Use, as an agent reducing the viscosity of a paste consisting of a thermoplastic or thermosetting resin, and of calcium carbonate, of a calcium carbonate characterised in that said carbonate is dry-ground in the presence of a grinding aid agent, firstly, and in that the grinding aid agent is, secondly, a copolymer consisting: a) of at least one anionic monomer chosen from among acrylic acid, methacrylic acid and their mixtures, b) and of at least one non-ionic monomer, of formula: in which: - m, n, p and q are integers such that: m, n, p ≤ 150, q ≥ 1, and 5 ≤ (m+n+p)q ≤ 150, and preferentially 15 ≤ (m+n+p)q ≤ 120, - R 1 represents hydrogen or the methyl or ethyl radical, - R 2 represents hydrogen or the methyl or ethyl radical, - R represents a radical containing an unsaturated polymerisable function, preferentially belonging to the group of vinylics, as well as to the group of acrylic, methacrylic, maleic esters, as well as to the group of unsaturated urethanes such as acrylurethane, methacrylurethane, α-α' dimethyl-isopropenyl benzylurethane, allylurethane, as well as to the group of allylic or vinylic ethers substituted or not, or again to the group of ethylenically unsaturated amides or imides, or again to the group consisting of acrylamide and methacrylamide, - R' represents hydrogen or a hydrocarbon radical with 1 to 40 carbon atoms, preferentially 1 to 4 carbon atoms, R' being very preferentially the methyl radical, or a mixture of several monomers of formula, c) and eventually of at least one other monomer chosen from among (meth)acrylic anhydride, (meth)acrylamide, or from among the (meth)acrylic esters, such as preferentially the acrylates and methacrylates with 1 to 20 carbon atoms in their ester radical, such as very preferentially the methyl, ethyl, isopropyl, n-propyl, isobutyl, n-butyl, ter-butyl or 2-ethylhexyl acrylates, the methyl or ethyl methacrylates, the hydroxylated methacrylates such as the hydroxyethyl and hydroxypropyl methacrylates, or from among the aromatic vinylic monomers such as preferentially styrene, α-methylstyrene, styrene sulfonate, or acrylamido-2-methyl-2-propane-sulfonic acid, or from among the organophosphate monomers, such as preferentially the acrylate and methacrylate phosphates of ethylene glycol, or the acrylate and methacrylate phosphates of oxyethylene and/or oxypropylene glycol, and their mixtures. a) of at least one anionic monomer chosen from among acrylic acid, methacrylic acid and their mixtures, b) and of at least one non-ionic monomer, of formula: in which: - m, n, p and q are integers such that: m, n, p ≤ 150, q ≥ 1, and 5 ≤ (m+n+p)q ≤ 150, and preferentially 15 ≤ (m+n+p)q ≤ 120, - R 1 represents hydrogen or the methyl or ethyl radical, - R 2 represents hydrogen or the methyl or ethyl radical, - R represents a radical containing an unsaturated polymerisable function, preferentially belonging to the group of vinylics, as well as to the group of acrylic, methacrylic, maleic esters, as well as to the group of unsaturated urethanes such as acrylurethane, methacrylurethane, α-α' dimethyl-isopropenyl benzylurethane, allylurethane, as well as to the group of allylic or vinylic ethers substituted or not, or again to the group of ethylenically unsaturated amides or imides, or again to the group consisting of acrylamide and methacrylamide, - R' represents hydrogen or a hydrocarbon radical with 1 to 40 carbon atoms, preferentially 1 to 4 carbon atoms, R' being very preferentially the methyl radical, or a mixture of several monomers of formula, - m, n, p and q are integers such that: m, n, p ≤ 150, q ≥ 1, and 5 ≤ (m+n+p)q ≤ 150, and preferentially 15 ≤ (m+n+p)q ≤ 120, - R 1 represents hydrogen or the methyl or ethyl radical, - R 2 represents hydrogen or the methyl or ethyl radical, - R represents a radical containing an unsaturated polymerisable function, preferentially belonging to the group of vinylics, as well as to the group of acrylic, methacrylic, maleic esters, as well as to the group of unsaturated urethanes such as acrylurethane, methacrylurethane, α-α' dimethyl-isopropenyl benzylurethane, allylurethane, as well as to the group of allylic or vinylic ethers substituted or not, or again to the group of ethylenically unsaturated amides or imides, or again to the group consisting of acrylamide and methacrylamide, - R' represents hydrogen or a hydrocarbon radical with 1 to 40 carbon atoms, preferentially 1 to 4 carbon atoms, R' being very preferentially the methyl radical, c) and eventually of at least one other monomer chosen from among (meth)acrylic anhydride, (meth)acrylamide, or from among the (meth)acrylic esters, such as preferentially the acrylates and methacrylates with 1 to 20 carbon atoms in their ester radical, such as very preferentially the methyl, ethyl, isopropyl, n-propyl, isobutyl, n-butyl, ter-butyl or 2-ethylhexyl acrylates, the methyl or ethyl methacrylates, the hydroxylated methacrylates such as the hydroxyethyl and hydroxypropyl methacrylates, or from among the aromatic vinylic monomers such as preferentially styrene, α-methylstyrene, styrene sulfonate, or acrylamido-2-methyl-2-propane-sulfonic acid, or from among the organophosphate monomers, such as preferentially the acrylate and methacrylate phosphates of ethylene glycol, or the acrylate and methacrylate phosphates of oxyethylene and/or oxypropylene glycol, and their mixtures.