Optimized Usage of Fly Ash and GGBFS on High Performance Concrete

Introduction

Ground granulated blast furnace slag (GGBFS) and pulverized fly ash (PFA) have become major players in the advanced concrete technology and production of high-performance concrete such as self-compacting concrete (SCC), mass concrete and extremely impermeable concrete. Consequently, understanding the nature and influence of these materials on concrete performance is essential to properly use them in concrete production.

In this article, the author will focus on GGBFS and PFA properties, advantages & disadvantages and the perfect usage for each of them to reach optimization based on the author’s practical experience.

PFA

Fly ash is finely divided amorphous alumina-silicate powder produced in coal burning power plants. Class F fly ash is considered a true pozzolanic material with no hydraulic properties. In the presence of lime, fly ash can enhance concrete durability and strength development through producing additional C-S-H gel.

Advantages of fly ash include the following:

-Reducing heat of hydration and thus peak temperature as result of reducing cement amount in the mix when replaced by fly ash at replacement rates ranging from 20% to 35%.

- Enhancing concrete workability due to its rounded spherical particles thus reducing the amount of admixture used and save cost.

- Enhancing strength and durability of concrete.

Disadvantages of fly ash include the following:

-Decreasing compressive strength at early ages and even at 28 days age, however there will be a noticeable improvement of strength at 56 and 91 days age.

- Relatively low allowable replacement rate compared to GGBFS.

GGBFS

GGBFS is a by-product of iron manufacturing industries mainly consists of CaO (30-50%), SiO2 (28-40%), Al2O3 (8-24%) and MgO (1-18%). It is off-white powder with density of about 1200 kg/m3 and specific gravity of about 2.9.

GGBFS is considered a latent hydraulic material which reacts with water such as cement, nevertheless this reaction is slow and need activation which occurs by the Ca (OH)2 produced from hydration of cement, thus GGBFS cannot be used alone in concrete without cement. The replacement rate of cement by GGBFS can reach 70% in the very severe exposures.

Advantages of GGBFS include the following:

-Dramatic reducing of the heat of hydration and peak temperature by replacing cement in concrete at high rates up to 70% compared to maximum 35% for fly ash.

-The presence of three grades of GGBFS ( 80 , 100 , 120 ) with different fineness, thus different effect on concrete will widen and optimize its applications on concrete.

- Enhancing strength and durability of concrete.

Disadvantages of GGBFS include the following:

-Decreasing concrete workability especially at higher rates, thus increasing the chemical admixture dosage and cost.

GGBFS or PFA ?

Well! In fact, there is no fixed answer for this question. The author believes that the designer priorities, required specifications, available resources and climate conditions will determine the answer of this question together with a complete understanding of the nature, properties, advantages and disadvantages for each one of them.

For example: -

-If you are going to design self-compacting concrete, then fly ash will be better than GGBFS due to its excellent improvement of concrete workability and pumpability.

-If the specifications mandate to get the strength and durability requirements at 28 days not 56 days, then GGBFS grade 120 will be your best choice due its higher fineness which will help to get the required strength and durability at 28 days.

If the specifications allow you to test strength and durability at 56 days instead of 28 days, then fly ash will be your best choice to save cost.

-Suppose we are going to design a mass concrete with low peak temperature let’s say 65°c, then which will be more beneficial for us GGBFS or PFA?

Before answering this question, the author claims that for the same mix design, using 65% GGBFS will yield a peak temperature lower by 4-5 degrees than using 25% PFA for the same initial concrete temperature based on the author practical experience.

The answer of this question will differ according to your priorities, specifications and climate as mentioned above:

If you are in a moderate climate area where you can easily produce concrete with initial concrete temperature of 22 or less, then using PFA would be excellent choice with a dramatic cost savings.

If you are in a very hot climate area such as Gulf for example, and producing concrete with initial temperature less than 25 in summer is a dilemma, then you are obliged to use GGBFS and it will be the best choice.

Conclusion

In sum, both PFA and GGBFS are important materials in the advanced concrete technology and the optimized use of them will depend upon the mix requirements, priorities, available resources and climate.