Bleeding Characteristics of Fresh Concrete with GGBFS
The bleeding can be defining as the property of movement of water in the concrete mix to the top layer of the mix. This is a phenomenon seen in the fresh concrete which is an unsuitable factor. The fact is that all the concrete has the chances of bleeding. The problem arises when the rate of bleeding exceeds the rate of evaporation. After compaction, for a small time period, the bleeding rate will remain uniform. The bleeding is blocked with the formation of hydration products. These products are solids that come together which blocks the pathway for the water to flow as channel resulting in bleeding. Bleeding causes a change in properties of the concrete mix. This is because bleeding causes a change in water content in the mix. Tests showed that 40 to 70% of replacement of ordinary Portland cement with GGBFS lead to increase in the rate of bleeding. A higher replacement with GGBFS leads to increase in the bleeding.Workability Characteristics of Concrete with GGBFS
As given by ACI, the replacement of GGBFS will let us use an adequate amount of coarse aggregates without compromising the workability of the concrete. The thickness of the mix can be reduced by this method. A replacement of cement with GGBFS by an amount of 30 to 50 % showed a greater workability. Up to a limit the value increase, beyond which there is no increase of workability. It concludes that the slag concretes have better workability compared to ordinary Portland cement fresh concretes. The reasons for higher workability is due to the spherical and glass texture of particles surface.
Glass Count of GGBFS
The glass count is defined as the glass or noncrystalline component of the slag. It is potentially reactive in nature hence greater the proportion of glass greater is the rate of reaction. The x-ray diffraction (XRD) is a method used to determine the glass count in the GGBFS mix. It is recommended to have a glass count greater than 67 percent. Stiffening Times The GGBFS is slower in reaction with the water compared with the Portland cement. Hence the GGBFS concrete or cement takes a greater time for stiffening. This time is more when the replacement level is high. The below graph shows the increase in the stiffening time with the replacement as per BS 4550.
Fig. The graph representing the influence of slag content and temperature on the setting time of concrete
Chemical Reaction of GGBFS
The reason that GGBFS react very slowly with the water results in less hydraulic activity that decreases its demand in the application of concrete manufacturing. The GGBFS have a glassy texture that is the reason that locks the particle from reacting with water. The material possesses latent hydraulicity that an activation is required in order make the material reactive. The activators like sulfates or alkalis react with GGBFS which will result in the increase in the pH of the whole mix. When the pH values reach a critical value, the glass structure of the GGBFS is disturbed that will result in activation of the particles. This will hence make GGBFS react with water and bring up the hydration products. In the case where GGBFS is used as a replacement the Portland cement is the one which makes the slag particles activate for hydration. As the Portland cement consist of alkalis and sulfates. The hydration mechanism is a combination of two reactions. A primary reaction which involves the hydration of ordinary Portland cement with water giving calcium silicate hydrate with certain alkalis like Calcium hydroxide, sodium hydroxide and potassium hydroxide as shown in the below equation. The secondary reaction involves the alkalis is reacted with the ground granulated blast furnace slag.
- Temperature: with the increase in temperature the reaction rate increases and vice versa
- The quantity of the components: The increase in the cement component proportion will increase the reaction
- The properties: The chemical composition and the fineness of the components have an influence on the rate of reaction