"hot" cement....Ever heard of it?

[a3a]

Can somone explain to me what "hot" cement is. It has something to do with the industry, or certian compainies, using cement that was not processed correctly.

Thanks for any info on the subject. Any discussion on how the performance of the concrete is affected is also welcomed.

Thanks

 

[Ron]

Hot cement occurs when cement does not have an opportunity to adequately cool down after it exits the kiln and is delivered to the end user. Cement is transported and stored in sealed containers and when it is produced in the kiln and moved to initial storage, the temperature might be in the 175 to 180F range. If immediately transported to ready mix suppliers due to high demand, the cement can hold a lot of this temperature and thus be introduced to the mix at elevated temperatures.

Hot cement can result in slump loss if the cement also has characteristic false set, but outside that, doesn't seem to have a detrimental effect.

 

[JAE]

Ron,
How often does this occur in practice? Do ready-mix suppliers have quality assurance systems to check against this?

 

[Ron]

JAE...its fairly rare. Usually only happens with domestic cements used in relatively close proximity to the cement plant. Most ready-mix suppliers that I have worked with have no means of checking or monitoring this. It is usually checked as a result of problems noted in the mix, either by an inordinately high mix temperature compared to the expected temperature (usually within 10 to 15 degrees of the groundwater temperature for the area), or by an unexplained slump loss (all other control parameters, i.e. saturated aggregates,maintained), or unexpected false set in the cement (usually predictable by mill certificates for the cement).
Ron

 

[a3a]

Thanks Ron. My impression was that the cement was heated too much and this "burning" (if you will) decreased the quality of the cement to the point that it probably shouldn't be used for performance reasons. The manufacturer then mixes properly processed cement with the hot cement and sells it at a lower price so that it is not wasted--but not telling the user of the cement what happend. This is just piecing together a conversation that I was overhearing, but didn't get all the details. Is there practices along these lines that occurs.

Thanks again.

 

[Ron]

a3a...I suppose that could happen, but mill certificates on the cement would probably tip it off by having shortened false set time and other property changes.

All issues such as this are good to keep in mind as basic skepticism in the quality of any construction process or material. The only way we keep the bar high is to continually question issues.

Tx,
Ron

 

[craftsman]

Could you be refering to "hot"concrete? As a concrete contractor, this has happened to me more than one occasion. It can occur in at least two ways that I know of. First, if you are pouring early in the morning when the temps are low, the concrete company will steam the aggregate. Sometimes they go over what they need to. Then with this extra heat it starts the hydration process. Then the concrete sets up even as it is being delivered. The other way, which no concrete company will probably confess to,is when the concrete company has a sizeable amount of concrete left over from a delivery. They then had more concrete to this and deliver it. Again the same thing happens. The old concrete is already setting up.
I have had concrete delivered for floors for which I had to power float the floor before I could screed it, it was setting up so fast. I am sure it effects the structural integrity of the concrete but to what extent I do not know.

 

[Ron]

Craftsman...those are wives tales and "urban rumor". I've worked with a lot of different ready-mix suppliers over the years from "Mom and Pop" operations to internationally known suppliers, and as poor as some QA/QC programs in some of the plants, there is very rarely any brazen, fraudulent attempt to adulterate concrete. It's usually pretty easy to find and prove, and I think most of them know that these days.

 

[Pondy]

Hot cement is the unacceptable cement due to its reaching/exceeding the temperature at which Gypsum[CaSO4.2H2O],avalable in cement,gets converted into Plaster Of Paris[CaSO4.1/2H2O].This process starts at about 100 deg. cetigrade[Theoriticlly at 140 deg cent.]Cement is produced by finely grinding about95% clinker[produced in hot rotary furnace, called Kiln]alongwith about 5% Gypsum. This Gypsum is added to retard[slowing]hardening of cement when water is added in it.In absence of Gypsum 'false setting'of cement takes place when water is added.Therefore it is essential to keep cement temp below 100 deg cent.during its manuturing as well as during preparation of concrete.

 

[Jim6758]

We had substantial problems with "hot cement" in this area when demand was high and the plants couldn't keep up with it. A railroad merger complicated the issue when a lot of cement got lost on railroad sidings (They found it however when the price got high enough). We experienced an unusual cracking pattern (alligator like)which was substantially different from the "crazed" appearance when it has been mixing too long and the heat of hydration has raised the temperature up.

 

[Pondy]

Inorder to avoid damage to the properties of cement due to high heat during hydration ice is added while concrete preparation.

 

[Jim6758]

Ice is very expensive and almost no one uses it here in Texas, even in the hottest part of the summer.

 

[Pondy]

Dear Jim6758,For your problem POZZOLANIC CEMENT is the most ideal solution.It presents a lower hydration heat rate & is more suited for preparation of concrete fot large constructions, such as dames, as this significantly reduces the manifestation of cracks surrounding the heat source.

 

[Jim6758]

We don't often have problems with Hot Concrete due to hydration since we use mostly fly ash mixes and set retarders during the hot summer months. Of course, fly ash is a natural possolan, isn't it. Occassionally a driver will get lost or a superintendent will schedule the trucks too close together and they stack up waiting to discharge.

 

Dear Jim6758, fly ash and admixtures are also our remedy here in the tropics plus chilled water (at max 4C chiller capacity). But this is not usually a guarantee, for ambient temp of 34C, not to exceed the 32C max of ASTM C94, 11.9, note 16 or ACI 305R. For a3a, cement is usually processed in the kiln at about 1500C to get the necessary reactions. The burning or kiln process (clinker production) in the cement production is followed by clinker cooling, grinding or finish mill and the last is packaging. Thou there are cooling process this is not really cool as we think to facilitate easy grinding. So from the 1500C at the kiln finish cement are usually stored at cement plant silos at about 80-100C. This will take weeks before the cement temp in silos to cool down due to its huge mass and cement plants silos needs this temp for easy fluidization discharge. In short, there is no really cold cement (bulk) unless it is old stock. "Hot cement" is usually a concern of ready-mixed concrete suppliers like us for factors said by others in this discussion. Cold cement are those you find in bags/sacks.

 

[RAY72]

A Hot load also is known as a "in the field" term when the delivery truck has exceeded it arrival time...load was in the barrel too long.over specs. reject the truck!!