Decoding thermal performance through microscopy: multi-scale insights into perlite-enhanced sustainable concrete

• A cement-free, polymer-bound perlite composite demonstrated the feasibility of ultra-low-carbon insulation materials for non-structural building applications. • Porosity ranged from 28 % (S3) to 52 % (S2), with micropores (<2 µm), mesopores (2–50 µm), and macropores (>50 µm) identified. • Interfacial transition zone (ITZ) widths in cement-based samples were 10–30 µm (S3: 10–15 µm, S2: 25–30 µm), with gypsum forming ettringite and polymer creating a dual-layer ITZ. • Cracking depended on mix design, S2 showed shrinkage cracking at interfaces, S3 showed minimal cracking, and S6 remained crack-free (with occasional delamination). • Strength increased with 28-day density, S2 had 536 kg/m³ and 1.90 MPa, while S3 reached 917 kg/m³ and 7.41 MPa, intermediate samples followed the same trend. The concrete production process accounts for 8% of all global carbon emissions worldwide. This urge demands the development of construction alternatives away from cementious material. This experimental study develops six perlite-enhanced concrete admixtures and explores them using multi-scale scanning electron microscopy (100×–10,000×) combined with energy-dispersive spectroscopy to establish quantitative relationships between microstructure and thermal insulation potential. Mixtures included variable quantities of water, cement, perlite, and additives, including gypsum and polymer, resulting in water-cement ratios ranging from 0.57 to 1.88. Elemental analysis shows calcium content ranging from 0.45 mass% in cement-free Sample 6 to 26.71 mass% in Sample 2, while silicon varied from 0.07 to 14.97 mass%. Ca/Si ratios ranged from 1.19 to 3.08, indicating varying extents of pozzolanic reaction. Sample 6 contained 21.15 mass% carbon from the glue binder and 21.79% aluminum content. Porosity ranged from 28% in Sample 3 to 52% in Sample 2, correlating with water content and thermal conductivity estimates of 0.15–0.45 W/m·K. Interfacial transition zones measured 10–30 m in width, with narrower zones in low water-cement ratio samples indicating better particle-matrix integration. Gypsum-containing samples exhibited sulfur contents of 4.46 and 0.91 mass%, resulting in the formation of ettringite crystals that bridge the interfaces. Multi-scale imaging captured three pore populations: micropores (<2 μm), mesopores (2–50 μm), and macropores (>50 μm), each contributing differently to thermal transport. The cement-free formulation proved feasible for ultra-low-carbon applications, whereas cement-based samples reduced cement use by 25% with perlite. These microstructure-property insights help optimize perlite concrete for sustainable construction by balancing thermal insulation and structural performance through porosity and interfacial control.