Engineering Benefits of GGBS in Concrete

The significance of Ground Granulated Blast Furnace Slag (GGBS) lies in its greener way to become a substitute in concrete material. GGBS being a waste material, it needs a proper method for disposal. So incorporating this waste material of iron in concrete can reduce the depletion of conventional concrete components such as cement, fine aggregate, and coarse aggregate. Also the various do not decrease the strength of concrete.

Some engineering benefits of GGBS in concrete are as below:

Durability

Alkalis are consumed with GGBS during the hydration process; reduce the Alkali-Silica Reaction.

GGBS in concrete improves resistance to sulfate attack due to the reduction of Ca(OH)2 and lower the C3A content.

GGBS in concrete reduces the diffusion of chloride ions and hence improves the resistance against corrosion.

The total shrinkage of concrete containing ground granulated blast furnace slag (GGBS) was lower than 100% Portland cement.

Structural stiffness can be enhanced and load-deflection can be minimized by using GGBS in concrete.

GGBS used in concrete reduces the heat of hydration and minimizes thermal cracks.

Permeability and surplus lime liberated during the hydration of OPC is reduced by using GGBS in concrete

GGBS replacement in concrete reduces cement content for the same strength thus reduces the cost of concrete and hence the cost of the project.

IS 456:2000 recommends the addition of more than 50% GGBS where chloride is encountered along with sulfates in place of sulfate resistant cement.

Strength

Concrete made with ground granulated blast furnace slag (GGBS) generally exhibit higher flexural strength for a given level of compressive strength.

Concrete with GGBS has a higher proportion of the strength-enhancing Calcium Silicate Hydrate (C-S-H) gel than the concrete made with Ordinary Portland Cement.

Concrete made with GGBS continues to gain strength and double its 28-day compressive strength over a period of time.

Mechanism of GGBS

Cement + Water = C-S-H + Ca(OH)2

GGBS + Ca(OH)2 = C-S-H

Sustainability

Replacing the Portland cement with GGBS helps in reducing CO2 emission and in conserving non-renewable resources of limestone.

The use of GGBS in concrete recognized by LEED (Leadership in Energy and Environmental Design) and adds points towards its certification.

The high volume of the GGBS concrete system is environment friendly and the concrete so produced also demonstrates the attributes of high-performance concrete (HPC).

Performance of GGBS in Concrete

Fresh Concrete

Water demand: The replacement of cement with GGBS will reduce the unit water content necessary to obtain the same slump. This reduction of water demand will be increased with an increase in GGBS. The water requirement also depends on the fineness of GGBS. This is because of surface configuration.

The particle shape and size of GGBS is different than cement particle. The use of GGBS in concrete will not allow water to loss immediately as the hydration of Slag Cement is slower than the Ordinary Portland Cement.

GGBS allows for a water reduction of 3% to 5% in concrete without any loss in workability. Water should not be added to GGBS concrete after dispatch from the concrete plants as it reduces the strength and durability of concrete.

Placing, Compacting, and Pumping: GGBS makes concrete more fluid, making it easier to place into formwork easier to compact by vibration. GGBS concrete remains workable for longer periods allowing more time for placing and vibrating. Pumping is also easier due to better flow characteristics.

GGBS having a fineness of 6000 cm2/gm and above shows significant effects on concrete workability. GGBS concrete sets slower than the normal concrete. An increase in the percentage of GGBS used in concrete will increase the setting time of concrete.

Setting Time

The initial setting time of concrete is dependent on the concrete’s constituents, curing conditions, and application use. Concrete with up to 30% GGBS will exhibit a similar initial setting as concrete with Portland cement only. At replacement levels of 40% to 50% the initial set is likely to be extended by 1 to 2 hours and for concrete containing more than 50% GGBS setting time may be extended past three hours.

Longer setting times can have the advantages of allowing concrete to be worked for longer periods meaning time delays, including delays in transport, between mixing and using concrete are less critical. They also decrease the risk of cold joints in larger concrete pours.

The initial set of all concretes is extended in cold conditions and the effect of cold temperatures on extending the initial setting time of GGBS concrete is more pronounced. This effect can be 10 to 12 hours depending on conditions, concrete constituents, and the section size of the pour.

Bleeding

Concrete with up to 40% GGBS replacement doesn’t exhibit different bleeding characteristics from that of concrete made with Portland cement. For higher percentages of GGBS, there is a longer period of bleeding due to the increase in setting time of these mixes. Concrete should be allowed to bleed fully before its finishing. Early finishing can lead to the remixing of the surface layer of the concrete which can reduce surface integrity and lead to dusting and delamination.

Power Floating

Concrete produced with GGBS cement can be power floated in the same way as Portland cement only concrete. GGBS concrete stays in plastic for a longer time than non-GGBS concrete enabling the contractor to achieve a very good quality finish. Experience in Ireland has shown that if using greater than 50% ground granulated blast furnace slag (GGBS), particularly in cold weather, it may be necessary to change the finishing regime.

Concrete could be placed late in the afternoon and power floated first thing the following morning.

Curing      

Good curing practice is essential for all concrete. Horizontal surfaces in particular are susceptible to poor curing and problems such as plastic shrinkage cracking due to exposure to direct sunlight and strong drying winds. Properly cured ground granulated blast furnace slag (GGBS) concrete is more durable and ultimately stronger than concrete made with Portland cement only. Water in GGBS concrete takes slightly longer to combine chemically to form hydration products making GGBS concrete a little more sensitive to poor curing. To get the full benefit of GGBS in concrete, it is essential to protect against early loss of moisture over than first 7 days.

Color

Normally concrete has grayish in color due to its cement. Replacement of GGBS up to 30% to 40% does not show any color change. GGBS is white in color hence replacement level 50% and above show in a color change of concrete. Concrete becomes slightly whitish in color.

Admixtures

GBS concrete is compatible with all admixtures. However, care should be taken if retarding agents are specified as GGBS can have a retarding effect on the set.

Hardened Concrete

Strength

GGBS concrete has slightly slower strength development at an early age but will have equal if not greater strength at 28 days compared to non GGBS concrete. At 7 days GGBS concretes will have 50% to 60% of its characteristics strength compared to 70% to 80% for Portland cement only concrete at the same time. At 28 days GGBS concrete will have fully developed its characteristics strength and will continue to develop strength past 90 days. It is good practice to make 56 days cube when using GGBS concrete at 50% and above should there be any concern over later strength development.      

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