Magnesium cement comprising (a) magnesium compound MV selected from the group consisting of magnesium oxide, magnesium carbonate and magnesium hydroxide; (b) at least one magnesium salt selected from the group consisting of magnesium sulphate, chloride, nitrate, iodide and bromide; (c) silica sand; characterized in that (d) at least one comb polymer is present, having a linear polymer structure and side chains bonded thereto, the linear polymer structure having carboxylic acid groups and/or carboxylate groups, and the comb polymer being a copolymer of the formula where M independently at each occurrence is selected from H +, alkali metal ion, alkaline earth metal ion, di- or trivalent metal ion, ammonium ion or organic ammonium group; R independently at each occurrence is hydrogen or is a methyl group; R 1 independently at each occurrence is - q -R 4; R 2 independently at each occurrence is a C 1 - to C 20 -alkyl group, -cycloalkyl group or -alkylaryl group or is - q -R 4, where A independently at each occurrence is a C 2 - to C 4 -alkylene group; R 4 is a C 1 - to C 20 -alkyl group, -cyclohexyl group or -alkylaryl group, and q represents a value of 2 to 250, more particularly of 8 to 200, more preferably of 11 to 150; R 3 independently at each occurrence is -NR 5 R 6 or -OR 7 NR 8 R 9, where R 5 and R 6 independently of one another are H or a C 1 - to C 20 -alkyl group, -cycloalkyl group or -alkylaryl group or -aryl group or are a hydroxyalkyl group or are an acetoxyethyl (CH 3 -CO-O-CH 2 -CH 2 -), a hydroxyisopropyl (HO-CH(CH 3)-CH 2 -) or an acetoxyisopropyl (CH 3 -CO-O-CH(CH 3 -CH 2 -) group, or where R 5 and R 6 together form a ring of which the nitrogen is a part, so as to construct a morpholine or imidazoline ring, R 7 is an unsubstituted or substituted alkyl radical having 1 to 10 C atoms, R 8 and R 9 each independently are a C 1 - to C 20 -alkyl group, -cycloalkyl group, -alkylaryl group or -aryl group or are a hydroxyalkyl group, the sequential arrangement of the substructure elements s1, s2, s3 and s4 being alternating, blocklike or random, and a, b, c and d represent molar ratios of the structural units s1, s2, s3 and s4, with the ratio a/b/c/d = (0.1-0.9) / (0.1 -0.9) / (0 -0.8) / (0 -0.3), more particularly (0.1-0.9) / (0.1-0.9) / (0 -0.5) / (0 -0.1), preferably (0.1 -0.9) / (0.1 -0.9) / (0 -0.3) / (0 -0.06), with the proviso that a + b + c + d = 1 and the sum of c + d is preferably greater than 0. (a) magnesium compound MV selected from the group consisting of magnesium oxide, magnesium carbonate and magnesium hydroxide; (b) at least one magnesium salt selected from the group consisting of magnesium sulphate, chloride, nitrate, iodide and bromide; (c) silica sand; characterized in that (d) at least one comb polymer is present, having a linear polymer structure and side chains bonded thereto, the linear polymer structure having carboxylic acid groups and/or carboxylate groups, and the comb polymer being a copolymer of the formula where M independently at each occurrence is selected from H +, alkali metal ion, alkaline earth metal ion, di- or trivalent metal ion, ammonium ion or organic ammonium group; R independently at each occurrence is hydrogen or is a methyl group; R 1 independently at each occurrence is - q -R 4; R 2 independently at each occurrence is a C 1 - to C 20 -alkyl group, -cycloalkyl group or -alkylaryl group or is - q -R 4, where A independently at each occurrence is a C 2 - to C 4 -alkylene group; R 4 is a C 1 - to C 20 -alkyl group, -cyclohexyl group or -alkylaryl group, and q represents a value of 2 to 250, more particularly of 8 to 200, more preferably of 11 to 150; R 3 independently at each occurrence is -NR 5 R 6 or -OR 7 NR 8 R 9, where R 5 and R 6 independently of one another are H or a C 1 - to C 20 -alkyl group, -cycloalkyl group or -alkylaryl group or -aryl group or are a hydroxyalkyl group or are an acetoxyethyl (CH 3 -CO-O-CH 2 -CH 2 -), a hydroxyisopropyl (HO-CH(CH 3)-CH 2 -) or an acetoxyisopropyl (CH 3 -CO-O-CH(CH 3 -CH 2 -) group, or where R 5 and R 6 together form a ring of which the nitrogen is a part, so as to construct a morpholine or imidazoline ring, R 7 is an unsubstituted or substituted alkyl radical having 1 to 10 C atoms, R 8 and R 9 each independently are a C 1 - to C 20 -alkyl group, -cycloalkyl group, -alkylaryl group or -aryl group or are a hydroxyalkyl group, the sequential arrangement of the substructure elements s1, s2, s3 and s4 being alternating, blocklike or random, and a, b, c and d represent molar ratios of the structural units s1, s2, s3 and s4, with the ratio a/b/c/d = (0.1-0.9) / (0.1 -0.9) / (0 -0.8) / (0 -0.3), more particularly (0.1-0.9) / (0.1-0.9) / (0 -0.5) / (0 -0.1), preferably (0.1 -0.9) / (0.1 -0.9) / (0 -0.3) / (0 -0.06), with the proviso that a + b + c + d = 1 and the sum of c + d is preferably greater than 0.