Engineering Seminars: SLAG CEMENT

1. INTRODUCTION

Slag cement, or ground granulated blast-furnace slag (GGBFS), has been used in concrete projects in the United States for over a century. Earlier usage of slag cement in Europe and elsewhere demonstrates that long-term performance is enhanced in many ways. Based on these early experiences, modern designers have found that these improved durability characteristics help further reduce life-cycle costs and lower maintenance costs.

2. MANUFACTURING

Slag is a byproduct of processing iron ore to iron and steel in a blast furnace. Molten slag, which floats on the top of the molten iron ore, is separated and granulated. Granulation is the rapid quenching with water of the molten slag into solid, hydraulically reactive glassy granules.Granulation is done by using granulators as shown in fig1. These granules are then ground to a suitable fineness to produce slag cement or incorporate as an ingredient in the manufacture of blended cement.

Fig1: Granulator(www.slagcement.org.)

3. HYDRAULIC ACTIVITY

When Portland cement and water are mixed, a chemical reaction called hydration initiates, resulting in the creation of calcium-silicate-hydrate (CSH) and calcium hydroxide (CH). CSH is a gel that is responsible for strength development in Portland cement pastes. CH is a byproduct of the hydration process that does not significantly contribute to strength development in normal Portland cement mixtures.

Silicates in slag cement combine with CH byproduct of hydration and form additional CSH. This in turn leads to a denser, harder cementitious paste, which increases ultimate strength as compared to 100% Portland cement system.

4. PROPORTIONING

For concrete pavements, slag cement is typically used in proportions of 25 to 35 percent. It is normally substituted for Portland cement on a one to one basis by mass. The proportion of slag cement is usually dictated by requirements for strength, durability, time of set and the resistance of the concrete to alkali-silica reaction (ASR).Mixtures should be optimized for strength and durability using appropriate test methods.

Concrete mixtures containing slag cement should be proportioned using conventional design. Slag cement should be substituted for Portland cement on a one-to-one basis by mass. Specific gravity of slag cement ranges from 2.85 to2.94, depending upon the slag cement source-as compared to 3.15for Portland cement. The difference in specific gravity means a greater volume of slag cement will be used to replace the same mass of Portland cement. The larger percentage of fines usually allows for the use of a higher percentage of coarse aggregate.

Depending on the purpose of GGBFS proportions will be vary. For acid attack resistance 70% slag is used, to counter the problem of sulphate and chloride attack 40% to 70% slag is used, proportion of 50% is used to protect from the Alkali Silica Reaction (ASR).

5. WORKABILITY AND FINISHABILITY

Correctly designed concrete mixtures containing slag cement demonstrate good workability and finishability when compared with 100% Portland cement concrete systems. This is due to several factors including increased paste cohesiveness, glassy structure of slag cement, and low Initial water absorption. Slag cement mixtures can achieve required strength at cementitious levels while maintaining good workability and finishability for many of the same reasons, consolidation of slag cement concrete is generally easier than Portland cement concrete. When concrete with constant water-cement ratio are compared, those containing slag cement generally exhibit higher slump. Slump control is essential to preventing edge slump during slipform paving.

6. TIME OF SET

Concrete containing slag cement in excess of 25% replacement dosage generally has noticeably slower set times than ordinary Portland cement concrete. Time of set is related to the percentage of slag cement used in the mix, the temperature of the concrete, and the ambient temperature. At an ambient temperature of 73F, time of initial set is usually extended by one to three hours. At temperatures above 85f, the time of set difference becomes insignificant.

7. BLEED RATES

There are minor differences between bleed rates and bleeding capacities of concrete containing ordinary Portland cement and slag cement. These differences are from variations in the fineness of the slag cements produced by each manufacture. Slag cements that are finer than Portland cement will generally cause a slower rate of bleeding than concrete made with ordinary Portland cement. Course slag cements may cause the same or even greater bleed rates and capacities than concrete made with ordinary Portland cement. In slipform paving mixtures where water – cement ratios are low, bleeding may not occur. In those situations specified curing methods should begin subsequent to finishing or a finishing aide employed.

8. CURING

Curing refers to maintaining a satisfactory moisture condition in concrete immediately following placing and finishing, and lasting for a prescribed period of time. Strength gain and durability are directly related to the degree of hydration of the cementitious materials. The strength and durability properties of any concrete will fully develop only if it is cured properly.

Curing methods for concrete with slag cement are identical to those of other concrete. The curing method is usually driven by the specification. The lack of a specification should not imply that curing is not needed. Curing practices vary, however, due to several factors including mix proportioning, ambient and environmental factors, and job site conditions. The common thread is that curing must begin as soon as feasible and be continuous.

9. ADVANTEGES OF SLAG CEMENT

· Reducing permeability- when slag cement substituted in 25% to 65%, it plays a vital role in reducing permeability in concrete. Thus corrosion resistance will increase.

· Mitigating sulphate attack – slag cement mitigate sulphate attack by reducing permeability, reducing overall amount of tri-calcium aluminates, reacting with excess calcium hydroxide.

· Mitigating Alkali-Silica reaction –it is the reaction between the alkalis in the Portland cement and siliceous aggregates. In worst incarnation, ASR can cause severe concrete cracking and deterioration.

· Increasing strength and durability- Calcium Silicate Hydrate is responsible for strength development in concrete

· Increased corrosion resistance- corrosion resistance is due to reduced rate of permeability of the slag cement concrete.

· Reduced thermal stress in mass concrete- It is by reducing amount of Portland cement, reducing cementitious content, reducing early rate of heat generation.

10. DISADVANTAGES OF SLAG CEMENT

· Strength gain is slow

· In cold weather condition the low heat of hydration of slag cement coupled with moderately low rate of strength development, can lead to frost damage.

11. APPLICATIONS OF SLAG CEMENT

· Slag cement in residential concrete.

The durability, versatility and beauty of concrete are increasingly attractive to homebuyers.

· Producing concrete blocks with slag cement.

Slag cement consistently enhances certain qualities of concrete block, no matter how it is produced, providing architecturally appealing lighter colour, a finer, tighter surface texture or more swipe etc.

· Slag cement in high performance concrete.

· Producing pre-cast and pre-stressed concrete with slag cement.

· Slag cement in concrete pavements.

12. CONCLUSION

Slag cement can enhance concrete structures by improving workability in the plastic state, and increasing strength and reducing permeability in the hardened state. The correct amount of slag to use on a particular project depends on the materials and admixtures used, as well as ambient conditions during concreting. With careful attention to detail during the concrete mixture design and construction portions of a project, a successful and durable structure can be produced with slag cement. However, it is critical that contractors and specifiers are aware of the difference between ordinary Portland cement mixtures and slag cement mixtures.