Homework SourseAlkalisilica reaction is a common degradation in portland ce
Alkali-silica reaction is a common degradation in portland cement concrete. The reaction involves the dissolution of certain siliceous aggregates in the high pH environment. The silica reacts with hydroxyl ions in the pore fluid in concrete to form an expansive gel that causes cracking as it imbibes water. It is well known that quartz is not a reactive aggregate, but opal is highly reactive.
A. Explain why this is the case.
B. If you broke up bottle glass and used this as an aggregate in concrete,
would you expect alkali-silica reaction?
Solution
Question
Alkali-silica reaction is a common degradation in portland cement concrete. The reaction involves the dissolution of certain siliceous aggregates in the high pH environment. The silica reacts with hydroxyl ions in the pore fluid in concrete to form an expansive gel that causes cracking as it imbibes water.
It is well known that quartz is not a reactive aggregate, but opal is highly reactive.
A. Explain why this is the case.
B. If you broke up bottle glass and used this as an aggregate in concrete,
would you expect alkali-silica reaction?
A. Explanation
Silica, is a component of many rocks, but all forms of silica do not react significantly with the pore solution(alkaline) of concrete and so all siliceous aggregates do not produce concrete damaging Alkali -aggregate reaction.
In the case of quartz it is stable whereas opal is highly reactive, while both are silica minerals with identical chemical composition primarily comprising of Silica (SiO2).
The main reasons are the difference in the solubility of the silica and the crystalline structure of these two minerals. While opal has solubility of about 1000mm/l , quartz has very low solubility of less than 150 or so. Also, Opal has a highly amorphous crystalline structure which renders it unstable at high pH values and so aggregates containing significant quantities of the mineral opal are expected to react and result in expansion when used in concrete, provided there is sufficient alkali present. However quartz will not react in damaging manner because of less solubility and better structure, regardless of the alkali content of the concrete.
B. Bottle glass like soda- lime glass has 75% SiO2 silica which is reactive and also it being amorphous in nature , alkali -aggregate reaction is expected if it is used as aggregate in concrete.
