Looking for sustainable alternatives to traditional cement in construction? GGBS and fly ash are two commonly used supplementary cementitious materials in construction that are gaining popularity due to their environmentally-friendly properties.
What is GGBS?
GGBS, or Ground Granulated Blast-furnace Slag, is a byproduct of the iron-making industry. It is produced by quenching molten slag from a blast furnace with water or steam, which leads to rapid cooling and the formation of glassy particles. These particles are then ground into a fine powder, which can be used as a partial replacement for Portland cement in concrete. GGBS has a high content of silicates and aluminates, which contribute to its pozzolanic and hydraulic properties. It is commonly used in the construction of dams, bridges, tunnels, and high-rise buildings due to its durability and resistance to chemical attacks.
What is Fly Ash?
Fly ash is a byproduct of coal-fired power plants. It is a fine powder that is produced when coal is burned at high temperatures, and it is collected from the flue gas using electrostatic precipitators or baghouses. Fly ash is composed of spherical particles that are primarily made up of silica, alumina, and iron oxide. It is commonly used as a partial replacement for Portland cement in concrete due to its pozzolanic properties. Fly ash improves the workability of concrete and reduces the heat of hydration, which can lead to lower shrinkage and cracking. However, the quality of fly ash can vary from batch to batch, which can affect the performance of concrete.
Let’s explore the benefits and potential uses of GGBS and Fly Ash in construction.
Variation in material quality
Fly ash and GGBS are commonly used as supplementary cementitious materials in concrete production. However, fly ash has a significant drawback in that its quality can vary greatly from batch to batch and even within the same source over time. This inconsistency can lead to unexpected changes in the performance of fresh or hardened concrete, which can be a source of frustration for producers. On the other hand, GGBS exhibits less variation from batch to batch, providing concrete producers with a reliable source of high-quality material that enables them to create more durable mixes.
How does this impact the cost and quality?
| Material Used | Maximum % | Cost Per Kg (Rs) | Cost Reduction | Remarks |
|---|---|---|---|---|
| 100% OPC | 100% | 7.00 | 0% | |
| Fly Ash | 15% | 1.25 | 13-15% | Due to less water demand an additional 5% can be saved using fly ash |
| GGBS | 50% | 3.50 | 20-25% |
Note: OPC stands for Ordinary Portland Cement and GGBS stands for Ground Granulated Blast Furnace Slag.
Replacement range with cement
Fly ash can be used to replace a range of 15% to 35% of the weight of cement in the mix, as per IS1489 PART-01. On the other hand, GGBS can replace a range of 20% to 70% of cement, as per IS-455. However, using more than 35% fly ash in the mix can significantly impact concrete strength and setting time, whereas GGBS can be used up to 70% without such effects. To achieve the same strength as a control mix made with only Ordinary Portland Cement (OPC), GGBS can be used between 25% to 50%, while fly ash can be used between 10% to 15%.
Admixtures usage – Water Reducing Admixtures
The spherical shape of fly ash particles contributes to the flowability of concrete, reducing the need for water-reducing admixtures to achieve the desired slump or flow. Additionally, when the water-to-cement ratio is lower, the strength gain of the concrete is higher.
In contrast, GGBS particles have an angular shape and typically require a mild reduction or increase in the use of water-reducing admixtures.
Slump retention:
In general, fly ash mixes exhibit better slump retention than GGBS mixes when other factors that affect slump retention are held constant. The spherical shape of fly ash particles creates a ball-bearing effect, contributing to its superior performance in this aspect.
Strength development
At all stages of curing, GGBS mixes generally achieve faster strength development than fly ash mixes with the same replacement percentage. This is because GGBS is a hydraulic material that produces CSH without the need for a secondary reaction.
CH consumption
GGBS mixes typically contain a lower percentage of CH due to the high level of cement replacement (30% to 70%) in these mixes, coupled with the fact that the GGBS reaction does not generate CH.
Durability
When it comes to durability, fly ash is typically replaced at a range of 15% to 35%, while GGBS is replaced at a range of 25% to 50%. Generally, GGBS mixes (25% to 50%) produce more durable concrete than fly ash mixes (15% to 35%). Numerous studies demonstrate superior performance in RCPT, water permeability, and surface and bulk resistivity for GGBS mixes compared to fly ash mixes. This is attributed to the high replacement rate of GGBS, which results in less CH and higher CSH in concrete.
Mass concrete
Over the last decade, GGBS and fly ash have been primarily utilised to regulate the core temperature in massive structural elements such as raft foundations. The replacement range for fly ash in this application is typically between 25% to 35%, while GGBS usage ranges from 50% to 70% based on the required strength and maximum core temperature. Although Fly Ash mixes may show lower core temperatures than GGBS mixes at times, GGBS mixes have a significant advantage over fly ash mixes: they produce higher tensile strength at a faster rate than fly ash mixes during the early stages of concrete development.
This advantage should be taken into account as cracks (thermal or non-thermal) occur when the stress in concrete exceeds its tensile strength. In the case of mass concrete, both core and differential temperatures are crucial, as is the development of tensile strength in the early age of concrete. Compared with fly ash mixes, GGBS mixes possess this advantage.
Self-compacting concrete
Fly ash is an excellent option to produce cost-effective SCC with high flowability (650 to 800 mm) due to its spherical shape and ball-bearing effect. It significantly enhances the filling ability, and passing ability, and reduces the segregation and bleeding potential of SCC, making it a preferable substitute for the control mix (OPC only) due to reduced viscosity. On the other hand, GGBS plays a different role in SCC by increasing its viscosity compared to the control mix, which helps in reducing the segregation potential of SCC. This makes GGBS the best option for casting vertical elements such as columns and walls with a slump flow between 550 to 650 mm, as it tolerates variations in aggregate gradations, admixtures, and water dosages during production. Combining 15% to 20% fly ash with GGBS in SCC could be a great idea.
Pumpability in high-rise buildings
The spherical shape of fly ash enables it to be pumped over longer distances with less pressure, making it ideal for use in high-rise buildings. While GGBS requires more pressure for pumping, it can help to minimise segregation. Using a combination of GGBS and fly ash would be a great approach for producing concrete that can be pumped easily over long distances while also minimizing segregation, making it a perfect fit for high-rise structures.
Concrete placing and finishing
The placement and finishing of fly ash are comparatively easier than control mixes, which consist of only OPC. This is primarily because fly ash has a spherical shape, while the addition of GGBS increases the paste volume, thereby making it easier to place and finish compared to OPC mixes.
The environmentally-friendly characteristics of GGBS and Fly Ash concrete
The production of cement is a significant contributor to global carbon dioxide emissions, accounting for 5% of the total emissions. The manufacturing process of cement produces 1 ton of carbon emissions for every ton of cement produced. In contrast, the production of GGBS emits only 35kg of carbon for every ton of GGBS produced, which is less than 4% of the carbon footprint of cement production. By using GGBS instead of cement, we can create Green concrete that prioritizes reducing carbon emissions in construction.
Using fly ash instead of cement can also help reduce carbon dioxide emissions. For instance, replacing one pound of cement with fly ash can save one pound of carbon dioxide emissions. To illustrate, a concrete floor slab for a typical two-car garage that contains around 4,000 lbs. of cement can save 1,000 pounds of carbon dioxide by substituting 25% fly ash. This amount is equivalent to the carbon emissions produced by an average automobile in one month.
Note: The information presented above represents the author’s personal opinions, which have been informed by their technical background, practical experience, and research conducted on the use of GGBS and fly ash.
