A set control composition for cementitious systems comprises a retarder (a) selected from (a-1) polymeric polycarboxylic acids selected from homopolymers and copolymers of α,β-ethylenically unsaturated carboxylic acids; and copolymers of at least one α,β-ethylenically unsaturated carboxylic acid and at least one sulfo group containing monomer; and salts thereof, whose milliequivalent number of carboxyl groups is 3.0 meq/g or higher, preferably 3.0 to 17.0 meq/g, and having a molecular weight 25,000 g/mol or less, assuming all the carboxyl groups to be in unneutralized form, (a-2) phosphonic acids and salts thereof, (a-3) low molecular weight polycarboxylic acids and salts thereof, and mixtures thereof, (b) at least one of (b-1) a borate source and (b-2) a carbonate source, wherein the carbonate source is selected from inorganic carbonates having an aqueous solubility of 0.1 g·L−1 or more at 25° C., and organic carbonates, in a weight ratio of b) to a) in the range of 0.1 to 10, (c) a polyol having at least 3 alcoholic hydroxyl groups in its molecule, in a weight ratio of c) to a) in the range of 0.2 to 4, and (d) a dispersant. The set control composition effectively improves workability of cementitious systems for prolonged periods of time without compromising early compressive strength. The compositions show sufficient open time, i.e., the time until initial setting, good workability during said open time as characterized, for example by adequate slump flow over time, and fast setting. The invention further relates to a construction composition comprising i) a cementitious binder comprising one or more calcium silicate mineral phases and one or more calcium aluminate mineral phases, ii) optionally, an extraneous aluminate source, iii) a sulfate source, and iv) the set control composition. The construction composition contains 0.05 to 0.2 mol of total available aluminate, calculated as Al(OH)4−, from the calcium aluminate mineral phases plus the optional extraneous aluminate source, per 100 g of cementitious binder i), and the molar ratio of total available aluminate to sulfate is 0.4 to 2.0.