An acrylic acid polymer having a weight average molecular weight Mw in the range from 3500 to 2000 g/mol and a polydispersity index Mw/Mn≤2.5, wherein 30% to 60% of the acid groups of the acrylic acid polymers have been neutralized with calcium ions, 30% to 70% of the acrylic acid polymers have been neutralized with sodium ions and 0% to 10% of the acid groups of the acrylic acid polymers have not been neutralized.