Comparing influence of inert biochar and silica rich biochar on cement mortar – Hydration kinetics and durability under chloride and sulfate environment

Abstract

This study investigated influence of biochar, prepared from rice husk and waste wood, on hydration kinetics and resistance to chloride and sulfate attack of cementitious composite. The resistance in terms of strength retention, mass change, expansion and micro-structural changes was studied and compared to similar properties of mortar under normal wet curing. Findings suggest that rice husk biochar (RHB) contain 15–16% amorphous silica, although its carbon content (42%) and specific surface area are significantly lower than wood biochar (79% carbon and surface area of 227 m2/g). Both types of biochar at 1 wt% dosage increase rate of hydration at early stage than control, attributed to high surface area, finer size than cement and nucleation of cement hydrates on biochar surface. Both RHB and MWBC at 1–2 wt% significantly reduce water permeability than control, which is also reflected in 15–18% improvement in strength at 7-day, 42-day and 120-day age of cement mortar. Lower permeability due to addition of both biochar types reduces rate of mass gain due to sodium chloride absorption, contributing to 9% higher strength than control after 120-day exposure period. Based on strength, water-cement ratio and cement content, the tested mortars are suitable for exposure class of XS2 (EN 206), where the concrete is always submerged in water. When exposed to sodium sulfate solution, 1 wt% of RHB and 1 wt% of MWBC are found to improve strength by 14–17 % than control after 120-day, while minimizing loss in strength due to sulfate attack. Micro-structural analysis shows pore blocking action of biochar mitigates micro-structural damage due to sulfate attack. This offer higher dimensional stability, reflected from 62 to 68% less expansion of MWBC-mortar and RHB-mortar compared to control. These findings suggest suitability of biochar-mortar application under moderate sulfate environment.