Thermal properties of volcanic ash and pumice
Conference Paper · August 2013
DOI: 10.13140/RG.2.1.1916.8165
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Rimma Grigorjevna Motenko Lomonosov Moscow State University 37PUBLICATIONS 134CITATIONS
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MINERAL DEPOSIT RESEARCH FOR A HIGH-TECH WORLD · 12TH Biennial Meeting 2013. Proceedings, Volume 4
Thermal properties of volcanic ash and pumice
Elena Kuznetsova
SINTEF Building and Infrastructure, Trondheim, Norway
Rimma Motenko
Lomonosov Moscow State University, Moscow, Russia
Svein Willy Danielsen
SINTEF Building and Infrastructure, Trondheim, Norway
Abstract. This paper presents the results on thermal conductivity of volcanic ash and pumice collected from Kamchatka, Far East of Russia, and from Iceland, in dependence on different humidity and density. The research shows that the humidity of the samples is the key factor for thermal conductivity, both in the frozen and thawed state. And this in turn depends on porosity/density and chemical composition.
Keywords: thermal conductivity, volcanic ash and pumice
1 Introduction
People started to use volcanic materials for construction and industrial purpose already many centuries ago, and still do. Uses range from cut slabs of tuff for construction to very fine ash for polishing automobiles.
According to Heiken G. (2005) volcanic materials can be grouped as (1) pumice and ash, (2) pozzolan (fine-grained or zeolitized ash), (3) quarried tuff, (4) perlite, (5) basaltic scoria and lava, and (6) volcanic clays and zeolites.
Pumice is a natural material of volcanic origin produced by the release of gases during the solidification of lava (Neville 1995). Volcanic pumice has been used as aggregate in the production of lightweight concrete in many countries of the world. So far, the use of volcanic materials was dependent on the availability and limited to the countries where it is locally available or easily imported. Satisfactory light weight concrete having good insulating characteristics, but also with high absorption and shrinkage can be manufactured using volcanic pumice (Neville 1995, Hossain 2004)
Volcanic activities are common phenomena in various part of the world, especially like Iceland, Kamchatka on Far East of Russia, Papua New Guinea and others.
In this paper we are going to present some results on thermal conductivity of volcanic ash and pumice from Kamchatka area in Russia and from south of Iceland.
2 Methods
Scanning electron microscope (SEM) images of the samples and chemical composition of the volcanic glass and some minerals were obtained at the petrology
department of Moscow State University by EPMA using
“Jeol JSM-6480LV” with inversions spectrometer
«INCA-Energy 350».
Transmission Electron Microscopy (TEM) analysis was performed in the NTNU/SINTEF Gemini Centre using a JEOL 2010F instrument equipped with an Oxford Instruments SDD x-ray detector for composition analysis.
Thermal conductivity (ʎ, (W/(mK))) was calculated as: ʎ = С·p·а, where C – specific heat capacity (J/(kgK)), p – rock density (g/cm3), a – temperature conductivity (m2/sec).
The temperature conductivity (a) was determined by using I type regular mode method (ɑ-calorimeter) (Ershov, 2004). We realize detecting of temperature conductivity (a) by heating and cooling of ground in environment with constant temperature (outside of area with intensive phase changes). The temperature range was 0…+20 and -22…-12 ºС. All measurements were done two times. Precision of measurements is about 10%.
Specific heat capacity (C) was calculated as sum of ground components (rock matrix, water, ice). Thermal capacity was set to 4200 J/(kgK) for water and 2100 J/(kgK) for ice. Thermal capacities for rock matrix were measured on ITS-400 by monotonous heating. Precision of measurements is about 10% (Platunov, 1972).
The content of unfrozen water in frozen ground was determined by a combination of cryoscopic and contact methods (over the range from 0 to –15°C). The contact method is based on the principle of equilibrium in the ice, water and steam. The total moisture content was obtained by this method in the initially dry plates corresponding to the equilibrium content of unfrozen water at a given temperature. Cryoscopic method is based on the assumption that amount of the unfrozen water at the temperature t is equal to moisture content of the soils at the freezing (thawing) temperature (Ershov, 1979).
3 Volcanic pumice and ash
Samples for laboratory study were collected in Kamchatka peninsula, Far East of Russia, and in the south of Iceland.
For Kamchatka's samples natural humidity and density are ranging from 10 to 65% and from 0.9 to 1.5 g/cm3 respectively. Hygroscopic water and particle density are ranging from 0 to 4% and from 2.1 to 2.8
TH
g/cm3. The chemical composition of volcanic glass in ash samples are rhyolitic and andesitic, in volcanic pumice – basaltic composition. The age of volcanic material is ranging from 35 to 9000 years.
