What is the chemical formula of cement?

[oddcitations]

Homework Statement

If limestone (calcium carbonate) and clay (assume aluminum silicate) are mixed together and heated then the result is cement. Cement is a mixture of calcium oxide, carbon dioxide, aluminum oxide, and calcium silicate. These bags can be purchased at any building supplies store. Write the balanced equation for this reaction.

Homework Equations

CaCO3 + Al2(SiO3)3 --> ...? Do I just do a double displacement, or does cement = CaO + Al2O3 + Ca2SiO4?

The Attempt at a Solution

4CaCO3 + Al2(SiO3)3 --> CaO + 4CO2 + Al2O3 + 3CaSiO4

C- 4
Ca - 4
Al - 2
Si - 3
O - 21

C- 4
Ca - 4
O -24
Al - 2
Si-3
I have no idea what to do for the oxygens. :\

 

[Phrak]

It appears the right hand side of your formula should consist of Alite (CaO)₃·SiO₂, Belite (CaO)₂·SiO₂ and tricalcium aluminate (CaO)₃·Al₂O₃ with evolved CO₂.

Thank you for prompting me to take an interesting tour of Wikipedia on the chemistry and production of Portland and similar cements.

 

[Borek]

IMHO question is poorly worded, it almost qualifies as a "question that should be never asked".

There is no CaO in the reaction products, and your calcium silicate formula is wrong. Correct these things and you should be able to balance the reaction.

Phrak: your approach (while correct) won't work here. This is about way more simplified case.

 

[Phrak]

IMHO question is poorly worded, it almost qualifies as a "question that should be never asked".

There is no CaO in the reaction products, and your calcium silicate formula is wrong. Correct these things and you should be able to balance the reaction.

Phrak: your approach (while correct) won't work here. This is about way more simplified case.

Could you be more specific? I'm confused by what you mean.

Secondly, I looked up Kaolinite clay (what china and porcelline are made of), on a whim, and discovered that it is aluminum clay. Its formula is Al₂Si₂O₅(OH)₄, as required by the problem, yet the OP suggests that the professor stipulates aluminum silicate, Al₂(SiO₃)₃.

 

[Borek]

Welcome to the world of chemistry education

Basic idea is - kaolinite is a kind of silicate. There are many similar compounds that are classified as silicates (see mineralogy section of wiki article), they slightly differ by composition, but their structures clearly show we deal with materials belonging to the same family (say like Lissajous curves look different, yet the are in a way all identical). However, it is difficult to explain why these minerals (with different formulas) are all classified as silicates, when someone who just started their eduction knows just the most basic version - silicate is a salt of silicic acid, period. Hence common approach is to simplify the problem - cement is made of aluminum silicate, so let's assume that aluminum silicate is just of a simplest possible form - Al₂(SiO₃)₃.

Problem is, these simplifications often go too far and they do more harm than good. But that's a different story.

 

[Phrak]

Welcome to the world of chemistry education

Oh, come on you love chemistry. So why the grumpy face-thing?--and I'm not that naive about it. But we are about 9 hours out of sink, so I'll have to consider you more thoroughly in the morning over here.

 

[chemisttree]

IMHO question is poorly worded, it almost qualifies as a "question that should be never asked".

There is no CaO in the reaction products, and your calcium silicate formula is wrong. Correct these things and you should be able to balance the reaction.

Phrak: your approach (while correct) won't work here. This is about way more simplified case.

You can balance this equation if you use the http://www.understanding-cement.com/bogue.html" as has been suggested by Phrak. Cement chemists aren't really that concerned with the actual chemistry of the anhydrous calcium aluminates or silicates, just the ratio of CaO to SiO2. It is a nomenclature peculiar to the cement industry but is nonetheless very useful.

Thus, tricalcium silicate is represented as (CaO)₃·SiO₂ (abbreviated as C₃S) whereas the actual form is certainly different than this formula would suggest. Borek's representation of aluminum silicate as Al₂(SiO₃)₃ is just as easily represented as Al₂O₃·(SiO₂)₃(abbreviated as AS₃).

 

[oddcitations]

IMHO question is poorly worded, it almost qualifies as a "question that should be never asked".

There is no CaO in the reaction products, and your calcium silicate formula is wrong. Correct these things and you should be able to balance the reaction. <-Merci boucoup!

Phrak: your approach (while correct) won't work here. This is about way more simplified case.

There is no CaO in the reaction products
That's interesting, but why? The question said Calcium oxide was involved. o.o

Chemistree + Phrak, that is all very facinating and if I were somewhere working in the cement industry I would be extremely impressed, but, I'm a moron and my chemistry teacher expects me to live up to those expectations. :\ I can't suddenly start speaking chemistry, then he'll a) have the false satisfaction of thinking he's inspired kids, and b) expect me to know stuff. Nevertheless, thank you very much. :]

 

[Phrak]

There is no CaO in the reaction products
That's interesting, but why? The question said Calcium oxide was involved. o.o

Chemistree + Phrak, that is all very facinating and if I were somewhere working in the cement industry I would be extremely impressed, but, I'm a moron and my chemistry teacher expects me to live up to those expectations. :\ I can't suddenly start speaking chemistry, then he'll a) have the false satisfaction of thinking he's inspired kids, and b) expect me to know stuff. Nevertheless, thank you very much. :]

Meet a fellow moron, but let me get this straight. You don't wish to come up with the right answer because of 'political' concerns. And this is just a chem class, not liberal arts. So you want Borek to help you find the right wrong answer. How's that?

Edit: Wait, this could get even better unless I'm egregiously mistaken. Take the right wrong answer and screw it up, just a little, so the professor can happily correct you with his superior-ness. If this is a better academic strategy you should really seek education elsewhere unless this is your gig. Not all schools are equal.

