It is mostly preferred to use a single brand of cement while producing concrete and in 1 cement silo.
If you demand for using more cement brands, the RMC manufacturer will tell you about the admixture compatibility issues.
Do you know what is mean by admixture compatibility issue. in most of the admixture other than accelerators, admixture only helps in improving the workability of the concrete and does not take part in the process of strength gaining.
So when it comes to admixture compatibility test, the flow of cement with specified water cement ratio and dosage of admixture is measured using marsh cone.
Higher the rate of flow, lower the dosage requirement of admixture, lower the rate of flow, it requires higher dosage of admixture.
Why this happens when cement is made up of more than 90% of lime.
It comes back to normal consistency or standard consistency of cement where we calculate the basic water demand of cement to form a paste of required consistency under control conditions.
The water required for achieving the standard consistency (paste of desired properties) is nothing but the water cement ratio which we use for finding it out.
Say if cement consistency is 29% - it have demand of 29% of water by weight to make it in paste form of desired properties ( i.e. - when cement paste is prepared and tested in Vicat apparatus by lowering the plunger into cake, plunger should be at 5 to 7mm from the base of cake - when this situation achieved, the water required to do that is termed as standard consistency).
The standard consistency of the cement can vary from 25% to 35 % and beyond. it can depend on the fineness of the cement, age of the cement, and temperature of the cement, finest can required more water, aged cement can required low water and temperature in which cement is stored or cement temperature if its higher, it will demand more water.
All cements are processed and manufactured in almost same way and can have different performance modifier like fly ash, ggbs etc. and can also have different gypsum sources with wet and dry produced gypsum powders. there is also a difference between the fineness of cement due to its grinding process. some manufacturer meets the requirement and some go beyond to make it more better.
When we use different brands of cement, there water demand can be different from each other which will impact the water cement ratio of concrete which we produce from it.
Say cement 1 has standard consistency of 25% and cement 2 has standard consistency of 35%. you can see there is difference of 10% water demand to just form a paste of it. so cement 1 can perform better at lower water cement ratio and cement 2 requires higher water cement ratio.
If we produced same concrete with same water cement ratio, cement 1 concrete mix will exhibits more workability compare to cement 2 mix because, cement to had additional 10% water demand (W/C of 0.1 goes to fulfill that demand) which in most of the cases treated as admixture compatibility issue, because it is not giving the desired workability.
Here the comparison starts with 2 brands of cement and made a statement like cement 2 is not compatible with admixture brand - is it that case ? - no but it can be explained practically by doing trials and showing performance.
When we have single brand cement, there will not be a fluctuations in water demand of cement and tracking of it becomes easier because, when you see there is more water demand, you can co-relate it with the standard consistency of cement which was checked at plant at the time of receiving of cement.
This makes easier to increase the water cement ratio with facts.
When there are multiple brands having varying standard consistency, it is difficult to adjust the water cement ratio as we never know about which cement is actually going in concrete and what water cement ratio need to be taken for it. if you see a cement silo, then you will see there is a small opening at the bottom of the cone from where the cement is loaded into screw conveyor. the cement wont get discharge like first come first go due to the angle of repose and shape of the silo and cavity created at center which can make fall of any side of cement into it and can not be identified from outside, how cement is flowing out from silo.
This can give a varying results in workability and strength of the concrete.
When you decide to check the admixture compatibility with cement brands at third party laboratory, you will come to know that, the compatibility test is run by making same tests on various brands of cement without considering its consistency.
All cement brands will be tested for same amount of water and admixture and flow in seconds were recorded. it will only give indication for fixing optimal dosage for each brand of cement with fixed water cement ratio.
Practically this wont help to do anything other that, understanding cement behavior of water demand.
This can make a big mess when you have cement brands which possesses the extreme ends standard consistency which can reflect in additional water demand in brand of cement which literally breaks all laws for water cement ratio fixed for concrete mix design.
Best approach of using multi brand cement is to use multiple silos for RMC plant and single brand should be stored in each silo.
Get the test done on each cement and concrete mix done by referring to base standard mix design whose all properties are known.
Adjust the water cement ratio for each brand of cement and if required do adjust the admixture dosage too.
Hope your query got answered via above lines, if you still have something on your mind regarding same, you can reply to this post.
