It begins with nucleation (fast process) followed by a slow process of particle growth.
In a typical process, silicon alkoxide viz., tetraethyl orthosilicate (TEOS) is hydrolyzed by water in presence of an acidic catalyst.
These reactive silanol (Si–OH) groups of the hydrolyzed silicon alkoxide go through condensation reactions which depend on the pH and temperature of the solution, to form siloxane (Si–O–Si) bonds, producing a 3D porous gel structure.