PORTLAND BLAST FURNACE CEMENT (PBFC)

 
 

Lafarge Portland Blast Furnace Cement is a special blended cement with low heat of hydration characteristics for mass concreting.

 

 

The product

Lafarge Portland Blast Furnace Cement is a special blended cement with low heat of hydration characteristics for mass concreting. Concrete produced using Portland Blastfurnace Cement have chloride and sulphate resisting properties with improved durability.It is a cementitious construction material comprising of a combination of selected high quality ground granulated blast furnace slag and ordinary portland cement, conforming to BS 4246 (low heat of hydration) and BS 146.Our standard product, Blue Circle Portland Blast Furnace Cement, PBFCS, has a slag replacement of 65-70% and conforms to BS 4246. 

Essentially, granulated blast furnace slag consisting of calcium, aluminium, silica and iron, the same components found in cement but in different proportions, is ground to a fineness greater than Ordinary Portland Cement (OPC). The ground granulated blast furnace slag is dry-blended together with OPC as per requirements. The slag's latent hydraulic properties are activated in the presence of calcium hydroxide which is released when cement is mixed with water.This produces a denser calcium silicate hydrate which possesses substantial properties that are superior to OPC.

 

 Properties/advantages of PBFC over OPC

 

a) Higher Ultimate Strength

 

Although PBFC has a slower strength gain compared to OPC in the early stages, it will eventually have a higher ultimate strength. The strength patterns can however be controlled through concrete mix design, cement and slag fineness as well as slag proportioning.The longer term and ultimate strengths are better with increase in slag addition.

 

b) Lower Heat of Hydration

 

Where the minimum dimension of the concrete to be placed at a single time is greater than 500 mm and especially where the cement content is 400 kg/m3 or more, measures to reduce the temperature such as using material with a slower release of heat hydration should be considered.Cement generates large amount of heat during the hydration process. Slag hydration is an exothermic reaction but it does not react as quickly as OPC and thus generates heat more slowly.Peak concrete temperatures are reduced.This is an advantage when thermal cracking would be a problem.

 

c) Better Sulphate/ Chloride resistance

 

Deterioration of concrete by chemical attack can occur by contact with gases or solutions of many chemicals, but it is generally the result of exposure to acidic solutions or to solutions of sulphate / chloride salts.Solutions of naturally occuring sulphates of sodium, potassium, calcium or magnesium, as may be present in some soils and groundwaters, can cause expansion and disruption of concrete due to their reaction with C3A content in the cement.Concrete made with blast furnace slag cement had a lower content of cement thus the C3A content present is low.

 

d) Better Workability

 

PBFC has, for a given slump, a better workability.Our field practice confirms that PBFC also processes better slump retention.PBFC, due to its lower density produces more paste for a given weight.This acts as a lubricant in the mix and enhances the workability of the mix.The glassy nature of slag does not extract water from the mix.It also blocks pores and reduce bleeding.Slag helps to disperse flocculated cement particles.

 

e) Less Bleeding

 

Bleeding is the separation of solids in concrete from the water.Solids settle and water escapes through the pores and remains on the surface, which causes weakness of the top layer.The fine slag particles block pores which tends to produce a concrete which bleeds less.

 

f)  Long Slump Retention due to the slower hydration process

 

g) Greater Pore Filling Capacity

h) Reduced Permeability

i)  Greater Durability

j)  Reduced Risk of Alkali-Silica Reaction With Aggregates

 

k)  Lighter Colour And Better Finishing

 

Application/usage of PBFC

 

1 - Mass concrete foundations and dam which require low heat of hydration

 

The control of concrete temperature and the heat generated by the hydration of cement is important to prevent thermal cracks in the concrete mass.

 

In a large concrete pour, the control of concrete temperature and the heat generated by the hydration of cement is extremely important to prevent thermal cracking from occuring in the concrete mass.Although specifying a lower concrete temperature and use of insulation material around the concrete will help to reduce the difference of temperature between the core of the concrete and the surface temperature, this is not an ultimate solution.We need to moderate the heat generated by the Ordinary Portland Cement during the hydration process with another material without sacrificing the ultimate strength required.Ground granulated blast furnace slag had been extensively used as partial replacement for Ordinary Portland Cement for this purpose.As a result Portland Blast Furnace Cement (PBFC), a blend of ground granulated blast furnace slag and Ordinary Portland Cement was introduced to the construction industry.

 

It has now been established worldwide that PBFC will generate lower heat than Ordinary Portland Cement during the hydration process but will still be able to achieve the required strength. This is due to the low content of Ordinary Portland Cement in the blend as well as slower hydration characteristics of the blast furnace slag.However, in order to achieve the optimum advantage/blend of PBFC, the fineness of both the ingredient and mix proportion had to be consistent and controlled.We had studied various manufacturing methods and had come to a conclusion that the 2 ingredients OPC clinker and blast furnace slag with different hardness had to be ground separately in order to control their respective fineness.The ground products are then blended accurately in a blender to produce PBFC.

 

Our Company has selected the blending method to produce the Low Heat Portland Blast Furnace Cement in order to provide our customers with a consistent product.

 

2 - Marine concrete structures exposed to sea water and sulphates

 

PBFC with slag percentage of 60 - 70% or greater significantly improves water impermeability and resistance to Chloride attack.This is due to the fine nature and physical structure of slag.

 

The ability of PBFC to withstand Sulphate attack is due to: 

 

  • Reduced concrete permeability
  • Reduce amount of Calcium Hydroxide (hydration product) and reactive aluminates
  • Ca(OH)2 + Na2SO4.10H2O  --à Ca SO4.2 H2O + 2NaOH + 8 H2O
  • Calcium Hydroxide + Sulphates ---à Calcium Sulphate (Gypsum)
  • C3A + Sulphates ---à Calcium Sulfoaluminate (Ettringite) 

 

3 - Water retaining structures where improvement in impermeability is essential

 

4 - Sub-structure, bored piles, pre-case piles and products positioned in soil

      containing chlorides or moderate content of sulphates

 

5 - Concrete which may consist of alkali reactive aggregates

 

6 - Waste treatment facilities

 

7 - Grouting