Using waste products is a promising and sustainable alternative to conventional non-renewable peat and nutrient-rich renewable materials. Their products are gaining importance for closing the loop in the ornamental plant industry. Porous inorganic materials have recently become potential bulking agents (BA) for sewage sludge (SS) owing to their high porosity, surface area, passivation capacity, high recycling potential, and potting soil components. The main purpose of the present work was to investigate the feasibility of pumice (PU) and expanded perlite (EP) on composting parameters, nutrient bioavailability and suitability of SS to serve as an ornamental substrate. The addition of PU and EP at 50% (v/v) resulted in higher porosity, optimal initial moisture content, higher temperature rise (56.24 ± 0.13 °C, 56.21 ± 0.11 °C, respectively), and higher CO₂ evolution (39.41 ± 0.17%, 41.70 ± 0.22% daily peaks). Composting with inorganic BA at EP-50 and PU-50 mixtures was beneficial owing to high nitrogen content (3.82 and 3.70%, respectively) and readily bioavailable nutrients (270 mg kg⁻¹ phosphates and 1835 mg kg⁻¹ potassium). The use of PU and EP was found helpful in improving the slow-release nutrient properties of the compost. The overall results indicated that composting SS with PU and EP is a viable approach to achieve good composting properties and a good nutrient-providing profile if the compost is used as a component for potting soil components or garden soil amendments.