Properties of Hardened Concrete with Blast Furnace Slag (GGBFS)

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The physical properties of Ground Granulated blast furnace slag (GGBFS) provides advantages to the concrete in fresh state as well as in its hardened state. It is always recommended for a concrete mix design, that would provide us with a denser mass of concrete, i.e. a solid that is free from voids. GGBS play an important role in making the concrete mass free from voids and hence making the structure less permeable.

Figure-1: Blast Furnace Slag

There are many applications of concrete with blast furnace slag such as Dartford Bridge which is shown in Figure-2.

Figure-2: Dartford bridge, United Kingdom

This article shed light on the properties of hardened concrete incorporating blast furnace slag.

Properties of Hardened Concrete with Blast Furnace Slag

• Setting Time of GGBFS concrete
• Compressive strength of concrete containing slag
• Curing of blast furnace slag concrete
• Color of concrete incorporating slag
• Flexural strength of Blast furnace slag concrete
• Permeability of concrete incorporating slag
• Young’s modulus of elasticity of concrete incorporating slag
• Drying shrinkage of concrete containing slag
• The change in properties of concrete microstructure with the replacement of cement by GGBFS.

Setting Time of GGBFS concrete

The GGBFS have lesser demand for water and there is a chance of an increase in the setting time of the concrete. When the replacement amount of cement by GGBFS increases, the setting time also increases. A study conducted by Hogan et. Al showed that for 40%, 50% and 60% replacement of the GGBFS amount would almost increase one hour more than the setting time of OPC (For both initial and final setting time).

Compressive strength of concrete containing slag

Slag concrete compressive strength is mainly based on number of factors for instance slag type, finesses, activity index, and the amount employed in concrete mixtures in addition to other factors for example cement type and water to cementitious material ratio. By and large, compressive strength of slag concrete gradually increases from 1-5 days and lower than that of concrete without slag, but slag concrete strength matches strength of controlled concrete from 7-28 days. Moreover, slag concrete compressive strength surpasses strength of concrete with zero slag after 28 days. Furthermore, low early strength achievement of slag concrete can be tackled by inclusion silica fume. The improvement of early age strength development depends on the amount of utilized fume. Lastly, Figure-3 illustrates the compressive strength development for controlled concrete and concrete replacement with different percent of slag. It can be noticed that the 40% replacement provided the best performance compared with controlled concrete and other replacement percentages.

Figure-3: Strength Development of Slag Concrete

Curing of blast furnace slag concrete

Inadequate curing of concrete substantially affects degree and rate of hydration and consequently formation of strength-production hydration will be slow. The detrimental consequences of insufficient curing are more profound and outstanding in concrete incorporated with high percentage of slag. Therefore, to avoid uncertainty in the strength and durability, concrete incorporating more than 30% slag is cured for longer period compare with concrete with no slag. Finally, the extension of slag concrete curing time is based on number of factors including ambient temperature, amount and types of cement, the temperature of utilized cement, and percentage of cement replacement.

Color of concrete incorporating slag

Due to light color of blast furnace slag, color of slag concrete is lighter in comparison with conventional concrete. Moreover, deep blue-green color shown by interior part of slag concrete and can be noticed from slag concrete broken parts, for example, after compressive strength test. This color would be lost after adequate exposure to air. The degree of the color is based on curing condition, percentage of blast furnace slag employed, and oxidation degree.

Flexural strength of Blast furnace slag concrete

At 7 days and beyond, the flexural strength of slag concrete is equal or exceeds controlled concrete flexural strength. The larger flexural strength of blast furnace slag concrete is the result of the stronger bond which is achieved in cement-slag-aggregate system due to slag particle shape and surface texture. Figure-4 shows slag particles.

Figure-4: Blast Furnace Slag Particles

Permeability of concrete incorporating slag

It is claimed that, concrete incorporating slag perform better in terms of permeability compare with controlled concrete because blast furnace slag in cement paste decreases the size of pores and consequently the permeability of slag concrete is declined. Moreover, it is shown that, concrete incorporating up to 75% slag demonstrated satisfactory performance in seawater.

Young’s modulus of elasticity of concrete incorporating slag

There is not considerable difference between modulus of elasticity of slag concrete and that of concrete without slag if strength of both types of concrete are the same. This result is reported by several researchers such as Stutterheim and Nakamura et. al..

