A method of making a low density additive, the method comprising the steps of; (a) providing particles of a low density inorganic material comprising calcium silicate hydrate having a porous structure by the sequential steps: a. filling a reactor with tap water and warming the water up to 70°C, adding lime, clay and silica to form a slurry including 8wt % solids, the solids comprising 21 wt% calcium hydroxide, 73wt% silica, 6wt% bentonite and stirring at high stirring speed to maintain the solids in a suspended condition. b. heating the slurry to 173°C at a saturated absolute steam pressure of 896.3KN/m 2 (130 psi) for 90 minutes; c. cooling the slurry to ambient temperature and pressure thereby obtaining sample A; and d. decanting the slurry to form a slurry with 15wt% solids; (b) contacting the slurry of the low density inorganic material from step (a) with between 1 to 10%wt based on the dry mass of the low density material of hydrolyzed n-octyltriethoxysilane or potassium methyl siliconate at ambient temperature and pressure, wherein contacting is carried out by gentle agitation at ambient temperature and pressure for 15 minutes and then maintained at ambient conditions without stirring for 24 hours thereby obtaining sample B and (c) contacting the particles from step (b) with an activation agent wherein the activation agent is calcium oxide of 90wt% purity wherein the calcium oxide is added in amounts equivalent to between 2 to 20wt% of the low density calcium silicate hydrates on a dry basis wherein contacting is carried out by gentle stirring for 2.5 hours at ambient temperature and pressure thereby obtaining sample C. (a) providing particles of a low density inorganic material comprising calcium silicate hydrate having a porous structure by the sequential steps: a. filling a reactor with tap water and warming the water up to 70°C, adding lime, clay and silica to form a slurry including 8wt % solids, the solids comprising 21 wt% calcium hydroxide, 73wt% silica, 6wt% bentonite and stirring at high stirring speed to maintain the solids in a suspended condition. b. heating the slurry to 173°C at a saturated absolute steam pressure of 896.3KN/m 2 (130 psi) for 90 minutes; c. cooling the slurry to ambient temperature and pressure thereby obtaining sample A; and d. decanting the slurry to form a slurry with 15wt% solids; a. filling a reactor with tap water and warming the water up to 70°C, adding lime, clay and silica to form a slurry including 8wt % solids, the solids comprising 21 wt% calcium hydroxide, 73wt% silica, 6wt% bentonite and stirring at high stirring speed to maintain the solids in a suspended condition. b. heating the slurry to 173°C at a saturated absolute steam pressure of 896.3KN/m 2 (130 psi) for 90 minutes; c. cooling the slurry to ambient temperature and pressure thereby obtaining sample A; and d. decanting the slurry to form a slurry with 15wt% solids; (b) contacting the slurry of the low density inorganic material from step (a) with between 1 to 10%wt based on the dry mass of the low density material of hydrolyzed n-octyltriethoxysilane or potassium methyl siliconate at ambient temperature and pressure, wherein contacting is carried out by gentle agitation at ambient temperature and pressure for 15 minutes and then maintained at ambient conditions without stirring for 24 hours thereby obtaining sample B and (c) contacting the particles from step (b) with an activation agent wherein the activation agent is calcium oxide of 90wt% purity wherein the calcium oxide is added in amounts equivalent to between 2 to 20wt% of the low density calcium silicate hydrates on a dry basis wherein contacting is carried out by gentle stirring for 2.5 hours at ambient temperature and pressure thereby obtaining sample C.