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In the precast industry very early age strength development is crucial for productivity, and it is for this reason that industry has been reluctant to embrace the use of ground granulated blast-furnace slag (GGBS) as a supplementary cementitious material, despite its lower carbon dioxide (CO2) emissions. This paper explores the replacement of different base cements (CEM II A-L and CEM I (42·5 R)) with slags of different fineness in mixes containing a novel accelerating admixture at different curing temperatures (20, 35, 45 and 55°C for 16 h); results are also compared with cement–slag mortars containing lime (Ca(OH)2; 6%) and calcium sulfate (CaSO4; 3%) for activating the early age strength. These studies are conducted for a 30% replacement of cement with slag, to establish the benefits offered for very early age strength attainment in the precast concrete industry.

The precast industry, due to its specific early-age strength requirements for hollowcore (HC) slab products, has been reluctant to introduce alternative cementitious materials such as ground granulated blast-furnace slag (GGBS) due to the slower strength development of such materials. This paper reports on a study undertaken at a precast production facility to examine how to overcome the consequences of strength loss due to the replacement of up to 50% of CEM I (42·5R), as usually used in cold weather conditions, with GGBS, for environmental or durability reasons. The GGBS concrete slabs were activated using thermal, chemical and mechanical means to enhance their early-age strength gain. The results were compared at early ages in terms of the temperature profile during curing, cube compressive strengths, load-carrying capacities for transportability requirements, ultimate load and short-term creep. In addition, 28 d densities and strengths were compared for in situ cubes and cores extracted from HC slab samples. The experimental results showed a favourable potential for replacing CEM I (42·5R) cement with up to 50% GGBS in winter without compromising on factory productivity.