Hardened Concrete Properties with Blast Furnace Slag (GGBFS)

Ground Granulated Blast Furnace Slag (GGBFS) offers benefits to both fresh and hardened concrete due to its physical properties. In terms of fresh concrete, GGBFS is highly recommended for mix designs aiming to achieve a denser concrete mass, eliminating voids and ensuring solidity. This is crucial for enhancing the overall quality of the structure. Additionally, GGBFS plays a significant role in reducing permeability, making the concrete mass void-free and increasing its resistance to the passage of liquids or gases.

Figure-1: Blast Furnace Slag

The Dartford Bridge, depicted in Figure-2, is a notable example of the diverse applications of concrete incorporating blast furnace slag.

Figure-2: Dartford bridge, United Kingdom

The article explores the characteristics of hardened concrete containing blast furnace slag.

Hardened Concrete Properties with Blast Furnace Slag

GGBFS (Ground Granulated Blast Furnace Slag) is a commonly used material in concrete production. One important aspect to consider when working with GGBFS concrete is the setting time, which refers to the time it takes for the concrete to harden and become solid. The addition of GGBFS can slightly extend the setting time of concrete compared to traditional cement-only mixes.

When it comes to the compressive strength of concrete incorporating slag, GGBFS has been found to enhance the overall strength performance. The inclusion of slag can lead to improved compressive strength, making the concrete more durable and capable of withstanding greater loads.

Proper curing is essential for the development of strength in blast furnace slag concrete. Curing involves providing the right conditions, such as maintaining moisture and temperature, to allow the concrete to properly hydrate and gain strength over time. Effective curing practices are necessary to maximize the strength potential of blast furnace slag concrete.

The color of concrete incorporating slag can vary depending on the specific composition and percentage of slag used. Generally, the addition of GGBFS can lighten the color of the concrete, resulting in a lighter gray shade compared to traditional cement-based concrete.

In terms of flexural strength, blast furnace slag concrete tends to exhibit favorable performance. The incorporation of GGBFS can enhance the flexural strength of concrete, making it more resistant to bending or cracking under load. This improved flexural strength contributes to the overall durability and structural integrity of the concrete.

Concrete incorporating slag has shown reduced permeability compared to cement-only concrete. The addition of GGBFS can result in a denser microstructure, reducing the passage of water and other substances through the concrete. This lower permeability can improve the durability and resistance to chemical attacks of the concrete.

The Young’s modulus of elasticity refers to the stiffness or rigidity of a material. Concrete incorporating slag generally exhibits a slightly lower Young’s modulus compared to traditional cement-based concrete. The presence of GGBFS contributes to a more elastic behavior, allowing the concrete to better absorb and distribute loads.

Drying shrinkage, which refers to the volume reduction of concrete as it dries, can be influenced by the inclusion of slag. Concrete containing slag tends to exhibit reduced drying shrinkage compared to cement-only concrete. The addition of GGBFS can help mitigate the shrinkage and minimize potential cracking or deformation.

Replacing cement with GGBFS in concrete can lead to changes in the properties of the concrete microstructure. The microstructure of concrete incorporating slag generally exhibits improved hydration and increased density. This results in enhanced strength, reduced permeability, and improved durability. The specific changes in the microstructure depend on factors such as the slag content and the curing conditions applied.

GGBFS Concrete Setting Time

GGBFS (Ground Granulated Blast Furnace Slag) is a material that requires less water compared to other substances when used in concrete. However, one drawback of using GGBFS is that it can potentially increase the setting time of the concrete. As the amount of cement replaced by GGBFS increases, the setting time also tends to increase. A research study conducted by Hogan et al. found that when GGBFS was used to replace 40%, 50%, or 60% of the cement content, the setting time of the concrete was extended by approximately one hour, both in terms of initial setting and final setting time, compared to ordinary Portland cement (OPC).

