For preparing surface-reacted calcium carbonate and its use

A process for preparing a surface-reacted calcium carbonate in an aqueous environment, the process comprising the following steps:

• a) providing a calcium carbonate;
• b) providing 5 wt.-% to 50 wt.-%, based on the weight of calcium carbonate, of at least one acid having a pKa of less than or equal to 2.5, wherein the corresponding acid anion is capable of forming water-insoluble calcium salts;
• c) providing 1 wt.-% to 20 wt.-%, based on the weight of calcium carbonate, of at least one water soluble non-polymeric organic and/or inorganic weak acid and/or a hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid, wherein the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid is selected from the group consisting of boric acid, citric acid, sodium dihydrogen citrate, potassium dihydrogen citrate, disodium hydrogen citrate, dipotassium hydrogen citrate, sodium dihydrogen borate, potassium dihydrogen borate, disodium hydrogen borate, dipotassium hydrogen borate, and any mixture thereof;
• and
• d) contacting the calcium carbonate of step a) with (i) the at least one acid having a pKa of less than or equal to 2.5 of step b), and (ii) the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid of step c), in an aqueous environment, in the presence of gaseous CO2, which is formed in situ or provided from an external source, to obtain a surface-reacted calcium carbonate having a BET specific surface area of at least 30 m2/g.