Effect of Processed Volcanic Ash as Active Mineral ...

Abstract

In the last quarter of , there was a very significant eruption of the Cumbre Vieja volcano on the island of La Palma, belonging to the Canary Islands, Spain. It generated a large amount of pyroclastic volcanic materials, which must be studied for their possible applicability. This work studies the properties and applicability of the lava and volcanic ash generated in this process. The need for reconstruction of the areas of the island that suffered from this environmental catastrophe is considered in this study from the point of view of the valuation of the waste generated. For this purpose, the possibility of using the fine fraction of ashes and lava as a supplementary cement material (SCM) in the manufacture of cement is investigated. The volcanic material showed a chemical composition and atomic structure suitable for replacing clinker in the manufacture of Portland cement. In this study, the cementing and pozzolanic reaction characteristics of unprocessed volcanic materials and those processed by crushing procedures are analysed. To evaluate the cementitious potential by analysing the mechanical behaviour, a comparison with other types of mineral additions (fly ash, silica fume, and limestone filler) commonly used in cement manufacture or previously studied was carried out. The results of this study show that volcanic materials are feasible to be used in the manufacture of cement, with up to a 22% increase in pozzolanicity from 28 to 90 days, showing the high potential as a long-term supplementary cementitious material in cement manufacturing, though it is necessary to carry out crushing processes that improve their pozzolanic behaviour.

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Keywords: volcanic ash, cement, pozzolanic behaviour, chemical composition, mechanical behaviour

1. Introduction

After 50 years of quiescence in La Palma Island (Canary Islands, Spain), the Cumbre Vieja volcano&#;historically the most active volcano in the Canary Islands&#;began an eruptive episode on 19 September , forcing the evacuation of residents, destroying infrastructure worth more than EUR 400 m, and affecting 1.212 hectares and 92.7 km of roads with solidified lava and ash [1]. All this volcanic solidified lava and ash, together with growing environmental awareness and a circular economy, considering that the construction industry is perceived as a major contributor to environmental degradation [2] that consumes 40% of the raw materials extracted [3], makes the study of lava and ash for its application in building materials very interesting. These materials, formed from the cooling of magma from the volcanic eruption, are known as pyroclastic materials and have very heterogeneous physical properties, varying in particle size from microns (ash) to metres (solidified lava) [4], and can have a dense or vesicular structure [5,6].

Dingwell et al. [7] differentiated typical volcanic ashes as pyroclastic debris no larger than 2 mm, however, many authors carry out crushing and sieving procedures for the utilization of volcanic ash [8,9,10,11,12]. Lemougna et al. [10] ground volcanic ashes to pass a 400 μm sieve; Leonelli et al. [11] dry-milled the analysed volcanic ashes to a fineness of 150 mm; Tchakoute et al. [12] ground and sieved the ashes to a powder of 80 μm. Some authors have determined the influence of the particle size of volcanic ash for use as a construction binder. According to Moufti et al., [13], finely pulverised ash with a particle size of less than 45 mm and a content of 10% by mass has a compressive strength similar to a control sample. On the other hand, Khan et al. [14] reported that 15% substitution of natural pozzolans with finely ground cement had a lower strength compared to controls. An important property of this type of material is that it has pozzolanic activity, i.e., in contact with water it can behave as a hydraulic binder, just like cement [15,16].

Currently, the use of lava and volcanic ash has been evaluated by different authors for use as construction material; the main applications have been as ceramic material, geopolymers, cement, and concrete [17,18,19,20]. Zhang et al. [21], manufactured and analysed bricks fired with a mixture of volcanic ash and black cotton soil between &#; °C, showing good compressive strength (60 MPa), a small percentage of dimensional variation, and similar bulk density to conventional brick. Serra et al. [22] reported the use of ash as a flux for feldspar replacement in clay-based materials and observed appropriate brick texture and mechanical properties compared to traditional materials used in brick manufacturing.

The high content of aluminosilicates for the synthesis of geopolymers has attracted the interest of a large number of studies of this type of mineral in the production of geopolymer materials, either as the sole source of aluminosilicate material [23,24] or combined with other types of materials such as metakaolin [25].

