (39) for natural pozzolan Class N. However, the structural water content in this type of pozzolan causes a LOI greater than 10% as required by the standard.

The TGA curve (Figure 6 - solid line) of the ground ZVB shows a continuous mass loss during heating, attaining a total mass loss of 11.93%. Between 20 and 110 °C, the mass loss is assigned to desorption of free water into the zeolite structure (hygroscopic water), while the mass loss in the 110–230 °C range is attributed to desorption of very loosely held water (40,41). A broad endothermic peak is recognized in the DSC curve (heat flow) in the 20–230 °C range due to these two processes (42). The mass loss above 230 °C could be due to strongly bound water (43) or the dehydroxylation of zeolite (44). According to Pechar and Rykl (43), mordenite is stable up to 510 °C without significant changes in its structure.

Figure 7 shows the FT-IR spectrum of the ground ZVB. The main bands were assigned according to Jansen et al. (45), Deshpande & Bhoskar (46) and Ostroumov & Corona-Chávez (47). The 3620 and 3454 cm⁻¹ bands are well defined and are assigned to the OH-stretching vibration, while the 1643 cm⁻¹ band is due to H-O-H bending of water. The vibration of T-O bending is assigned to 1223 and 791 cm⁻¹ (external asymmetric and symmetric stretching, respectively), 1051 and 727 cm⁻¹ (internal asymmetric and symmetric stretching, respectively), and 469 cm⁻¹ bands (possibly bending). The bands at 627, 577 and 523 cm⁻¹ could be attributed to double-ring tetrahedral vibrations. Minor peaks observed at 2879, 2232 and 1440 cm⁻¹ could not be assigned.

Table 4 gives the results of CEC and exchangeable cations of the ground ZVB (average of two determinations). According to Mumpton (48), the CEC of mordenite is 2.29 meq/g (calculated from unit-cell formula). Therefore, considering the CEC determined, ~25.8% of mordenite in ZVB can be estimated, which is similar to the percentage determined by the staining technique.