Approach to hydrochar as a partial cement substitute in mortars

Hydrochars derived from organic waste have shown promise as sustainable additives for low-carbon cements. While mortars incorporating hydrochar show reduced thermal and electrical conductivity, their low compressive strength limits their use to non-structural applications such as masonry and low-load pavements. To further explore the potential of hydrochar, stabilized organic waste from a mechanical-biological treatment plant was hydrothermally treated at 195 ºC for 3 hours at pilot scale. The resulting hydrochar displayed distinct physicochemical properties compared to wood-based biochar, a widely studied carbon admixture in mortars. Pastes with 2.5 wt% cement (CEM I 52.5 N) replaced by hydrochar or biochar were monitored for their hydration over 1 hour to 28 days. X-Ray diffraction revealed early portlandite formation in control and biochar containing-samples, whereas those with hydrochar showed no such reflections until 1 day, indicating delayed hydration. Fourier-transform infrared spectroscopy confirmed a reduced presence of Ca(OH)₂ in pastes with hydrochar during the first 7 hours. Scanning electron microscopy showed delayed and less developed C-S-H formation compared to the counterparts. Advanced statistical techniques (Principal Component Analysis (PCA) and Partial Least Squares (PLS) regression) provided additional insights. Incomplete pores filling during hydration resulted in a relevant porosity of 0.5-10 μm in the hydrochar-containing mortars, leading to a compressive strength of around 16 MPa, about 70% lower than that of the control after 28 days-curing. Several alternative approaches are proposed to overcome these limitations and unlock the full potential of hydrochars as supplementary cementitious materials. • Hydrochar is unstable in the highly alkaline cementitious medium • Extracted or degraded organic compounds significantly delay early cement hydration • Delayed cement hydration favours large pores, reducing mortar compressive strength • Hydrochar modification is needed to enhance its effectiveness as a cement substitute • PCA and PLS as tools for assessing the efficacy of secondary cementitious materials