The at least one second anionic surfactant is thus not in the form of a magnesium salt.The at least one second anionic surfactant may be chosen, for example, from salts, including salts of alkali metals and alkaline earth metals other than magnesium, such as sodium salts, ammonium salts, amine salts, and aminoalcohol salts of the following compounds: alkyl sulphates, alkyl ether sulphates, alkylam