The penetration of chlorides is affected by the content of Al2O3. The increase Al2O3 in cement reduces the penetration of chlorides in concrete.

Greetings,

As the answer above, the influence of the cement in chloride penetration is due to the Al2O3, the CaO has a little influence. The Al2O3 interact with H2O forming CSHA, then when the Cl- ingress to the pore solution reacti with the CSHA forming friedel´s Salt. This mechanism is responsable for minimazing the chloride ingress by chemical bounding.

Babar Ali as the above answers indicate, the amount of CaO may be indirectly related to Cl penetration, though I'm not aware of any reason that there should be a direct relationship. Besides the above-mentioned formation of Friedel's Salt, another mechanism may be related to the content of alite (C3S), belite (C2S), and/or secondary cementitious materials (whether added by the cement manufacturer or the concrete batch plant/lab), as these are important binding phases in closing the pore structure of concrete. The CaO content will, to a large extent, be a function of the relative content of these phases. Depending upon the age at measurement, these different phases will have varying effects on Cl penetration (e.g., slag decreases diffisuvity greatly, but requires a long time to react, so if you test at 28 days, you may not see the full effect).

May I ask: why are you measuring Cl penetration as a function of CaO content? Is there a particular application that you had in mind?

You can read this in the book "Durability of Concrete" by J. Stark, B. Wicht

High alkaline content in concrete forms a protective layer which will resist the chloride penetration in concrete.

The below mentioned link will help you,

https://link.springer.com/journal/11527.

Chloride attack on concrete is one of the important aspects of durability of concrete. It primarily affects the reinforcement of concrete and cause corrosion.

Chlorides can be introduced into the concrete either during or after construction as follows.

Before construction Chlorides can be admitted in admixtures containing calcium chloride, through using mixing water contaminated with salt water or improperly washed marine aggregates.

After construction Chlorides in salt or sea water, in airborne sea spray and from de-icing salts can attack permeable concrete causing corrosion of reinforcement.

The chloride in the presence of water and oxygen reacts with alkaline protected layer around the reinforcement and removes it.

Higher the CaO content, then from a total % composition basis, the Al2O3 content will be lower. Conversely as Bala mentioned, the alkalinity of the concrete pore water will be incrementally higher with higher CaO content, proving somewhat higher passivation of the steel.

CaO content and Cl- peneration are two independent parameters.

As higher the CaO content as better the passivation of any steel (reinforcement). Furthermore, the penetration of CO2 will be reduced (permeability reduction due to the formation of CaCO3 accompanied by a volume increase of ~ 10% within the pores).

An effectice reduction of Cl- penetration into concrete is possible by an addition of 3 - 5 % Silica Sol solution (Nanosilica)< standard product: 30% SiO2 in a translucent neutral water-based solution/particle size < 7 nm>. Cement/concrete permeabilities will be reduced distinctly, and the durability of concrete structures may be enhanced considerably.