Furthermore, numerous studies corroborate the suitability of volcanic ash for partial replacement of cement, paste, and mortar or in the manufacture of concrete [25,26,27,28,29]. For example, Celik et al. [27] reported that a high-volume mass replacement of Portland cement (OPC) with volcanic ash produces concrete with good workability, high compressive strength, and high resistance to chloride penetration. Al-Fadala et al. [27] analysed the mixture of volcanic ash and cement according to international standards, to evaluate the use of this material, and concluded that it met the technical requirements to be used for certain percentages of volcanic ash from a chemical, physical, and mechanical point of view. Regarding treatments applied to volcanic ash prior to its use, Khan et al. [28] showed that pozzolanic activity increased with the fineness of the material; however, a heat treatment applied to volcanic ash was not positive. Other studies, such as that of Abdullah et al. [30], showed that volcanic pumice powder improved the compressive strength of self-compacting concretes made with it, thus demonstrating the influence of the degree of fineness of volcanic ashes on the mechanical properties. Al-Swaidani and Aliyan [31] studied the durability of mortar and concrete made with different slag substitutions, showing great interest in properties related to chloride ion penetration, acid attacks, and corrosion of reinforcing steel, and concluded that the volcanic slag studied was suitable for use as a natural pozzolan in accordance with international standards.

Therefore, taking into account that cement, and especially the process necessary to produce it, contributes significantly to climate change, emitting 8% of total CO2 emissions worldwide, the aim of this work is to study the use of ash from the Cumbre Vieja volcano as a replacement for cement in the production of Portland cement as well as its effects on the manufacture of mortar. The physical, chemical, mechanical, and environmental properties, in accordance with international specifications, have been studied. This study shows the long-term pozzolanic potential of volcanic ashes and how the application of a crushing treatment influences the mechanical properties of cement mortars. A comparative study has been carried out with other types of commonly used mineral additions. This study shows the possibility of applying the fly ashes accumulated to date after the natural catastrophe that occurred on the island of La Palma, which would lead to the elimination of their accumulation and generate low-emission cement with good mechanical properties.

3. Results and Discussion

3.1. Pozzolanity and Frattini Tests

Figure 9 shows the results obtained for the [CaO] and [OH&#;] concentrations of each of the mixtures analysed at 8 and 15 days according to the standardised test. The results were compared with the portlandite solubility curve.

Figure 9.

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Results of Frattini test at 8 and 14 days.

Figure 9 shows the values of [Ca]2+ and [OH&#;] oxides, which decrease in solution as a consequence of the depletion of calcium hydroxide, after the pozzolanic reaction of each of the mixes analysed at 8 and 15 days, according to the standardised test. The results were compared with the portlandite solubility curve.

The analysed mixture is considered to comply with the test, i.e., to be pozzolanic, when the concentration of calcium ions is lower than the saturation concentration indicated by the reference curve. It was observed that all ash mixtures analysed were above the curve, unlike the results obtained in other studies [41], in which volcanic ashes showed high pozzolanicity as well as fly ash and silica fume [42,43].

The crushing processing of the volcanic ash led to an improvement in the pozzolanic capacity of the material; in the short term, the material was not considered to be pozzolanic, but the values were close to the solubility curve. Previous studies showed that mechanical activation of ashes increases the reactivity of the pozzolanic material [44].

As shown by other authors, crushing volcanic material to be used as a supplementary cementitious material improved the pozzolanic properties of the mixes [45,46,47]. The approximation to the solubility curve of the crushed material is due to the fact that higher amounts of calcium silicate hydrate (C-S-H) and calcium aluminate silicate hydrates (C-A-S-H) gel phases were formed, and finer sizes of the material lead to a higher amount of these phases.

3.2. Resistant Activity Index

It can be seen in Table 7 that the processed volcanic ash improves its resistance compared to unprocessed volcanic ash; that is, the degree of fineness has a very relevant influence on the resistance obtained. If FVA-2 and CVA-2 were compared with the mixture in which FA was used, it remains slightly below, not reaching 85% in the case of volcanic ash. However, a much higher increase than the mixture with LF is obtained, which allows one to think that they are usable in the manufacture of cement and as a mineral addition to concrete.

Table 7.

Results of compressive strength in resistant activity index.

