GGBS - Ground Granulated Blast Furnace Slag

GGBS - Ground Granulated Blast Furnace Slag

We all know that we are surrounded by the concrete forests and concrete is a mixer of cement, fine aggregates, course aggregates and water. concrete play a vital role in the development of infrastructures because we all know that the India is a under construction country there are two many constructions are constructing in present like buildings, industries, bridges, highways, dams, reservoirs and canals etc. are consuming a large quantity of concrete. On the other side, In a concrete, cement plays an important role to acquire desirable properties and strength. which is scarce and expensive. Hence we have to find economically alternatives in the production of concrete. GGBS has a characteristics like cement. so we can use it as a partial replacement of cement.

Ground Granulated Blast furnace Slag (GGBS) is a by product from the blast furnaces used to make iron. These operate at a temperature of about 1500 degrees centigrade and are fed with a carefully controlled mixture of iron ore, coke and limestone. The iron ore is reduced to iron and the remaining materials from a slag that floats on top of the iron. This slag is periodically tapped off as a molten liquid and it is to be used for the manufacture of GGBS it has to be rapidly quenched in large volumes of water. The quenching optimizes the cementitious properties and produces granules similar to coarse sand. This "granulated" slag is then dried and ground to a fine powder.

CHEMICAL COMPOSITION OF GGBS

The chemical composition of a slag varies considerably depending on the composition of the raw materials in the iron production process. Silicate and aluminate impurities from the ore and coke are combined in the blast furnace with a flux which lowers the viscosity of the slag. In the case of pig iron production the flux consists mostly of a mixture of limestone and forsterite or in some cases dolomite. In the blast furnace the slag floats on top of the iron and is decanted for separation.

Typical chemical composition:-

Calcium oxide = 40% 
Silica = 35% 
Alumina = 13% 
Magnesia = 8% 

It is a granular product with very limited crystal formation, is highly cementitious in nature and, ground to cement fineness, and hydrates like Portland cement. 

Typical physical properties:- 
Color : off white 
Specific gravity : 2.9 
Bulk density : 1200 Kg/Cu.m
Fineness: 350 to 550 Sq.m/kg

USE OF GGBS IN MAKING OF CONCRETE :

  • GGBS is used to make durable concrete structures in combination with ordinary Portland cement and other pozzolanic materials.
  • GGBS has been widely used in Europe, and increasingly in the United States and in Asia (particularly in Japan and Singapore) for its superiority in concrete durability, extending the lifespan of buildings from fifty years to a hundred years. 
  • Two major uses of GGBS are in the production of quality-improved slag cement, namely Portland Blast furnace cement (PBFC) and high-slag blast-furnace cement (HSBFC), with GGBS content ranging typically from 30 to 70% and in the production of ready-mixed or site-batched durable concrete. 
  • Concrete made with GGBS cement sets more slowly than concrete made with ordinary Portland cement, depending on the amount of GGBS in the cementitious material, but also continues to gain strength over a longer period in production conditions. This results in lower heat of hydration and lower temperature rises, and makes avoiding cold joints easier, but may also affect construction schedules where quick setting is required.


PROPERTIES OF GGBS :
  • Better workability, making placing and compaction easier. 
  • Lower early age temperature rise, reducing the risk of thermal cracking in large pours. 
  • Elimination of the risk of damaging internal reactions such as Alkali-aggregate reaction.
  • High resistance to chloride ingress, reducing the risk of reinforcement corrosion. 
  • High resistance to attack by sulfate and other chemicals. 
  • Considerable sustainability benefits. 

EFFECTS OF GGBS ON PROPERTIES OF CONCRETE:

1. Setting time of concrete :
The setting time of concrete is influenced by many factors, in particular temperature and water/cement ratio. With GGBS, the setting time will be extended slightly, perhaps by about 30 minutes. The effect will be more pronounced at high levels of GGBS and/or low temperatures. An extended setting time is advantageous in that the concrete will remain workable longer and there will be less risk of cold joints. This is particularly useful in warm weather.

2. Need of water :
The differences in Rheological behaviour between GGBS and Portland cement may enable a small reduction in water content to achieve equivalent consistence class.

3. Consistency (Slump) :
While concretes containing GGBS have a similar, or slightly improved consistence to equivalent Portland cement concrete, fresh concrete containing GGBS tends to require less energy for movement. This makes it easier to place and compact, especially when pumping or using mechanical vibration. In addition, it will retain its workability for longer.

4. Temperature according to age :
The reduction involved in the setting and hardening of concrete generates significant heat and can produce large temperature rises, particularly in thick section pours. This can result in thermal cracking. Replacing Portland cement with GGBS reduces the temperature rise and helps to avoid early age thermal cracking. The greater the percentage of GGBS, the lower will be the rate at which heat is developed and the smaller the maximum temperature rise.