Compressive Strength (Concrete Containing Slag)

The compressive strength of slag concrete depends on several factors, such as the type of slag, fineness, activity index, and the amount used in concrete mixtures. Other factors, including the type of cement and the water-to-cementitious material ratio, also contribute to the strength. Typically, slag concrete exhibits a gradual increase in compressive strength from 1 to 5 days, which is lower than that of concrete without slag.

However, the strength of slag concrete matches that of regular concrete from 7 to 28 days. Additionally, after 28 days, the compressive strength of slag concrete surpasses that of concrete without any slag. To address the issue of lower early strength in slag concrete, the inclusion of silica fume can be effective. The extent of improvement in early age strength development depends on the amount of silica fume used. Lastly, Figure-3 demonstrates the development of compressive strength in both regular concrete and concrete with varying percentages of slag replacement. Notably, the 40% slag replacement showed the best performance compared to regular concrete and other replacement percentages.

Figure-3: Strength Development of Slag Concrete

Blast Furnace Slag Concrete Curing

Inadequate curing of concrete has a significant impact on the degree and rate of hydration, leading to a slower formation of strength. This effect is particularly pronounced in concrete that contains a high percentage of slag. Insufficient curing can have detrimental consequences on the strength and durability of the concrete.

To ensure consistent strength and durability, concrete with more than 30% slag content requires a longer curing period compared to concrete without slag. The duration of curing for slag concrete depends on various factors, including ambient temperature, the amount and types of cement used, the temperature of the cement during application, and the percentage of cement replacement with slag.

Color of Concrete Incorporating Slag

Blast furnace slag is lighter in color compared to traditional concrete, resulting in a lighter appearance of slag concrete. The interior portion of slag concrete exhibits a distinctive deep blue-green color, which becomes evident when the concrete is broken, such as during a compressive strength test. However, this color gradually fades when the concrete is sufficiently exposed to air. The intensity of the color is influenced by factors such as the curing conditions, the proportion of blast furnace slag used, and the degree of oxidation.

Flexural Strength (Blast furnace slag concrete)

Slag concrete surpasses or matches the flexural strength of controlled concrete after 7 days. This superiority in flexural strength is attributed to the enhanced bond within the cement-slag-aggregate system, primarily due to the unique shape and surface texture of slag particles. Figure-4 visually represents the appearance of these slag particles.

Figure-4: Blast Furnace Slag Particles

Permeability of concrete incorporating slag

Concrete incorporating slag has been found to exhibit improved permeability compared to controlled concrete. This is attributed to the presence of blast furnace slag in the cement paste, which reduces the size of pores and consequently decreases the permeability of the slag concrete. Additionally, research has demonstrated that concrete incorporating up to 75% slag shows satisfactory performance when exposed to seawater.

Young’s modulus of elasticity of concrete incorporating slag

Multiple researchers, including Stutterheim and Nakamura et al., have reported that when the strength of slag concrete and concrete without slag is equal, there is no significant disparity in their modulus of elasticity.

Drying shrinkage of concrete containing slag

According to Hogan and Meusel’s research, the drying shrinkage of slag concrete is greater compared to concrete without slag. This is primarily attributed to the low specific gravity of slag, which leads to an increased volume of the concrete paste when slag replaces a portion of the cement by weight.

Microstructure of GGBS Concrete

The microstructure of concrete is a fascinating aspect that greatly influences its properties and performance. When Ground Granulated Blast Furnace Slag (GGBS) is used as a partial replacement for Portland cement in concrete, it imparts unique characteristics to the microstructure. Let’s delve into the microstructure of GGBS concrete and explore its key features.

GGBS is a byproduct of the iron and steel industry, obtained by rapidly quenching molten iron slag from a blast furnace with water or steam. This process results in the formation of glassy granules that are then ground to a fine powder, making it suitable for use as a supplementary cementitious material in concrete.

When GGBS is incorporated into concrete, it chemically reacts with calcium hydroxide (a byproduct of cement hydration) to form additional cementitious compounds. These compounds, known as calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H), contribute to the overall strength and durability of the concrete.

