Effect of Processed Volcanic Ash as Active Mineral ...

In this section, the physical, chemical, and mineralogical properties of raw and processed volcanic ash and volcanic lava were studied to evaluate their pozzolanic potential and their effect as an additive in the development of new cements.

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In addition, two artificial pozzolanic materials, silica fume and fly ash, which are widely used in the cement industry, were studied as a reference, along with limestone filler.

For the performance of this research, an ordinary commercial Portland cement of type CEM I 42.5R was used. A study of the main cement composition elements was carried out by fluorescence study. The main composition and density of OPC used is shown in Table 1

The fly ash (FA) used in this study is a commercial artificial pozzolan used in the manufacture of cements. FA comes from the combustion of coal in power generation plants and is collected in filters by electrostatic precipitation. As can be seen in the laser granulometry, as shown in Figure 2 , FA is the finest material analysed, with an average retained size of approximately 20 microns.

The silica fume analysed in this study has a particle average size of approximately 40 microns, as shown in Figure 2 , as well as an actual density of 2.24 g/cm. It is composed entirely of amorphous SiO, as can be observed in Table 2 and in the XRD pattern in Figure 4 . For this reason, silica fume has a great pozzolanic potential, which is observed with a SiOpercentage higher than 60% and is widely applied in the manufacture of cements and concretes [ 34 35 ].

Silica fume (SF), or microsilica, is an inorganic product consisting of fine spherical particles formed from the reduction of quartz with carbon in the silicon metal and ferro-silicon manufacturing processes in electric arc furnaces. The dust produced is a byproduct collected in baghouses and silica dust collectors.

A study of limestone filler, as a mineral addition without activity, was carried out to compare the effect of using volcanic material. It is a material of inorganic nature and mineral origin composed mainly of calcium carbonate (at least 75%), with a clay content of less than 1.2%. As shown in Table 2 , its main composition is CaO.

2.1.5. Volcanic Lava and Volcanic Ash

In this section, the physical, chemical, and mineralogical properties of the volcanic materials analysed are shown, and, in the following section, their pozzolanic potential compared to FA and SF is evaluated in order to determine the possibility of applying them in cementitious materials.

Three volcanic materials were analysed: one sample of volcanic lava and two samples of pyroclasts.

- Volcanic lava, from the solidified magma ejected by the volcano and collected from solidified lava flows close to the coastline, is called VL.

- Lapilli pyroclastic are particles between 2 and 64 mm in size ejected from the crater during ejection. Due to their size, lapilli pyroclastic precipitate by gravity in the areas near the crater, where the samples were collected, and are referred to as CVA (coarse volcanic ash).

- Ash type pyroclastic are particles smaller than 2 mm expelled during ejection; due to their size they can be deposited over long distances. They were collected near the coastline of the island and are referred to as FCV (fine volcanic ash).

3 and 2.90 g/cm3, with the lowest density in the FVA and the lowest in the CVA, due to the more compact granulometry of the fine ash, which gives them a higher density. Volcanic lava has an intermediate density value of 2.72 g/cm3; similar values have been shown in studies of volcanic ash from other eruptions. [

Analysing the data shown in Table 2 for the three volcanic materials, it is observed that the densities of the volcanic ash vary between 2.30 g/cmand 2.90 g/cm, with the lowest density in the FVA and the lowest in the CVA, due to the more compact granulometry of the fine ash, which gives them a higher density. Volcanic lava has an intermediate density value of 2.72 g/cm; similar values have been shown in studies of volcanic ash from other eruptions. [ 15

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The three materials present a practically identical composition, due to the fact that they come from the same volcanic material in the interior of the earth, varying in the process of expulsion and subsequent deposit and cooling of the materials. VL, FVA, and CVA present a composition with silica as the major element, with values between 14%&#;19%, followed by Fe, with values between 8%&#;9%, Al and Ca, with values between 6%&#;8%, and Na and Mg, with values close to 3%.

However, the reactive SiO2 content is similar in both types of ash, in the order of 45%, but higher than 60% for volcanic lava, indicating a higher pozzolanic potential in this material.

FVA, CVA, and LV were studied by using X-ray diffraction (XRD). XRD data were collected at room temperature using Cu-Kα radiation (λ = 1. Å) operated in the reflection geometry (θ/2θ). Data were recorded from 10° to 60° (2θ) with a step-size of 0.02. The X-ray tube was operated at 40 kV and 40 mA. Analysing the main component determined by X-ray fluorescense for the three volcanic materials, the XRD pattern shown in Figure 5 , and the legend of the majority phases found ( Table 3 ), it is observed that they were mainly composed of pyroxenes belonging to the inosilicate family, such as diopside and augetite, followed by feldspars of the tectosilicate family, where the presence of andesine, albite, and anorthoclase stand out. In addition, other crystalline phases were observed in the form of titanium oxides (rutile) and silicon oxide (quartz). Although the composition of volcanic materials depends on several factors, such as location and type of eruption, similar compositions have been found in volcanic ashes analysed by other authors [ 36 37 ].

The morphology of the volcanic material was determined with scanning electron microscopy (SEM), complemented with EDX to complete the compositional studies. A Hitachi S electron microscope (Tokyo, Japan) was used for the morphology study. For the determination by energy dispersive spectroscopy (EDX) of the chemical composition of the samples, a Bruker Nano XFlash silicon drift detector was used.

Figure 6 shows the micrographs of FVA (a), CVA (b) and VL (c). A non-uniform microstructure is observed with the presence of larger angular particles in CVA and smaller ones in FVA. The presence of crystals was observed in all three volcanic materials analysed.

The existence of large quartz crystals, as observed in Figure 6 , corresponds to the mineralogy of the volcanic material ( Figure 5 ). The higher proportion of calcium and aluminum observed by XRD patterns ( Table 4 ) would explain the formation of inosilicates and tectosilicates ( Table 3 ) and corresponds with what has been observed by other authors who carried out analyses of volcanic material.

Volcanic lava is extracted from the lava flows by mechanical means, which involves obtaining particle sizes of several centimetres in diameter. Furthermore, volcanic ash (FVA and CVA) present coarser granulometry than FA and SF, as shown in Figure 7 , which prevents their direct application as a mineral addition in the manufacture of new cements.

For this reason, a size reduction process is carried out on the samples to obtain the necessary particle size for the application as a mineral addition. The processing applied is as follows:

(1)

Drying of the material in an oven at 60 degrees Celsius.

(2)

Previous size reduction in a jaw crusher. Reduction in the initial fraction to a size of less than 4 mm (VL and CVA).

(3)

Grinding by impact mill with different abrasive loads and processing times.

The processes applied on volcanic ash and volcanic lava were two, from more abrasive (P1) to less abrasive (P2). This micronisation process aims to have a sufficient specific surface area to act as a cementitious material. The processes were carried out by introducing a determined quantity of material and abrasive load in a standardised friability test machine, subjecting them to a determined number of turns for their correct pulverisation.

After the size reduction process, five processed materials were obtained. The nomenclature of these materials is shown in Table 5

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