An inorganic water resistant binder composition comprising liquefied sodium silicate, which can be used to fabricate lightweight, water resistant hardcore panels, is provided herein. Specifically, the inorganic water resistant binder composition comprises 50.0-94.0 parts by weight of liquefied sodium silicate, which comprises 76.0-80.0 parts by weight of silicon, 19.6-23.0 parts by weight of sodium, 0.15-0.35 parts by weight of potassium, 0.20-0.30 parts by weight of aluminum, and 0.05-0.15 parts by weight of iron; 5.0-30.0 parts by weight of nano silica; and 0.5-10.0 parts by weight of a silane composition, which enhance water resistance of the binder composition. A process is provided for fabrication of a lightweight, highly water resistant hardcore panel by mixing the components of the binder composition described above with inorganic additives, such as an inorganic filler and a strength enhancer.