The addition of GGBS alters the microstructure of concrete in several ways:

Hydration Products: GGBS reacts with water and forms additional hydration products. The reaction leads to the development of a refined and denser microstructure compared to plain cement concrete. This densification improves the impermeability and durability of the concrete, reducing the ingress of harmful substances such as chlorides and sulfates.
Reduced Calcium Hydroxide Content: GGBS reacts with calcium hydroxide, which is a byproduct of cement hydration. The consumption of calcium hydroxide results in a reduction of its content in the microstructure. This reduction leads to a decrease in the presence of free lime, which is known to be susceptible to chemical attacks and can cause long-term durability issues.
Porosity Reduction: The incorporation of GGBS promotes the refinement of pore structure within the concrete microstructure. This refinement results from the formation of additional C-S-H and C-A-H gels, which occupy pore spaces and contribute to the overall densification. The reduced porosity improves the resistance to permeability and enhances the long-term durability of the concrete.
Strength Enhancement: GGBS contributes to the development of strength in concrete through the formation of additional cementitious compounds. These compounds provide a greater number of binding points within the microstructure, resulting in improved mechanical properties. The refined microstructure and increased density also contribute to higher compressive strength and improved resistance to various forms of deterioration.
Alkali-Silica Reaction (ASR) Mitigation: GGBS has been found to effectively mitigate the alkali-silica reaction, which is a chemical reaction that can cause expansion and cracking in concrete due to the reaction between alkalis in cement and certain types of reactive silica in aggregates. The incorporation of GGBS in concrete significantly reduces the availability of alkalis, thereby minimizing the potential for ASR.

Scanning Electron Microscope View of Ordinary Portland Cement and GGBFS Concrete

In the study conducted by Li and Zhao, the microstructure of ordinary Portland cement concrete (OPC) and GGBFS (Ground Granulated Blast Furnace Slag) replaced concrete was observed using scanning electron microscopy. The aim was to investigate the combination effect of GGBFS and fly ash in the concrete.

In OPC concrete, a significant number of capillary pores ranging in size from 0.05 to 60 μm were observed, along with the presence of calcium hydroxides. Additionally, ettringite formation was observed, albeit in a limited number. Ettringite is formed through the hydration of tricalcium aluminate (C3A) with water and gypsum. However, it is important to note that ettringite is not stable and does not contribute to the strength of the concrete. It exists in the form of long crystals.

On the other hand, the GGBFS replaced concrete exhibited a different microstructure. The number of ettringite crystals formed was relatively low. Furthermore, the capillary pores observed in this concrete were smaller, ranging from 10 to 50 μm, and they were present in a smaller quantity. These pores were found to be filled with the pozzolanic reaction products, such as the C-S-H gel. The presence of these reaction products contributes to the overall densification and improved durability of the concrete.

To study the combined effect of GGBFS and fly ash, Li and Zhao conducted experiments with a combination called GGFAC, which included 300 kg/m3 of cement, 125 kg/m3 of fly ash, and 75 kg/m3 of GGBFS. The control specimen was ordinary Portland cement concrete (PCC), which contained 500 kg/m3 of cement. Scanning electron microscopic views of OPC and GGBFS concrete at 7 days and 360 days were obtained, as shown in Figure 3 and Figure 4, respectively. The microstructure views of the study are depicted in Figure 7 and Figure 8.

The examination of the microstructure provides valuable insights into the effects of GGBFS replacement and the combination with fly ash in the concrete. It demonstrates the potential of GGBFS to contribute to the refinement of the capillary pore structure and the formation of reaction products that enhance the overall strength and durability of the concrete. These findings support the use of GGBFS as a sustainable and effective supplementary cementitious material in concrete construction.

**Fig.5: Scanning Electron Microscope View of OPC concrete**

**Fig.6: SEM of GGBS Replaced Concrete**

**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.