Reducing cement consumption in mortars by waste-derived hydrochars

Waste-derived hydrochars are presented for the first time as promising materials to reduce the consumption of natural resources and the carbon footprint of the cement industry, while eliminating waste and sequestering a high amount of carbon in civil infrastructures. Rice husk (RH) and stabilized organic waste from a mixed municipal waste mechanical-biological treatment plant (SOW) were subjected to hydrothermal carbonization at 200 °C for 2 h and the resulting hydrochars were thoroughly evaluated as cement substitutes in fresh and hardened mortars. Compared to the control, mortars with 1.25–5 wt% of cement replaced by hydrochar from stabilized organic fraction caused a decrease in compressive strength of about 50–60% at 28 days of curing, while flexural strength was diminished by about 38–47%. The use of rice husk-derived hydrochar led to a reduction of 32–47% in compressive strength and of 22–34% in flexural strength. With compressive and flexural strengths of 27–41 and 3.31–4.92 MPa, respectively, blended mortars (28 days) display good prospects for use in plastering, rendering, masonry, partition panels and low-load paving. On the other hand, substituting 5 wt% of cement by hydrochar decreases the thermal conductivity and increases electrical resistivity of the mortar by 25–30%, which enhances thermal insulation properties and potential durability. This approach opens a new avenue for large-scale application of biowaste hydrochars as secondary raw materials for sustainable construction.