For Iceland's samples humidity and density are up to 40% and 2.75 g/cm3 accordingly. The chemical composition of the volcanic glass – basaltic. The age of material is approximately 100,000 years.
4 Alteration processes
Pyroclastic ash is the least stable solid phase of sediments that is predisposed to different mineral transformation in all lithogenetic stages. Volcanic ashes mainly consist of volcanic glass as the major component.
They also include plagioclase, feldspar, quartz and pyroxene (Dahlgren et al. 1993). As a result of volcanic activity, volcanic glass is transported to a great distance and is then weathered forming secondary minerals.
Among the alteration products caused by weathering and diagenesis are smectites, halloysite, kaolinite, allophane, palagonite and others.
For samples from Kamchatka IR-absorption spectra allowed diagnosing the composition of ashes with mafic (basaltic and andesitic) glass as allophane, and the composition of ashes with acidic (rhyolitic) glass - as opal; for samples from Iceland IR-spectra showed appearance of palagonite (Kuznetsova et al. 2010).
4.1 Allophane
Allophanes (from Greek allophanes – "turning out to be different") are amorphous minerals of variable chemical composition. Wada (1989) gives the approximate chemical formula of allophanes as Al2Si2O5·nH2O. They belong to sheet silicates because they have similar chemical composition and some common structural peculiarities with them, but differ from them through the lack of crystalline texture. Allophane is fully amorphous and was found for the first time as a product of volcanic ash transformation. Allophane consists of hollow spheres with the outer diameter of about 4-5 nm (Henmi & Wada 1976).
On the Figure 1 there is presented the glass particle from Kamchatka's samples, where neoformation of allophane is taking place from the borders of the vitric material towards the inside.
4.2 Opal
Opal (from Sanskrit upalah – stone) is amorphous silica, silicon dioxide hydrate, the chemical formula of which is SiO2·nH2O, where n usually varies from 0.5 to 2.
Two types of opal silicon are the most common for volcanic ashes: pedogenic (more known as laminar opal silicon) and biogenic (diatoms). Laminar opal silicon emerges at the early weathering stages and is characterized by the existence of spheric bunches of reticulate-like spheres of hydrated silicon. Opal silicon is formed in silicon-rich environment from over-saturated solutions which appear due to surface evaporation and,
probably, water freezing in soils (Shoji et al. 1993;
Nanzyo 2002). Its quantity falls with the increase of the volcanic soil age as a result of particles weathering.
During our research it was assumed that the unit particle of opaline silica is roughly spherical in form and that these spheres are closely packed in microaggregates (Figure 2), as described by Shoji and Masui (1971). The opaline silica particles are extremely thin; the size of each peculiar sphere can be up to 10-20 nm. An eroded edge of the opaline silica can be seen which suggests that dissolution occurs during weathering. Most of the cells in the completely altered soft zone are filled with spherical bodies.
Figure 1. Allophane crust around glass particle; Gl – volcanic glass
Figure 2. Transmission electron micrographs glass particle with opaline silica around; the boundary between altered and unaltered zones is visually distinguishable
4.3 Palagonite
“Palagonite” (gel to fibro palagonite depending on the degree of crystallinity) is a mixing of phases resulting from the process of palagonitisation which is an alteration of the glass by sea-water or by unsalted-water, submitted to different temperature or pressure conditions. The highly hydrated gel palagonite, related to low temperature alteration, is optically similar to the allophanic material: a clear yellowish to brownish isotropic and commonly concentrically banded material, in which the primary vitric morphology is preserved.
MINERAL DEPOSIT RESEARCH FOR A HIGH-TECH WORLD · 12TH Biennial Meeting 2013. Proceedings, Volume 4 Palagonite was described as the first stable product
resulting from the alteration of glass below water, forming a ring of insoluble material at the glass-fluid interface. In gel-palagonite spherical structures of 20 to 60 nm were observed by high resolution electron microscopy and interpreted as precursors of smectites.
Amorphous “gel”, palagonite is related to Si, Al, Mg, Ca, Na, K losses, H2O gain and immobile Ti, Fe behavior (Gerard & Stoops, 2005).
On figure 3 there is presented volcanic glass particles from Iceland's samples. Palagonite borders are surrounding the glass material and the process is very similar with allophane formation.
Figure 3. Palagonite crusts around glass particles in the samples from Iceland; Gl – volcanic glass
5 Thermal conductivity
Figure 4 presents the dependences of thermal conductivity versus humidity for both volcanic ashes and pumices in the thawed and frozen states. Samples nos. 1- 21 – volcanic ash and nos. 1'-4' – volcanic cinder, collected from Kacmhatka, nos. 1*-3* - volcanic ash and no. 4* - volcanic pumice, collected from Iceland.
