Valorisation of "La Palma" Volcanic Ash for Making Portland-Blended, Alkaline and Hybrid Portland-Alkaline Cements

Abstract

The present work evaluates the feasibility of using volcanic fly ash (VFA) generated by the eruption of the Tajogaite volcano on the island of La Palma (Spain) in 2021, as a precursor in the preparation of cementitious materials with different Portland cement (PC) replacement levels (0%, 30%, 70% and 100%), in the absence (Blended Cement, BC) and presence of an alkaline activator (Hybrid Alkaline Cement, HAC, and Alkaline Cements, AC). Hydration kinetics (isothermal conduction calorimetry), paste mechanical strengths and reaction products were characterised by XRD, FTIR, TG/DTG and BSEM/EDX. The results obtained indicate that the strengths developed by the hybrid alkaline cements (HAC) are higher than those of the blended cements (BC), especially at the age of 2 days, where 25 MPa were obtained with the replacement of 70% PC by VFA. Alkaline cements (AC, 100% VFA) that were prepared with 8 M NaOH solution as the activator reached 40 MPa after 2 days. It was observed that in all the binders, depending on the initial composition of the binder mixture and the percentage of replacement and/or activator, VFA reacts to form cementitious gels, C-A-S-H and N-A-S-H type, which supports its use as a mineral addition to blended cement or as a precursor in the preparation of alkaline and hybrid alkaline cements.

Keywords: alkaline cement; blended cement; hybrid alkaline cement; volcanic fly ash.

Figure 1

Landscape generated after the eruption of the Tajogaite volcano (La Palma, Spain).

Figure 2

(a) Particle size distribution of both PC, original VFA and VFA after milling; (b) SEM photograph of original VFA; and (c) SEM photograph of milled VFA.

Figure 3

(a) XRD patterns and (b) FTIR spectra of Portland cement (PC) and the volcanic fly ash (VFA). Legend: d: diopside (MgCaSi₂O₆) (COD 9004319); a: augite (Ca,Mg,Fe)₂(Si,Al)₂O₆ (COD 9006247); q: cristobalita (SiO₂) (COD 9001578); m: magnetite (Fe₂O₃) (COD 9006247); b: bytownite (Ca,Na)(Si,Al)₄O₈) (COD 9011200); i: ilmenite (FeTiO₃) (COD 9000910); py: pyroxene (Fe_0.44Mg_0.56SiO₃) (COD 9001577); A: alite (3CaO·SiO₂) (COD 1540705)); B: belite (2CaO·SiO₂) (COD 9012793)); C₃A: tricalcium aluminate (3CaO·Al₂O₃) (COD 9014359)); C₄AF: ferritic phase (4CaO·Al₂O₃·Fe₂O₃) (COD 9015955); g: anhydrite (CaSO₄) (COD 5000040); c: calcite (CaCO₃) (COD 9016022).

Figure 4

Compressive strengths (MPa) for all pastes: CEM, BC, HAC and AC cements.

Figure 5

XRD patterns of (a) CEM, (b) BC-3, (c) HAC-3, (d) BC-7, (e) HAC-7, (f) AC after 2 and 28 days of curing. Legend: d: diopside (MgCaSi₂O₆); a: augite (Ca,Mg,Fe)₂(Si,Al)₂O₆); q: cristobalite (SiO₂); b: bytownite ((Ca,Na)(Si,Al)₄O₈); i: ilmenite (FeTiO₃); A: alite (3CaO·SiO₂); B: belite (2CaO·SiO₂); C₃A:tricalcium aluminate; e:ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂(H₂O)₂₆) (COD 9015084); p: portlandite (Ca(OH)₂) (COD 1008780); c: calcite (CaCO₃); AFm: ((3CaO·Al₂O₃·CaSO₄·12H₂O) (COD 9013423), bs: basanite (Ca₃H₃.₆O₁₃.₈S₃) (COD 9012211), ms: magnesium silicate (MgSiO₃) (COD 9016052).

Figure 6

FTIR patterns of CEM, BC-3, HAC-3, BC-7, HAC-7 and AC after 2 and 28 days of curing.

Figure 7

Thermogravimetric analysis of (a) CEM, (b) BC-3, (c) HAC-3, (d) BC-7, (e) HAC-7 and (f) AC after 2 days and 28 days of curing.

Figure 8

BSEM micrography of (a) AC after 2 days of curing (×500), (b) AC after 28 days of curing (×500), (c) partially attacked VFA particle in AC 28d (×3000).

Figure 9

BSEM micrography of (a) BC-3 after 2 days of curing (×500), (b) BC-3 after 28 days of curing (×500), (c) HAC-3 after 2 days of curing, (d) HAC-3 after 28 days of curing (×500).

Figure 10

BSEM micrography of (a) BC-7 after 2 days of curing (× 500), (b) BC-7 after 28 days of curing (× 500), (c) HAC-7 after 2 days of curing, (d) HAC-7 after 28 days of curing (× 500).

Figure 11

Ternary diagram CaO-SiO₂-Al₂O₃ of elemental EDX analysis of the gel phase.

Figure 12

(a) Heat Flow (J/g.h) (b) Total heat (J/g) for the different cementitious systems (CEM; BC and HAC) (where g represents the grams of binder (VFS+ CEM + Activator).