A Comparative Study on the Properties of Volcanic Rocks from the Aegean Islands, Greece, for Utilization as Pozzolanic Additives in Cement ^†

Abstract

This comparative study investigates the pozzolanic potential of volcanic rocks sourced from the Aegean Islands, Greece, as additives in cement. The utilization of volcanic materials as pozzolanic additives has gained interest due to its ability to enhance the performance and sustainability of cement-based materials. Pumice of Pleistocene age from Gyali Island, Pleistocene and Pliocene rhyolites from Kos Island, the Upper Miocene zeolitized rhyolite of Samos Island, and pumiceous tuffs of Upper Miocene age from Chios Island were sampled, and their chemical, mineralogical, and physical properties were analyzed. The results were compared to identify the most suitable for use as pozzolana in cement-based applications. The reactive silica determinations showed that all the rocks studied satisfy the EN 197 1-2 specification for reactive SiO₂ content > 25%. Despite their lower Chapelle values, the Pliocene and Pleistocene rhyolites (perlites) of Kos Island (Kefalos area) exhibit reactive silica values higher than 25%, acceptable LOI, and, due to their homogeneity and specifications compliance along with their wide spatial extend, may be considered the most promising among the rock formations studied as a natural pozzolana resource suitable for the cement industry.

1. Introduction

Research on pozzolanic materials, either as additives or as substitutes for cement, has been carried out for many decades, and the need to drastically reduce CO₂ pollution resulting from the clinker production cycle has been intensified and defined by international entities in the frame of the climate crisis. The cement industry accounts for about 9% of the total CO₂ global emissions [1,2,3].

The utilization of natural pozzolans as cement substitutes is considered an efficient alternative way to significantly reduce CO₂ emissions due to the non-calcinating process that is required for the typical raw materials (mainly limestone and clays) in cement production. It is noted that for the production of 1 ton of common cement (OPC), composed of more than 90% clinker, 0.93 tons of CO₂ are produced, while the production of 1 ton of blended cement by the substitution of clinker by natural or artificial pozzolanic materials can reduce CO₂ emissions to 0.25 tons [4]. In addition, the appropriate, optimal increase in the substitution rates of additives against clinker can bring about a reduction of CO₂ emissions of up to 24% on an annual basis, i.e., 650 million tons [3].

Major cement industries aim to net zero the CO₂ footprint of cement production by 2050, according to the CEMBUREAU 2050 roadmap, a goal that is supported by the substitution of cement by natural or synthetic cementitious materials, through which CO₂ emissions are estimated to be reduced by 1.3 Gt globally per year.

The long-term volcanic activity in the Aegean region during the Eocene/Oligocene-Pleistocene in a complex geotectonic environment favors, or hinders in some cases, the formation of natural pozzolans. Hydrothermal flows alter the host rocks promoted by the regional tectonics that enhance lava and fluid movements [5] and accelerate the diagenetic alteration of minerals over short distances [6]. These processes take place simultaneously, either converting rhyolitic tuffs to zeolitic minerals such as clinoptinolite and mordenite or altering the glass and feldspars to kaolinite or other clay minerals, thus favoring the pozzolanicity of these formations.

As it is well known since antiquity that many volcanic rocks of the Aegean region (e.g., [7,8,9,10]) have pozzolanic properties, our research for natural cementitious materials has been focused on young volcanic formations (i.e., Pleistocene pumices from Yali Island, Pleistocene and Pliocene rhyolites of Kefalos and Kos Island, volcanic tuffs of Kefalos and Kos plateau, Upper Miocene zeolitized rhyolite of Samos Island, and Upper Miocene pumiceous tuffs of Chios Island) (Figure 1;

Table 1. Locations, rock types, and ages of the formations studied as pozzolana resources.

Sample:	P-G	VT-PK	VT-KK	R-Pc-KK	R-Po-KK	ZR-S	PT-C
Location:	Yali Island	Kos Plateau, Kos Island	Kefalos, Kos Island	Kefalos, Kos Island	Kefalos, Kos Island	Samos Island	Chios Island
Rock type:	Pumice	Volcanic tuff	Volcanic tuff	Rhyolite	Rhyolite	Zeolitized Rhyolite	Pumiceous tuff
Age:	Pleistocene	Pleistocene	Pleistocene	Pleistocene	Pliocene	Miocene	Miocene
References:	[11,12]	[13,14,15]	[13,14,15,16,17]	[15,16,17]	[15,16,17]	[18,19,20]	[21]

).

The aim of this work is the identification and preliminary evaluation of volcanic lithological formations in the modern and older Aegean volcanic arc, demonstrating pozzolanic properties and exhibiting sufficient reserves for exploitation as pozzolanic raw materials in the cement industry.