Drying shrinkage of concrete containing slag

As per the research carried out by Hogan and Meusel, drying shrinkage of slag concrete is larger than drying shrinkage of concrete without slag. Due to low specific gravity of slag that increases volume of concrete paste when slag replaces part of cement on weight basis.

Microstructure of GGBS Concrete

Studies have shown that GGBS help in decreasing the pores within the concrete thus making the concrete denser. The hydration reaction of GGBS gains two reactions. The first reaction involves the activation of GGBS particles to make them prepare for hydration. This is done by the alkali environment of calcium hydroxide (Ca (OH)₂) created by the primary reaction of cement with water. This alkali environment facilitates the formation of hydration product by the pozzolanic reaction carried out by the GGBS and the alkali. This gives C-S-H gel in the paste initially. This rate of formation is slow down and strength development is carried out with time. The hydration product C-S-H makes the concrete mass denser. More the GGBS replacement, more the C-S-H formation, hence denser the concrete. Denser concrete help in denser microstructure and lower porosity. Low porosity is a factor that has resistance to water penetration, thus conveying a guaranty on the durability of the concrete. Compared with the hydration products of Ordinary Portland Cement(OPC), there is the difference in the rate of products produced in GGBFS concrete. The hydration products Ca (OH)₂ due to the primary reaction will activate the slag reaction to form a low amount of CaO / SiO2 ratio or C/S ratio. This also reduces the AFm products (Products formed by the hydration of alumina and calcium hydroxide in the cementitious products) also. It is found that the pozzolanic activity increases the C/S ratio, because of the unstable C-S-H and the Ca (OH)_2. The use of GGBFS not only reduces the porosity but also change the pores to be finer nature. This will help in change the mineralogy of the hydration of cement. This promotes the reduction of chloride ion penetration.

Scanning Electron Microscope View of Ordinary Portland Cement and GGBFS Concrete

A study conducted by Daube et.al (1983) observed that the addition of GGBFS modifies the hydration products and the porosity of the concrete. This was observed clearly by the view obtained through a scanning electron microscope (SEM) as shown in figure 5 and figure 6. The view was taken for both ordinary Portland cement concrete as well as GGBFS concrete. The replacement amount was 60% of the concrete by GGBFS.

Fig.5: Scanning Electron Microscope View of OPC concrete

Fig.6: SEM of GGBS Replaced Concrete

It was observed that a great number of the capillary pore of size (0.05 to 60?m), as well as Calcium hydroxides, were formed in ordinary Portland cement concrete (OPC). The view of GGBFS replaced concrete showed that ettringite is formed in few number. The ettringite is formed by the hydration of tricalcium aluminate (C3 A) with water and gypsum. The ettringite is not a stable. It consists of long crystals and does not have any strength contribution. The capillary pore formed are of 10 to 50 ?m which are in less amount. These are even filled by the pozzolanic reaction products like the C-S-H gel. The combination effect of GGBFS and Fly ash were studied by Li and Zhao. The combination was GGFAC. The control specimen was Portland cement concrete (PCC).Figure 3 and 4 shows the scanning electron microscopic view of OPC and GGBFS concrete at 7 days and 360 days. The PCC contained 500kg/m3 of cement and the GGFAC contained 300 kg/m3 of cement, 125kg/m3 of fly ash and 75kg/m3 of GGBS. The microstructure view of the following study was shown in figure 7 and 8.

Fig.7: The SEM micrograph of PCC at 7 days and 360 days respectively

Fig.8: The SEM micrograph of GGFAC at the age of 7 and 360 days respectively

From the figure-7, it is clear that the PCC contain a larger amount of needle-shaped ettringite and calcium hydroxide. The sample also observed a lot of pores within it as shown. Figure-8 showing the microstructure of GGBS and fly ash combination showed a greater change in microstructure. The main products where the C-S-H gel and few number of ettringite. There was no sign of fly ash particles/ This might be due to complete reaction of Fly ash. The microstructure of PCC was observed to be more compact. A large number of calcium hydroxide which is plate-like crystals were observed, which is itself in large excess amount is undesirable for concrete performance. Read More: Ground Granulated Blast Furnace Slag in Concrete & its Advantages Fresh Concrete Properties with Ground Granulated Blast Furnace Slag Durability Properties of Concrete with GGBFS