Effects of activator powders on hybrid binders based on volcanic ash and portalnd clinker based cement

Abstract

Concrete remains the most widely used construction material worldwide. However, the high energy consumption and CO₂ emissions associated with Portland cement production have driven the search for more sustainable alternatives. Volcanic ash (VA), a naturally occurring pozzolan found in abundance in regions such as Tauranga, New Zealand, presents promising potential as a partial replacement for cement.

This thesis investigates the performance of value-added products from the Takitimu North Link (TNL) site, using chemical activation with sodium sulphate (Na₂SO₄) and calcium hydroxide (Ca(OH)₂) in hybrid binder systems. A central composite experimental design was employed to systematically vary the VA content and activator dosages. Mortar mixes with 25%, 50%, 75%, and 100% VA replacement were tested (Series A), with particular focus on the 25% blend. This composition underwent further testing with varying activator dosages to determine the optimal Na₂SO₄/Ca(OH)₂ (NS/CH) ratio (Series B). Compressive strength tests were conducted at 7, 28, and 90 days to assess mechanical performance.

Additionally, microstructural characterization techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) were used to identify hydration products and evaluate binder densification.

The results showed that a 25% replacement of Portland cement with VA, activated with 14% Na₂SO₄ at an NS/CH ratio of 1, delivered optimal compressive strength and microstructural development. While higher replacement levels generally reduced early-age strength due to weaker pozzolanic activity, appropriate activator dosages improved long-term performance significantly.

Overall, the study demonstrates that chemically activated volcanic ash can substantially improve the sustainability profile of concrete by reducing Portland cement usage and its environmental impacts. The use of optimized hybrid binders incorporating volcanic ash offers a viable solution for developing sustainable, resilient, and cost-effective construction materials in New Zealand and other volcanic regions worldwide.