2. Materials and Methods

Samples presented in this paper were selected from a series of samples from various volcanic formations in the Aegean Islands after a preliminary evaluation regarding their mineralogy, spatial extent, and environmental issues.

Bulk samples of about 50 kg each were collected from the field, dried, crushed in gravels of about 1 cm diameter, and homogenized in order to receive about 1 kg of representative material. They were then pulverized for 2 min in a tungsten carbide mortar for the experimental work and tests.

The methods applied for the study and the evaluation of the samples included XRD, XRF, optical and scanning electron microscopy, energy dispersive microanalysis, and Blaine [22]. Furthermore, the direct method of measuring pozzolanicity, concerning the consumption of portlandite by each sample studied, was the Chapelle method [23].

3. Results and Discussion

It is well known that natural pozzolanas are usually fine-grained rocks of biogenic or volcanic origin that have a suitable mineralogical and chemical composition. The most common are the glassy and pumice tuffs, the zeolitized tuffs, and, eventually, some diatomaceous rocks [24]. Providing that the exploitation of diatomaceous rocks as cement additives has obstacles such as the higher water demand [25], the current study has focused on young volcanic formations from the Aegean islands.

The particle size distribution (PSD) and the special surface area (SSA) data (Blaine) of the pulverized samples are presented in

Table 2. Particle size distribution (PSD) and special surface area (SSA) data of the pulverized samples.

	P-G	VT-PK	VT-KK	R-Pc-KK	R-Po-KK	ZR-S	PT-C
PSD %
<90 μm	97.3	90.6	96.4	91.7	94.4	96.9	99.0
<35 μm	90.4	67.2	77.8	61.7	69.1	75.5	87.0
SSA cm2 gr−1	8170	8870	9010	5840	6930	2043	7600

.

The mineralogical composition of the studied samples, as determined by optical and scanning electron microscopy and X-ray diffractometry, is presented in

Table 3. Qualitative and semi-quantitative mineralogical composition of the bulk samples as determined by optical microscopy and X-ray diffractometry. Please note that the glassy phase is not included. - is used if the phase is absent (i.e., not detected), and + is used for the relative presence of the mineral.

	P-G	VT-PK	VT-KK	R-Pc-KK	R-Po-KK	ZR-S	PT-C
Opal A/C-CT	+++	++	+++	+++	+++	++++	++
Quartz	+++	++++	+++	+++	++++	-	++++
Feldspars	+++	+++++	++++	+++	++++	+++++	+++
Micas	-	+	-	+	+	+	-
Zeolites	-	-	-	-	-	++++	+
Clay minerals	-	-	-	-	-	+++	+

.

The pumice formation from Yali Island (P-G) is glassy and cryptocrystalline, presenting elongated cavities and pores (≈100 μm) formed by the outflow of gases during the rapid cooling of the lava flows to ambient temperature, imparting a fluid texture due to its oriented pores.

The texture under a microscope of the volcanic tuffs of Kos Plateau (VT-PK) and Kefalos (VT-KK) resembles a pumice texture. Moreover, they contain the minor mineral phases ilmenite, zircon, and monazite.

The Pleistocene rhyolite formation of Kefalos of Kos (R-Pc-KK) is characterized by the presence of a glassy phase with a perlitic texture. A few biotite crystals are observed.

The Pliocene rhyolite of Kefalos of Kos (R-Po-KK) is more compact and exhibits a pearlitic texture with few pores. It is glassy to merocrystalline, sometimes porphyritic, with a few phenocrysts of augite, sanidine, biotite, and banded plagioclase.

In the Samos zeolitized rhyolite formation (ZR-S), the volcanic glass is being replaced by zeolites (mordenite and clinoptinolite) with characteristic elongated to fibrous and tabular crystals, respectively, along with clay minerals. A few zircon crystals have been observed.

The Chios pumiceous tuff (PT-C) is merocrystalline with zeolitic and clay mineral phases, replacing the initial glass. Furthermore, feldspar crystals are detected.

The chemical composition of the samples, presented in

Table 4. Chemical composition (w/w %) of the bulk samples.

	P-G	VT-PK	VT-KK	R-Pc-KK	R-Po-KK	ZR-S	PT-C
SiO2	72.0	73.2	65.5	74.9	73.1	73.3	71.9
Al2O3	11.1	13.2	11.9	12.6	12.8	11.3	13.4
Fe2O3	1.1	1.7	1.3	0.8	1.0	1.3	0.7
CaO	1.9	1.4	5.4	0.5	0.7	1.3	1.0
MgO	0.4	0.4	0.4	0.1	0.2	0.2	0.1
K2O	4.2	3.9	3.9	4.3	4.2	4.0	3.6
Na2O	3.5	3.7	3.2	3.8	3.9	0.7	3.1
SO3	<0.1	<0.1	0.1	<0.1	<0.1	<0.1	0.1
MnO	0.0	0.1	0.1	0.1	0.1	0.0	0.1
TiO2	0.1	0.2	0.2	0.1	0.1	0.1	0.0
L.O.I	5.2	2.2	8.0	2.8	2.1	7.5	5.1
Sum	99.6	100.0	99.9	100.0	98.2	99.7	99.1