Compressive Strength MPa (Age) % Regarding Control 28D % Regarding Control 90D Resistance Increase
28&#;90 days 7 28 90 CEM I (42.5) 41.5 46.2 49.1 - - 6.4% SF 35.6 42.4 45.5 91.8% 92.6% 7.3% FA 31.5 38.4 42.5 83.1% 86.5% 10.7% LF 29.7 33.8 36.2 73.2% 75.7% 7.1% FVA-NP 26.7 31.6 37.1 68.4% 75.5% 17.4% FVA-1 26.2 31.5 36.2 68.2% 73.7% 14.9% FVA-2 27.6 34.6 40.7 74.9% 82.8% 17.6% CVA-NP 19.7 31.1 37.8 67.3% 76.9% 21.5% CVA-1 24.3 33.7 38.6 72.9% 78.5% 14.5% CVA-2 27.6 35.4 41.1 76.6% 83.6% 16.1% VL 25.5 33.8 37.1 73.2% 75.5% 9.8% Open in a new tab

In Figure 10, only the five most significant mixtures have been included. It is clearly observed how there is a more relevant increase in resistance in the two samples made with processed volcanic ash (CVA-2 and FVA-2), compared to the control or in the mixture made with FA. This fact indicates that the volcanic ash gradually increases its resistance over time, with its growth being greater after 28 days, compared to the case of mixtures made with a conventional cement. This is due to an increase in pozzolanicity at 90 days, as observed in Figure 8, which contributes higher strength to the mortars made with FVA and CVA. On the other hand, in the mixture made with LF, it does not show significant growth after day 28 because it is a mineral addition with little pozzolanic activity.

Figure 10.

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Progress in resistance activity index.

Therefore, based on these results, it can be concluded that volcanic ashes processed with an adequate degree of fineness can be used in the manufacture of cement, presenting a higher reactive silica content at 42%, which is very important for the validation of these volcanic materials as a cement substitute. Lastly, it should be noted that the unprocessed volcanic ash presented similar results to the mortars made with LF at 90 days, with the behaviour being less than 28 days and the growth of resistance being between 28 and 90 days. For example, in both mixtures, FVA-NP and CVA-NP, it is possible to observe the increase in resistance between 28 days and 90 days, going from 31.1 MPa to 37.8 MPa in CVA-NP (increase of 21.5%), and from 31.6 MPa to 37.1 MPa in FVA-NP (increase of 17.4%).

These results clearly show that volcanic ash can be used as SCM, and, although it can be processed, improving behaviour, it can be applied in the manufacture of cements and as a mineral addition in concrete manufacturing.

3.3. Setting Time and Volumetric Expansion

Table 8 shows the results obtained for the initial and final setting times and volumetric expansion for all mortars. As can be observed, the OPC initial and final setting times were 105 and 190 min, respectively, consistent with high percentages of clinker and rapid hardening cement.

Table 8.

Setting time and volumetric expansion of mortar.

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Mixture Setting Time (min) Expansion (mm) Initial Final OPC 105 190 1.50 mm SF 110 180 1.40 mm FA 125 235 2.10 mm LF 85 175 1.30 mm FVA-NP 90 140 1.0 mm FVA-P1 105 155 0.9 mm FVA-P2 90 140 1.1 mm CVA-NP 90 140 1.0 mm CVA-P1 95 140 0.9 mm CVA-P2 90 130 1.0 mm VL-P 100 135 0.8 mm Open in a new tab

The addition of LF retarded both setting times. On the contrary, the addition of FA implies a lengthening of both times. This behaviour has been extensively studied and described for decades [48,49,50].

Analysing the results for the VL, FVA, and CVA samples, it is observed that the addition of these materials and their different processing slightly decrease the initial setting time as well as more noticeably reduce the final setting time; however, the times between the different volcanic materials remain practically stable. Other studies describe an opposite behaviour after adding these materials, with slight increases in setting times; however, due to the different origins, compositions, and properties of the volcanic materials, different effects can be observed [15].

