Structure and Hydration Kinetics of Silica Particles in Rice Husk Ash Studied by ²⁹ Si High‐Resolution Nuclear Magnetic Resonance

The structure, surface fractal dimension, and global hydration kinetics of silica particles obtained from rice husk ashes (RHAs) were studied with 29 Si‐NMR spectroscopy and spin–lattice relaxation techniques. Silica particles presented an amorphous fraction higher than 93%, with traces of silica‐organic bonding and crystal‐like domains. Fe‐impurities are located preferentially on the surface of the particles. From the effect of these paramagnetic ions on the spin–lattice relaxation of Q 3 and Q 4 silicate groups, the surface of the particles was characterized as being effectively two‐dimensional ( D =1.9±0.1). The hydration kinetics of the particles during the reaction with lime and water was monitored from 8 to 706 days. The process can be described by a power law, with the characteristic exponent higher than those measured for other cements. Also, Johnson–Mehl–Avrami expressions reproduce equally well the experimental data, with parameters compatible with growth habits and morphology known for C–S–H. Two types of Q 2 tetrahedra were identified in C–S–H, which can be attributed to the bridging and nonbridging silicate groups predicted by the “dreier‐kette” structural model of C–S–H.