# From Waste to Binder: Alkali Activation of Blended Brick and Metakaolin Residues for Design of Circular Construction Materials

## TL;DR

This study shows how waste brick powder and metakaolin residue can be combined to create a sustainable, low-carbon construction binder with good strength and environmental benefits.

## Contribution

The novelty is using complementary waste materials to overcome single-precursor limitations and create a circular construction binder.

## Key findings

- Blending 30–50% metakaolin residue with brick powder improves setting times and strength development.
- Blended pastes achieved compressive strengths up to 39 MPa and flexural strengths of 8 MPa at 90 days.
- The carbon footprint of the blends was 392–408 kg CO2e/m³, a 60% improvement over Portland cement.

## Abstract

Alkali-activated materials (AAMs) offer a promising low-carbon alternative to Portland cement, but their development has been dominated by fly ash and slag, whose availability is increasingly limited. This research explores waste brick powder (WBP) and metakaolin residue (RN), two abundant yet underutilized by-products, as blended precursors for sustainable binder design. The novelty lies in demonstrating how complementary chemistry between crystalline-rich WBP and amorphous RN can overcome the drawbacks of single-precursor systems while valorizing construction and industrial residues. Pastes were prepared with varying WBP/RN ratios, activated with alkaline solutions, and characterized by Vicat setting tests, isothermal calorimetry, XRD with Rietveld refinement, MIP, SEM, and mechanical testing. Carbon footprint analysis was performed to evaluate environmental performance. Results show that WBP reacts very rapidly, causing flash setting and limited long-term strength, whereas the incorporation of 30–50% RN extends setting times, sustains dissolution, and increases amorphous gel formation. These changes refine the formed reaction products, leading to compressive strengths up to 39 MPa and flexural strengths of 8 MPa at 90 days. The carbon footprint of all blends remained 392–408 kg CO2e/m3, thus providing about a 60% improvement compared to conventional Portland cement paste. The study establishes clear design rules for waste-derived blended precursors and highlights their potential as circular, low-carbon binders.