Concerning the results for the determination of volumetric expansion, all materials show values below the limits for cementitious specifications according to EN 197-1. The addition of SF has no significant effect on the expansion of the cementitious pastes [49], although the addition of FA does lead to an increase in expansion. The addition of volcanic material slightly reduces the volumetric expansion of the cementitious pastes and, as with the setting times, the values for all volcanic materials are similar. This decrease in expansion with respect to the mortars manufactured with FA may be mainly due to the increased absorption of the mortar pastes manufactured with FA [51,52]. The FVA and CVA samples presented low absorption (Table 2); therefore, the manufactured mortars presented low dimensional changes at early ages, due to the fact that there are no large pores or occluded water in the mortars that could modify their dimensions at initial curing ages.

4. Conclusions

In the present study, the effect of applying volcanic material (pyroclasts and volcanic lava) from the eruption of the Cumbre Vieja volcano in La Palma, Spain, as an active mineral addition for the manufacture of pozzolanic cements, is analysed. In addition, silica fume and fly ash from coal combustion were analysed as pozzolanic material references. After studying the physical, chemical, and mineralogical properties of volcanic materials and their application in mortars, the following conclusions are drawn:

- Volcanic material (fine ash, coarse ash, and lava) is mainly composed of SiO2, Al2O3, Fe2O3, and CaO. A natural pozzolan is essentially composed of reactive silicon dioxide (SiO2), aluminium oxide (Al2O3), and iron oxide (Fe2O3). Therefore, the material has suitable characteristics to be used as natural pozzolanic material as SCM.

- The three materials analysed (coarse ash, fine ash, and lava) have reactive silicon dioxide values well above the 25% required by the UNE-EN 197-1 standard for the application of natural pozzolan in the manufacture of cement. This demonstrates that their use is viable and complies with the minimum requirements established.

- The pozzolanicity study showed that the volcanic lava presented high pozzolanicity at early ages; however, the volcanic ash evolved more positively, obtaining high pozzolanicity at 90 days. This is a positive fact, since natural pozzolans cannot be evaluated only in the short term: it is necessary to evaluate their mechanical behaviour in the medium and long term.

- The unprocessed volcanic ash showed a resistance in the 28-day resistant activity index test that was lower than the rest of the SCM studied in this work, but the increase in resistance between 28 and 90 days was much higher, obtaining up to a 21.5% increase in resistance in the sample in the mortar mix made with CVA-NP.

- A relevant increase was observed in resistance in the processed volcanic ash, and the mixtures made with them increase their resistance over time, so the increase between 28 and 90 days was very relevant.

- In the long term (90 days), the compressive strength results of mortars manufactured with FVA and CVA increased considerably, exceeding the results obtained in the LF mixtures.

- In the long term, it is demonstrated that unprocessed and crushed volcanic ash can be used as a natural pozzolan for the manufacture of cement, obtaining higher results than a mortar made with limestone filler.

In view of the results, the pozzolanic potential of the volcanic ash from the La Palma eruption is feasible for the manufacture of cement, and it is possible to apply substitution percentages of SCM of up to 25%. This application shows the environmental and social benefits in relation to the volcanic process that occurred in on the island of La Palma, Spain, due to the large volume of fly ash generated during the eruption of the volcano.

Acknowledgments

The authors acknowledge the financial support provided by the company Sacyr, the promoter and principal researcher of the project, and especially Francisco Javier Mateos and Ana Esteban. The authors would also like to thank the project Development of &#;Smart&#; surfacing and repair Materials from low-carbon by products for more effective active and predictive safety. Advanced applications for Roads, SMATCAR funded by the Minister of Science and Innovation of Spain. In addition, the authors would like to thank María Isabel Sánchez de Rojas, a member of the Eduardo Torroja Institute for Construction Science.

Author Contributions

Conceptualization, F.A. and J.R.; methodology, M.R. and M.C.; formal analysis, J.L.D.-L. and J.R.; in-vestigation, F.A., J.R., M.R., J.L.D.-L. and M.C.; writing&#;original draft preparation, F.A. and J.R.; writing&#;review and editing, FA. and J.R.; supervision, F.A. and J.R.; project administration, F.A. and J.R. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Funding Statement

This research was funded by the project Development of &#;Smart&#; surfacing and repair Materials from low-carbon by products for more effective active and predictive safety. Advanced applications for Roads, SMATCAR funded by the Minister of Science and Innovation of Spain, grant number PID-RB.

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Data Availability Statement

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