To provide modified calcium carbonate that does not have demerits such as non-non-uniformity of textures caused by application of minus electric charge, plastic wire abrasion, or the like modifies surface electric charge of calcium carbonate to the vicinity of neutrality and remarkably improves a yield in comparison with a case in which an anionic dispersant is used when calcium carbonate is used as a papermaking internal additive.

The modified calcium carbonate slurry for papermaking internal addition comprises an aqueous slurry that is composed of 100 pts.mass of precipitated calcium carbonate and 0.01-10 pts.mass of a polymer having general formula (wherein R1-R4are each independently hydrogen atom or methyl; Y is methylene or carbonyl; X is methylene or phenoxymethylene; M is alkali metal atom; a and b are each an integer of 0 or not less than 1 but not 0 at the same time; c is an integer of 0 or not less than 1; d is an integer of not less than 1; and m is a polymerization degree required for making a mass-average molecular weight of 3,000-1,000,000) and has a solid content concentration of 20-70 mass%.

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