News Date:2015-09-14
The addition of GGBFS either in cement or in concrete improves the following properties of concrete related to its durability.
Sulphate resistance: Cements with added GGBFS contain less alumina, Ca(OH)2 and C3A compared with normal Portland cement and they also have less water demand. This result in a more impermeable cement paste for penetration of sulphate ions and provides conditions to prevent ettrengite formation. Around 65% or more slag addition makes the cement comparable to sulphate resisting cement.
Resistance to chlorides: The permeability of concrete decreases due to fine GGBFS particles and their hydration products in the cement paste. Such cement pastes also absorb more chloride ions. It has been determined that, compared with normal Portland cement pastes, slag cement pastes are 100 times less permeable against potassium ions.
Alkali-silica reaction: The use of at least 50% GGBFS in cement is effective in reducing alkali-silica reaction expansions.
Acid resistance: Concretes containing GGBFS have been found to be more resistant to diluted acids compared with those containing normal Portland or sulphate resistant cements.
Resistance to sea water: Concrete in sea water is subjected to sulphates and chlorides. Field studies in Belgium, Germany, Norway, England and France have been conducted on the performance of concrete structures built with different types of cement in sea water. There was less spalling and reinforcement corrosion in concretes built with slag cements (25-35% GGBFS). Slag cements with more than 50% GGBFS performed even better.
Reinforcement corrosion: Highly alkaline cement pastes protect embedded steel bars against corrosion. Carbonation or penetration of chloride ions into concrete can initiate corrosion. Cement pastes made with slag cements can effectively block the entrance of chloride ions and prolong the useful life of concrete structures.
Thermal behavior and cracking: The hydration of cement is an exothermic process. The heat produced can cause temperature gradients and cracking in massive structures and can also be harmful in rich concrete mixes, in rapid curing and in hot weather concreting. In slag cements, the heat producing components of clinker is partially replaced with GGBFS and heat of hydration is effectively reduced. Thus, adverse conditions mentioned above that would otherwise serve to harm the durability of concrete do not take place.
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