The mass concrete with strict temperature control requirement (e.g. mega basement or sub-structural works, tunnels etc); Corrosion-resisting concrete structures which are exposed to rigorous conditions (e.g. marine structures, sewerage treatment plant, concrete structures in acidic or alkaline area etc); Concrete structures in the areas with potential Alkali-aggregate reaction.
The replacement ratio of GGBS has significant impact on the strength development of concrete; hence, the user should determine the best range of replacement ratio through experiment, in accordance to the design and construction requirements.
Under the curing conditions, High Slag Cement Concrete with 30-50% replacement ratio would achieve 50 – 60% compressive strength of plain concrete in 3 days, 70 – 80% achieved in 7 days, and its compressive strength is equivalent to plain concrete’s in 28 days and continues to grow since then.
This is the unique characteristic of High Slag Cement Concrete, and it is a normal phenomena. That means the concrete gets very good curing in its early days, and this is good for its strength development. Usually, the blue colour will automatically fade off and become normal colour after being exposed to the air for 3 – 5 days. The reason blue/green colour appears is, minimal amount of FeS and MnS are formed during the GGBS hydration process, the hydrate appears in blue colour. With the exposure to the air for a period of time, the above chemical composition will then change to FeSO4 and MnSO4, and the blue colour will then disappear.
Hydrothermal curing is particularly good for High Slag Cement Concrete. Steam curing with slightly higher temperature as compared to OPC cement can be adopted in order to increase the strength of the High Slag Cement Concrete precast structures. However, high Slag Cement Concrete has slower early-age strength development than OPC concrete and it is sensitive to the influence of temperature, the user has to appropriately adjust or delay the time for concrete pre-curing (especially for the production during winter season). This is to ensure that the concrete structures possess sufficient preliminary strength before putting into the autoclave, such that the best steam curing effect is attainable.
High Slag Cement Concrete with higher than average replacement ratio (e.g. 30 – 40% and above) is particularly sensitive with the early curing conditions, and therefore strict requirements should be imposed, such as:
Early curing should be carried out before the concrete’s final solidification (e.g. spray the curing compounds, use wet burlap or cotton mats etc);
Moisture-retaining curing for at least 7 days;
When the replacement ratio is above 50%, moisture-retaining curing period should be extended to at least 14 days.
As compared to the plain concrete, High Slag Cement Concrete has the following basic characteristics:
It prolongs the solidifying time, prolongs slump retention, which is particularly beneficial for construction work during summer; The binder has higher cohesiveness which does not segregate easily, its pump ability is good; It reduces the concrete bleeding, and beneficial in reducing the sedimentation craze.
Due to the fact that the fineness of GGBS is far higher than OPC (e.g. GGBS: 420-450m2/kg, OPC: 330-350m2/kg), as compared to the plain concrete, the High Slag Cement Concrete requires a water content increase of 5%;When the user design the mixture of the concrete, he has to fulfill the workability and strength requirement of the concrete by adjusting the amount of additives used, and should not increase the amount of water used as he wishes;
Similar to the normal concrete, the mixture design of the High Slag Cement Concrete should go through the following steps by the relevant technical staffs: The preliminary concrete mix design Trial mixing in the laboratory
Trial mixing on the site
- All types of residential, commercial and industrial structures.
- All civil structural and non structural works.
- Water retaining and mass concrete works.
- Ideal for use in marine and industrial structures.
- Effluent Treatment Plants and Sewage Treatment Plants.