In this study, cryptocrystalline magnesite was converted into MgO nanosheets using thermal activation and ball milling. The microstructural properties of raw and thermally activated cryptocrystalline magnesite were assessed using advanced and state-of-art analytical equipments such as high-resolution scanning electron microscopy equipped with electron dispersion spectroscopy (HR-SEM-EDS), transmission electron microscopy coupled with EDS (TEM-EDS), the Brunauer–Emmett–Teller (BET), and X-ray Diffraction (XRD). Raw cryptocrystalline magnesite was observed to contain irregular particles lumped together but they were observed to change to nanosheets post calcination at 1000 ℃. The product nanosheets were comprised of octagonal structure-like with varying sizes. The surface area was observed to reduce from 19 m2.g−1 to 13 m2.g−1 for raw magnesite and activated magnesite, respectively. The pore size were observed to increase after the activation process hence confirming that this material is mesoporous. High pore volume may be attributed to thermal decomposition of the magnesium carbonate post emission of CO2. Furthermore, the chemical composition of raw and activated material were observed to change hence proving that the new material has been synthesized. Chemically, raw and treated materials were predominated by Mg, C and O, amongst the traces of Ca and Si. The product material was observed to have high neutralisation capacity by increasing the pH from 7.3 to 10.36 for 0 g and 0.1 g, respectively. Ultimately, the pH was observed to be ≥ 11 hence denoting the alkalinity potential of the material.