Allowable amount of Unhydrated Cement Particles???

[Kwaz]

I am currently trying to find out what the allowable amount (percentage) of unhydrated cement particles is acceptable in a concrete mixture. If you can help by specifying what code or reference (any American Code/Reference) specifies this allowable percentage, it would be greatly appreciated.

Thanks,
Kwaz

 

[Ron]

To my knowledge there is no code requirement for percent of hydrated particles. It is too difficult a parameter to check for it to be a commonly mandated item.

As a practical matter, a water-cement ratio of about 0.25 to 0.30 is all that's required to hydrate common amounts of cement (in theory); however, many factors control the amount of cement that actually hydrates in a given time. Unhydrated particles can be found in concrete that is years old.

As an example, hydration of enough particles to provide cohesion and develop strength might occur in a few hours, leaving many particles yet to be hydrated. As strength increases (hydration occurs), porosity decreases as the calcium molecules particles bind with water molecules and increase in size. As this occurs, fewer and few particles get hydrated, some of which get isolated by the calcium compound formations (other compounds occur as well and contribute, I am just using calcium as an example because of its abundance in cement).

 

[bluesman]

Just to add to what Ron said many factors affect the amount of the unhydrated cement like time, temperature, curing and fineness of cement besides the water content of the mix.

As the temperature increases the hydration of the cement increases and the HYDRATES (the result of the hydration of the cement) build up faster around the cement grain making it harder for the water to reach the cement or to be more exact harder for the ions to diffuse from the cement grain surface into the water.

Also when the hardened concrete isn't cured probably, much of the water will be lost, stopping the hydration of the cement.

when the fineness of the cement is increased, there is more surface for the hydrates to cumulate making it easier for more grains to achieve full hydration.

Because of the difficulty in measuring the percentage of the unhydrated cement particles, I doubt you will find a spec’s on that in any code.



Drop By !!

http://www.geocities.com/concretesite

 

[BryanStein]

Ron and Blues are on the money. There will be some quantity of unhydrated cement in all concrete. Studies have shown that a w/c ratio of 0.28 is required for 100% hydration in sealed laboratory specimens. In the field, good hydration is achieved above 0.40. Any w/c less than that would led to only 80% or so hydration.

Curing moisture and temperature are the primary issues that affect the % of unhydrated cement.

 

[Kwaz]

Thank you all for your insight. It was very useful for me.

- Chris

 

[hornet2]

There is no such thing as 100% hydration. Optimally 90% - 95% hydration is all that is reached. This has been proven by curing and regrinding mortars and recurring again.