5. Workability :
Concrete containing GGBS improves the workability of concrete. It provides ease of placing and compaction of concrete.

6. Strength gain with GGBS:
With the same content of cementitious material (the total weight of Portland cement plus GGBS ), similar 28 day strengths to Portland cement will normally be achieved when using up to 50% GGBS. At higher GGBS percentages the cementitious content may need to be increased to achieve equivalent 28 day strength. GGBS concrete gains strength more steadily than equivalent concrete made with Portland cement. For the same 28 day strength, a GGBS concrete will have lower strength at early ages but its long term strength will be greater, the reduction in early strength will be most noticeable at high GGBS levels and low temperatures. Typically a Portland cement concrete will achieve about 75 percent of its 28 day strength at seven days, with a small increase of five to ten percent between 28 and 90 days. By comparison, a 50 % GGBS concrete will typically achieve about 45 to 55 % of its 28 day strength at seven days, with a gain of between 10 and 20 % from 28 to 90 days. At 70 % GGBS, the seven day strength would be typically around 40 to 50 % of the 28 day strength, with a continued strength gain of 15 to 30 % from 28 to 90 days. Under normal circumstances, the striking times for concretes containing up to 50 % GGBS, do not increase sufficiently to significantly affect the construction programme. However, concretes with higher levels of GGBS will not always achieve sufficient strength after one day to allow removal of vertical formwork, particularly at lower temperatures, lower cementitious contents and in thinner sections.

7. Permeability and chemical stability :
The reaction between GGBS, Portland cement and water are complex. When Portland cement reacts with water, the insoluble hydration products (mainly calcium silica hydrates) form close to the cement particle. The more soluble product of hydration (Calcium hydroxide) migrates through the pore solution and precipitates as discrete crystals, surrounded by large pores. When GGBS particles are also present, both the GGBS and Portland cement hydrate to form calcium silicate hydrates. Additionally, the GGBS react with the excess of calcium hydroxide to form a finely dispersed gel, which fills the larger pores. The result is a hardened cement paste, which contains far fewer calcium hydroxide crystals and therefore has fewer large capillary pores. The reduction in free calcium hydroxide makes concrete chemically more stable, and the finer pore structure limits the ability of aggressive chemicals to diffuse through the concrete.

8. Corrosion of reinforcement by chloride :
Steel embedded in concrete is normally protected against corrosion by the alkalinity created inside concrete by hydrated cement. In such conditions, a passive layer forms on the surface of the steel and rusting is inhibited. However, if significant amounts of chloride are able to penetrate the concrete this protection can be destroyed and the embedded steel will rust and corrode. Because of its finer pore structure, GGBS concrete is substantially more resistant to chloride diffusion than Portland cement concrete. For reinforced concrete structures exposed to chlorides, the use of GGBS will give enhanced durability and a longer useful life. This applies in many situations, including highway structures (particularly bridge parapets), car parks subjected to de-icing salts and coastal environments. Generally the higher the proportion of GGBS, the greater will be the resistance to chloride penetration. Typically, use of 50% GGBS will give high resistance to chloride and use of 70% GGBS will give very high resistance.

9. Alkali-silica reaction :
Alkali ions (sodium and potassium) are present in Portland cement. They are readily soluble in water and are released into the pore solution of concrete when the cement hydrates. Here they can slowly react with certain types of silica in the aggregate to produce an alkali-silicate gel. In wet conditions this gel can absorb water, swell and exert sufficient pressure to crack the concrete. In some cases the resultant cracking is sufficient to endanger structural integrity. The consequences of ASR can be severe and there is no reliable cure for affected structures. Addition of appropriate percentages of GGBS is an effective means of minimising the risk of damaging ASR. Detailed recommendations are given in Building Research Establishment Digest 330:2004, “Alkali Silica Reaction in concrete”. BS8500:2006 now refers to this digest rather than including specific recommendations for ASR. With GGBS, the BRE digest requirements are normally easily satisfied. By incorporating GGBS, the BRE digest requirements to limit the reactive alkali content of the concrete are normally easily satisfied. With normal reactivity aggregates and GGBS percentages of at least 40 percent, the GGBS is deemed to make no contribution towards the reactive alkali content.

10. Effective micro-structure :
Micro Structure Concrete containing ground granulated blast furnace slag (GGBS) is less permeable and chemically more stable than normal concrete. This enhances its resistance to many forms of deleterious attack, in particular: 

  • Disintegration due to sulphate attack.
  • Chloride related corrosion of reinforcement.
  • Cracking caused by alkali silica reaction.

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