Thermal conductivity of aluminum sulfate-treated sewage sludge ash cement composites

Sustainable management of sewage sludge ash (SSA) requires high-value reuse pathways that simultaneously reduce waste disposal burdens and improve the thermal performance of cementitious materials. This study evaluates the thermal conductivity, pore structure, and mechanical performance of non-foamed cement composites incorporating aluminum-sulfate-treated SSA. SSA was used as a cement replacement at 40, 70, and 80 wt% to develop low-thermal-conductivity cementitious composites without chemical foaming agents. Chemical composition, microstructure, micro-area elemental distribution, bonding characteristics, and pore structure were characterized using X-ray fluorescence (XRF), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), Fourier transform infrared spectroscopy (FTIR), and mercury intrusion porosimetry (MIP). Thermal conductivity was measured using the transient plane source method and compared with series and Maxwell–Eucken porous-media model predictions, while compressive strength was determined after 28 days. Compared with the pure cement control, SSA incorporation increased porosity from 37.6% to 52.8–60.6%, increased total pore volume from 0.247 mL g⁻¹ to 0.448–0.654 mL g⁻¹, and reduced bulk density from 1.525 g cm⁻³ to 0.926–1.179 g cm⁻³. The measured thermal conductivity decreased from 0.189 W m⁻¹ K⁻¹ for the control to 0.098, 0.105, and 0.089 W m⁻¹ K⁻¹ for the 40, 70, and 80 wt% SSA composites, respectively, corresponding to a maximum reduction of 52.9%. The predicted conductivities agreed with TPS measurements within 2.0–3.4%. Compressive strength decreased from 5.41 MPa for the control to 1.37, 2.28, and 0.26 MPa for the 40, 70, and 80 wt% SSA composites, respectively. Among the investigated formulations, the 30:70 cement:SSA mixture provided the most balanced performance, with a thermal conductivity of 0.105 W m⁻¹ K⁻¹ and a compressive strength of 2.28 MPa. These quantitative results demonstrate the feasibility of using aluminum-sulfate-treated SSA as a direct component of lightweight insulating cement composites without chemical foaming agents.