Microstructure and mechanical properties of lignocellulosic magnesium potassium phosphate cement composites modified with activated biochar

Magnesium potassium phosphate cement (MKPC) is a rapid-hardening binder suitable for ambient-temperature fabrication of lignocellulosic composites. This study investigates potassium hydroxide (KOH)-activated wood biochar as a matrix-scale modifier in MKPC composites reinforced with wood particles or coir fibres. Wood biochar was activated at 400 °C for 1 or 2 h and used to replace 5 wt.% of the MgO–KH 2 PO 4 binder fraction, while wood particles or coir fibres were incorporated at 10 or 15 wt.% of the dry binder mass. KOH activation increased oxygen-containing surface functionalities and micro-/mesopore development relative to pristine biochar, while milling reduced particle-size differences among the biochar variants. At the binder scale, biochar incorporation modified early-age heat-flow response without substantially changing cumulative heat release. Phase and thermal analyses confirmed struvite-K as the dominant crystalline reaction product, while mercury intrusion porosimetry indicated redistribution of accessible pore-throat sizes toward finer entry radii. Elemental mapping showed increased spatial association between carbon-rich biochar domains and K–P-rich phosphate reaction products in the activated systems. At the composite scale, reinforcement architecture controlled mechanical behaviour, with coir-fibre composites exhibiting higher flexural strength and toughness than wood-particle composites. Activated biochar produced additional matrix-scale improvements in flexural performance, while recovery of internal bond strength was consistent with matrix strengthening rather than direct modification of the reinforcement–matrix interface. Water absorption was governed mainly by the reinforcement type and loading, whereas the change in thickness remained below 1% after 72 h of immersion.