It is reported that higher alkalinity is necessary for early hydration but higher alkalinity may for some products like ettringite, chloro- and sulfoaluminate phases etc. which weakens the cement matrix and reduced the strength of mortar and concrete.
Does decrease in alkalinity improves the strength of concrete? How alkalinity influence the mechanical behavior of concrete?
one more important thing, Although the alkalinity within the concrete pore structure can lead to ASR, the high pH value also provides a protective coating of oxides and hydroxides on the surface of the steel reinforcement. Without this layer, which is known as a 'passive' film, the steel would be exposed to the air and moisture in the pores, leading to rapid corrosion. It is the main chemical reason why reinforced concrete is a durable construction material. The layer is durable and self repairing, and it can last for hundreds of years if the alkalinity is maintained. However, the passive layer itself can be attacked by chlorides in salt and the alkalinity of the concrete can be reduced by reaction with atmospheric carbon dioxide, a process known as 'carbonation'.
alkali is important in cement and concrete but up to specific limit after which it become harmful and may lead deterioration of concrete as their reaction will be accompanied with volume increase which in turn cause crack and spalling, therefore the international standards and specifications limit the amount of alkali to less than 0.6 i think ,may be you should see the specifications for Portland cement for example or read about ASR which is the harmful alkali-silica reaction.
one more important thing, Although the alkalinity within the concrete pore structure can lead to ASR, the high pH value also provides a protective coating of oxides and hydroxides on the surface of the steel reinforcement. Without this layer, which is known as a 'passive' film, the steel would be exposed to the air and moisture in the pores, leading to rapid corrosion. It is the main chemical reason why reinforced concrete is a durable construction material. The layer is durable and self repairing, and it can last for hundreds of years if the alkalinity is maintained. However, the passive layer itself can be attacked by chlorides in salt and the alkalinity of the concrete can be reduced by reaction with atmospheric carbon dioxide, a process known as 'carbonation'.
As I understood the question, the focus is on the mechanical properties of the concrete. There are some studies about the influence of alkali addition on strength development, for example see Sant, G., Kumar, A., Patapy, C., Le Saout, G., & Scrivener, K. (2012). The influence of sodium and potassium hydroxide on volume changes in cementitious materials. Cement and Concrete Research, 42(11), 1447-1455.
A general trend is the alkali addition speeds up hydration of Portland cement at early ages and accelerates the development of strenght. After a few days, however, the strength tends to be lower than in the reference concrete.
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