Foundry waste reutilization: Anti-shrinkage geopolymer based on nano-clay and coal gangue

Abstract

The global challenge of utilizing foundry waste is particularly complex and severe in developing countries. To address this issue, a novel geopolymer was formulated using coal gangue (CG) powder, slag (SL), and nano lithium montmorillonite (LM) as raw materials and expansion agents. The aim was to mitigate the drying shrinkage of alkali-activated geopolymer and enhance the utilization of CG (foundry waste). This study employed the response surface methodology to design anti-shrinkage geopolymers and assess their shrinkage ratio and mechanical properties. The impact of CG on mechanical properties and the role of nanoclay in geopolymer formation and shrinkage mitigation were investigated. Results indicated that the gelable properties of magnesium-silicate in LM influenced geopolymerization in the SL/CG powder-based alkali system. The release of Mg2+ from LM and Al from raw materials (SL and CG) led to the formation of a hydrotalcite-like phase, which filled the pores in the geopolymetric mortars. The addition of 2.0 wt% LM reduced geopolymer mortar shrinkage by 64.74%, while near-zero shrinkage was achieved at 3.0–4.0 wt% LM dosage. Moreover, the new geopolymer exhibited excellent flexural strength (FS) of 7.50 MPa and compressive strength (CS) of 31.65 MPa. Considering both mechanical and shrinkage properties, the optimal mix ratio comprised 2.48 wt% LM content, 29.60 wt% CG content, and 140 mesh CG. Additionally, digital image correlation (DIC) technology was employed to monitor the fracturing process of geopolymer mortars, and a novel method for assessing the mechanical characteristics of such brittle materials based on strain data cloud from DIC technology was established.