Evidently, increasing of humidity and density resulted in increasing thermal conductivity coefficients.
It is because low-conductivity air is replaced by more high-conductivity liquid or ice. The highest increasing of conductivity for samples with highest density values is explained by greater part of soil skeleton.
To compare data for thermal conductivity is possible only for samples with the same dry density ρd. Lines are summarizing the values for the same dry density. With increasing humidity W (from 0 to 80%) and dry density ρd (from 0.7 to 1.7 g/cm3) thermal conductivity λ is increasing from 0.13 to 1.0 W/mK in thawed and from 0.14 to 1.27 W/mK in frozen states. For dry samples the values of thermal conductivity are close – 0.13-0.17 W/mK.
The dry density has a bigger influence on thermal conductivity in the thawed than in the frozen state. We also can see it on this example: the values for samples with dry density 1.0 and 1.1-1.2 g/cm3 are summarized by two lines in the thawed state and by one line in frozen state.
It is very important to mention the fact that the values for both ash and cinder are summarized by the same line
despite the fact that cinder has bigger grain size than the ash. The reason for this is the cinder particles having a very high porosity and these two factors, particle size and porosity, are compensating each other; the major influence on thermal conductivity is due to the soil dry density.
Figure 4. The dependence of thermal conductivity (λ) versus humidity (W) for volcanic deposits studied in the frozen (λf) and thawed (λth) states: nos. 1-21 – volcanic ash from Kamchatka, nos. 1'-4' – volcanic pumice from Kamchatka, nos.
1*-3* – volcanic ash and no. 4* - pumice from Iceland. Lines summarize the values for the same dry density ρd: 1 – 0.8–0.9 g/cm3, 2 – 1 g/cm3, 3 – 1.1-1.2 g/cm3, 4 – 1.3 g/cm3, 5 – 1.5 g/cm3, 6 – 1.6 g/cm3, 7 – 1.7 g/cm3
6 Unfrozen water content
The unfrozen water content in the frozen volcanic materials has been reported in a previous investigation (Kuznetsova et al. 2011). Figure 5 is a crossplot showing the unfrozen water content Ww and negative temperature for samples from Kamchatka containing either allophane or opal. For temperature below -3oC the content Ww changes from 2 to 13% for ashes containing allophane (area 1) and from 0 to 3% for ashes containing opal (area 2). For samples from Iceland, containing palagonite, the content Ww for temperature below -3oC is 2%.
MINERAL DEPOSIT RESEARCH FOR A HIGH-TECH WORLD · 12TH Biennial Meeting 2013. Proceedings, Volume 4 Figure 5. The dependence of unfrozen water content on
temperature for studied samples from Kamchatka. Area 1 - ash samples containing allophane, area 2 –ash samples containing opal
According to many investigations (e.g. Theng et al.
1982, Henmi & Wada, 1976), allophane consists of hollow spheres with the outer diameter of about 4-5 nm, with defects in the wall texture which consequently form micro-pores 0.3-2.0 nm and with active surface area estimated is 800 m2/g. The moisture retention capacity of allophanes is related to the presence of these very small pores and high active surface areas.
Palagonite is a proto-smectite, that's why it also has high active surface area and good absorption properties (Gerard & Stoops, 2005).
7 Conclusions
The main results obtained by the study are summarized below.
• The mineralogical analysis showed that for Kamchatka's samples: allophane was found in samples with andesitic and basaltic glass, opal - in samples with rhyolitic glass; for Iceland's samples: palagonite was found in samples with basaltic glass.
• In dry state, thermal conductivities for all volcanic deposits are very close and equal to 0.15-0.18 W/mK.
• On condition that the density (ρd) and humidity (W) are changing from 0.7 to 1.65 g/cm3 and from 10 to 80 % respectively the thermal conductivity (λ) increases from 0.37 to 1.0 W/(mK) in a thawed state and from 0.41 to 1.27 W/(mK) in a frozen state.
• Thermal conductivity values under fixed humidity and density are close for pumice and ash in both thawed and frozen states, which related to two competing effects: grain size and particle porosity.
Pumice particles have enclosed porosity which decrease their thermal conductivity and compensate the fact that cinder particles are bigger than ash particles.
• Volcanic deposits have very good thermo insulation properties.
• The content of unfrozen water is higher in volcanic ashes containing allophane than in those containing opal.
Thereby, the ash and pumice is found to be suitable as a thermal insulating material and has the potential to be utilized in manufacturing heat-insulating concrete and building blocks, especially where weather is cold and wet.
Acknowledgements
Experimental work was done at the Department of Geocryology of Lomonosov Moscow State University as a part of the first author's PhD work.
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