, reveals the acidic and alkaline nature of the studied volcanic formations. Silicon oxide is 65.5–74.9% w/w, Al₂O₃ ranges between 11.1 and 13.4% w/w, and the total alkalis (K₂O, Na₂O) were measured at 4.7–8.1% w/w. The lower sodium oxide content (0.7% w/w) was detected in the zeolitized sample (ZR-S) from Samos. Low MgO values (<0.5% w/w) were detected in all samples.

The volcanic tuff of Kefalos, Kos Island (VT-KK), the pumiceous tuff of Chios Island (PT-C), and the zeolitic rhyolite of Samos Island (ZR-S) exhibit the highest loss on ignition values (Table 4). It is noted that the incorporation of zeolite in cement has been found to contribute to the consumption of Ca(OH)₂, formed during cement hydration, resulting in the formation of cement-like hydrated products [26].

The Ca-Si-Al ternary diagram (Figure 2) presents the different average compositions of Portland cement, limestone, and supplementary cementing materials (SCMs) [27]. All studied samples are plotted into the field of the natural pozzolan composition.

The reactive silica values, as determined by the Chapelle test (

Table 5. Comparison of silica, aluminum, total alkalis content, and estimated reactive silica (EAS) by the Chapelle test.

	P-G	VT-PK	VT-KK	R-Pc-KK	R-Po-KK	ZR-S	PT-C
SiO2	72.0	73.2	65.5	74.9	73.1	73.3	71.9
Alkalis	7.7	7.6	7.1	8.1	8.1	4.7	6.7
Al2O4	11.1	13.2	11.9	12.6	12.8	11.3	13.4
EAS	54.0	54.0	58.0	45.0	46.0	52.0	54.0

), suggest that all the volcanics studied are in line with the EN 197-1 [27] specification for a reactive SiO₂ content > 25%.

Yali Pleistocene pumice is currently being quarried by Lava Mining and Quarrying Company, part of Lafarge Holcim Group, to be used for the production of lightweight pumice blocks, lightweight concrete, lightweight precast elements, lightweight insulations, coatings, and floor fillings, as a substrate for hydroponics, soil improvement media, for green roof ingredient substrates and soil cover in landscaping, in geotechnical applications, and as a polishing material in textile stone wash. This pumice is suitable for the cement industry as a natural pozzolanic source.

The Pliocene and Pleistocene rhyolites of Kefalos, Kos Island, demonstrated reactive silica values above 25% (Table 5) and low LOI (Table 4). Even though they exhibited the lowest estimated reactive silica (EAS) values, they are considered the most promising among the studied materials for exploitation as a natural pozzolanic source for the cement industry due to their homogeneity and specification compliance, along with their wide spatial extend.

4. Conclusions

The pozzolanic potential of volcanic rocks from the Aegean Islands, Greece, has been investigated. The chemical, mineralogical, and physical properties of the (a) Pleistocene pumice from Gyali Island, (b) Pleistocene and Pliocene rhyolites from Kos Island, (c) Upper Miocene zeolitized rhyolite from Samos Island, and (d) Upper Miocene pumiceous tuffs from Chios Island were studied. The results obtained were compared in order to identify which of those would be most suitable for use as pozzolana in cement-based applications.

The chemical composition of the samples revealed the acidic and alkaline nature of the studied volcanic formations. Concentrations of SiO₂ were measured between 65.5% and 74.9% w/w, Al₂O₃ ranged between 11.1 and 13.4% w/w, and the total alkalis were determined between 4.7 and 8.1% w/w. The lowest Na₂O content (0.7% w/w) was measured in the zeolitized samples of Samos. Low MgO values (<0.5% w/w) were detected in all samples. The volcanic tuff of Kefalos, Kos Island, the pumiceous tuff of Chios Island, and the zeolitic rhyolite of Samos Island demonstrated the highest LOI values.

The reactive silica determination suggests that all the studied lithological formations satisfy the EN 197 1-2 specification for a reactive SiO₂ content > 25% and can therefore be used as natural pozzolans in the cement industry.

The Yali Pleistocene pumice that is currently quarried and exploited for other applications seems to be suitable for the cement industry as a natural pozzolanic source.

Although the Pliocene and Pleistocene rhyolites of Kefalos, Kos Island, exhibited the lowest reactive silica values, their demonstrated combination of homogeneity, specification compliance, and wide spatial extend deems them the most promising for exploitation as natural pozzolanic resources in the cement industry.