# Valorization of Volcanic Ash and Stainless Steel Slag as Partial Replacements of Metakaolin in Geopolymer Binders

## TL;DR

Volcanic ash improves geopolymer strength more than stainless steel slag, offering a sustainable construction material.

## Contribution

Demonstrates volcanic ash as a superior partial metakaolin replacement in geopolymer binders compared to stainless steel slag.

## Key findings

- 50MK–50VA mix achieved 56.66 MPa compressive strength after 28 days.
- Volcanic ash outperformed stainless steel slag in mechanical and thermal properties.
- Geopolymerization was confirmed via XRD, MAS NMR, and FTIR analysis.

## Abstract

What are the main findings?
Volcanic ash showed higher reactivity than stainless steel slag in geopolymer systems.50MK–50VA achieved compressive strengths above 56 MPa after 28 days.

Volcanic ash showed higher reactivity than stainless steel slag in geopolymer systems.

50MK–50VA achieved compressive strengths above 56 MPa after 28 days.

What are the implications of the main findings?
Metakaolin (MK) was partially replaced by volcanic ash (VA) or stainless steel slag (SSS) in geopolymers.

Metakaolin (MK) was partially replaced by volcanic ash (VA) or stainless steel slag (SSS) in geopolymers.

The high environmental impact associated with ordinary Portland cement production has driven increasing interest in alternative low-carbon binder systems based on alkali-activated materials. In this context, geopolymers synthesized from metakaolin and supplemented with natural or industrial by-products represent a promising route toward more sustainable construction materials. In this study, the partial substitution of metakaolin (MK) with stainless steel slag (SSS, calcium rich) or volcanic ash (VA, silica-rich) in alkali-activated cements (AACs) synthesis was investigated by analyzing their physical, mechanical, and thermal properties. The structural evolution associated with alkali activation was assessed using X-ray diffraction (XRD) and 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR). Fourier transform infrared spectroscopy (FTIR) revealed a shift in the main Si–O–T (T = Si, Al) asymmetric stretching band toward lower wavenumbers (≈1000 cm−1), indicating changes in the aluminosilicate network consistent with geopolymer formation. Scanning electron microscopy (SEM) was used to examine the microstructural features of the hardened matrices. The results showed that samples containing 50 wt.% MK and 50 wt.% VA achieved the highest mechanical performance, with compressive and flexural strengths of 46.29 MPa and 16.2 MPa at 7 days, increasing to 56.66 MPa and 17.58 MPa at 28 days of curing, respectively. In contrast, the samples containing 50 wt.% MK and 50 wt.% SSS displayed lower strength development, reaching compressive and flexural strengths of 27.7 MPa and 9.6 MPa at 7 days and 41.01 MPa and 13.68 MPa at 28 days. Additionally, thermal conductivity decreased with increasing porosity and decreasing bulk density, highlighting the potential of these AACs as structurally efficient materials with improved thermal insulation performance.