Corrosion, alkali-silica reaction

Natural Catastrophes (earthquakes, floods) Climate/Environmental Factors (high soil chloride concentrations, rapid microbiological digestion, heavy use of de-icing salts) Conflicts (physical damage) Design/Construction Errors (chloride attack, reinforcement corrosion alkali silica reaction) and of course Age Effects (carbonation, long term erosion). 

It is important to repeat that we are only discussing corrosion repairs. Other problems, structural defects alkali-silica reaction (ASR) and so on may also need to be addressed at the same time and rehabilitation systems must be an integrated package of compatible, complementary systems. Some of the issues of compatibility are discussed later. Specifically the compatibility of patch repair systems and ASR with electrochemical treatments will be addressed. 

AIPO4 is isoelectronic with silica and, as such, readily forms glasses and Si02-like crystalline materials. As well, framework stmctmes similar to zeolites may be prepared by the use of amines as templates. Like zeolites, these are active in catalytic reactions such as methanol conversion to hydrocarbons (seeZeolites) As a ceramic material, AIPO4 is an infusible material that is insoluble in water but is soluble in alkali hydroxides. It is often used with calcium sulfate and sodium silicate for dental cements. AIPO4 is also used as a white pigment that also acts as a corrosion inhibitor. 

The alkali silicate glasses are easily corroded by aqueous acids and the main reaction is an exchange between H3O+ of the solution with the alkali M+ of the glass the corrosion rate increaseswith the ionic radii ofM+, that is, K > Na" " > Li" ". The chemical durability is greatly increased by addition of divalent CaO or trivalent AI2O3 oxides, leading to commercial soda-lime-silica compositions. 

Alkali reactivity results from sodium and potassium hydroxides present in cement reacting with silica, silicates, or carbonates in aggregates. Under certain conditions, the reaction can result in expansion of the aggregate and cracking of the concrete. This can lead to mis-alignment of components within structures and corrosion of reinforcement. 

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