Thermal and Mechanical Performance of Geopolymer Mortar with Thermally Activated Walnut Shell Ash

Geopolymer mortars for thermal energy storage (TES) require an appropriate combination of thermal and mechanical properties. Conventional phase change materials (PCMs) like paraffin cause significant strength loss and excessive thermal conductivity reduction under thermal cycling, failing to achieve this balance. Accordingly, the present study aims to introduce a practical method to activate waste walnut shell powder as a sustainable additive. This additive is intended to replace paraffin for improving thermal conductivity while minimizing strength reduction. Raw walnut shell powder (W), calcined walnut shell ash (WC), and paraffin (P) were used as partial replacements for ground granulated blast furnace slag (GGBFS) at replacement ratios of 1%, 3%, and 5% by weight in the geopolymer compositions . The study assessed mortar workability, compressive strength, direct tensile strength, flexural strength, specific heat capacity, and thermal conductivity. Compared with the reference geopolymer, WC mixtures (1–5%) showed a 4.8–16.1% reduction in compressive strength at 28 days, whereas P mixtures (1–5%) exhibited a 18.8–46.7% reduction. The specific heat of WC mixtures ranged from 1.04 to 1.21 J/g°C, comparable to P (1.05–1.24 J/g°C) and higher than the reference (0.96 J/g°C). Thermal conductivity decreased by 4.3–16.5% for WC versus 6.3–25.5% for P. Among all mixtures, 1WC was identified as the optimal formulation, offering the best balance between mechanical strength and thermal performance. The results suggest that walnut shell powder can serve as a promising sustainable alternative to paraffin in creating geopolymer mortar for TES applications while preserving satisfactory mechanical properties.