Ground granulated blast slag is a by-product of steel industry. Figure 1 showing the process of producing GGBS. using of GGBS improving concrete properties such as reducing concrete permeability, resisting sulfate attack, increasing concrete strength and reducing the heat of hydration. on the other hand, there is a concern related to the use of GGBS in a freezing environment and the possibility of concrete durability reduction due to use GGBS in a freezing environment.
Silica fume along with GGBS can be used to provide concrete with high strength and durability.
Strength of concrete
The compressive strength of concrete improved by increasing the amount of GGBS in a concrete mix. figure number 2 showing a result of the compression test for several concrete mixes with different amount of GGBS. this test conducted by Vera Mónica Ferreira Correia. we can notice from the figure below the significant improvement of compressive strength with increasing GGBS in the mix at the age of 56 days.
at 7 days we can notice that the mix with no GGBS attains higher compressive strength than other mixes. I mentioned before that GGBS tending to reduce the heat of hydration. therefore, adding more GGBS means reducing the rate of hydration and heat stresses on concrete.
rapid chloride resistance Figure number 3 showing the rapid chloride migration coefficient for several concrete mixes with different GGBS concentration. the test conducted by Vera Mónica Ferreira Correia. the increasing of GGBS lead to increase of concrete resistance against chloride ingress.
We can notice the significant reduction of rapid chloride migration coefficient at 56 days compared to 28days for concrete mixes with GGBS. the main reason is the low rate of hydration for concrete with GGBS. the hydration continued after 28 days. on the other hand, the rate of hydration for ordinary concrete decreased significantly after 28 days. the use of GGBS will produce denser concrete mix. therefore the permeability will be reduced and resistance for chloride increased. ingress of chloride may cause reinforcement steel rust. and this, in turn, will cause concrete spalling and reduce the durability of the structure.
Adding GGBS tends to improve workability of concrete. a test was conducted by (Santosh Kumar Karri1, G.V.Rama Rao2, P.Markandeya Raju3) for two concrete mixes with different proportions. table 1 showing the mix proportions. table 2 and 3 showing compaction factor and slump test results. we can notice clearly the improvement in concrete workability with increasing GGBS content in concrete mix.
Scaling under freezing and thawing freezing and thawing may cause surface scaling. in surface scaling, cementitious material of concrete is removed exposing the aggregates. continuation of this process may result in exposing steel and reducing structure strength and durability. the increase of GGBS leads to increase of surface scaling. mix 4 with 100% GGBS undergoes higher scaling than other mixes. therefore the using high amount of GGBS in a freezing environment may result in a reduction of structural stability.