 

[chemisttree]

The OP's initial statement states...

Cement is a mixture of calcium oxide, carbon dioxide, aluminum oxide, and calcium silicate.

If this is the professor's question, he is leading you toward a Bogue type treatment of the problem by defining cement as "...a mixture of calcium oxide, carbon dioxide, aluminum oxide, and calcium silicate."

Don't overthink it. Your initial answer is all you need to understand this question. BTW, the question mentions calcium silicate not dicalcium silicate or tricalcium silicate. In Bogue representation it is CS or CaO·SiO₂ but you can represent it... how?

Also, typical portland cement produces about 25% by weight hydrated lime (known as portlandite by cement chemists) when it is hydrated so it doesn't confuse the issue by representing cement as a mixture of calcium oxide plus other species. Of course it isn't true that cement is a mixture that includes carbon dioxide! That part is added to help you balance the equation.

 

[oddcitations]

Meet a fellow moron, but let me get this straight. You don't wish to come up with the right answer because of 'political' concerns. And this is just a chem class, not liberal arts. So you want Borek to help you find the right wrong answer. How's that?

Edit: Wait, this could get even better unless I'm egregiously mistaken. Take the right wrong answer and screw it up, just a little, so the professor can happily correct you with his superior-ness. If this is a better academic strategy you should really seek education elsewhere unless this is your gig. Not all schools are equal.

That's half right, kind of. I mean, I just want the answer that's most closely related to the question. It seems to get very complicated after that, and also doesn't seem as relevant as it could be. It's like having a normal bike then covering it with so much grease you fall off the handlebars everytime you use it. I probably should, but my other teacher's are quite respectable and genuienly inspiring. You win a little, you lose a little. Also, your moron-ness and my moron-ness are totally different definitions, don't even try. :]

If this is the professor's question, he is leading you toward a Bogue type treatment of the problem by defining cement as "...a mixture of calcium oxide, carbon dioxide, aluminum oxide, and calcium silicate."

Don't overthink it. Your initial answer is all you need to understand this question. BTW, the question mentions calcium silicate not dicalcium silicate or tricalcium silicate. In Bogue representation it is CS or CaO·SiO₂ but you can represent it... how?

Also, typical portland cement produces about 25% by weight hydrated lime (known as portlandite by cement chemists) when it is hydrated so it doesn't confuse the issue by representing cement as a mixture of calcium oxide plus other species. Of course it isn't true that cement is a mixture that includes carbon dioxide! That part is added to help you balance the equation.

What? My initial answer? My initial answer doesn't work at all! o_o Oh, hold on.
The calcium oxide is kind of like a by product of cement and that's why it's not included in the equation? I mean that can't be true. If carbon dioxide is included, I don't understand why CaO is not. I'm sorry, you need to make this very idiot - proof.

See, this is what I got:

CaCO3 + Al2(SiO3)3 --> CaO<-Not included because is a by product + CO2<-Fun fact! Carbon dioxide is not really included, it's kind of like a catalyst, it's just there to help you balance equations! + Al2O3 + CaSiO3

Gah I despise Chemistry.

 

[chemisttree]

What? My initial answer? My initial answer doesn't work at all! o_o Oh, hold on.
The calcium oxide is kind of like a by product of cement and that's why it's not included in the equation? I mean that can't be true. If carbon dioxide is included, I don't understand why CaO is not. I'm sorry, you need to make this very idiot - proof.

See, this is what I got:

CaCO3 + Al2(SiO3)3 --> CaO<-Not included because is a by product + CO2<-Fun fact! Carbon dioxide is not really included, it's kind of like a catalyst, it's just there to help you balance equations! + Al2O3 + CaSiO3

Gah I despise Chemistry.

Your initial answer was very close. You wrote:

4CaCO3 + Al2(SiO3)3 --> CaO + 4CO2 + Al2O3 + 3CaSiO4

All the species are there on the left and right but your formula for calcium silicate (bolded) isn't correct. Using the Bogue representation for calcium silicate (CS) you get CaO·SiO₂. How would you represent that? Hint: it isn't CaSiO4. Also, notice that you were able to balance all of the elements except one. Which one was that and why?

It's painful for me to see someone that is so close to an answer give up!


BTW, calcium oxide is certainly part of the products. In actual portland cement, this calcium oxide is associated with a ferrite phase (not part of this problem), an aluminate phase (not part of this problem by definition of your professor) and several silicate phases (not part of this problem) alluded to by Phrak. Focus only on what is in front of you... given you by this particular problem.

 

[oddcitations]

4CaCO3 + Al2(SiO3)3 --> CaO + 4CO2 + Al2O3 + 3CaSiO3

C- 4
Ca - 4
Al - 2
Si - 3
O - 21

C- 4
Ca - 4
O -21
Al - 2
Si-3

OH MY GOD. It's like my birthday all over again
I love your brain Chemstree! Thank you thank you thank you thank you! You're awesome! :]


"BTW, calcium oxide is certainly part of the products. In actual portland cement, this calcium oxide is associated with a ferrite phase (not part of this problem), an aluminate phase (not part of this problem by definition of your professor) and several silicate phases (not part of this problem) alluded to by Phrak. Focus only on what is in front of you... given you by this particular problem."

Next time I see him I'll make sure to give him a suspicious smirk while walking by to mock him for giving a not only an inaccurate question, but for picking something that's so general and inconsistant that the actual formula is difficult to pin down. >:[ That'll show him!

 

[Phrak]

Next time I see him I'll make sure to give him a suspicious smirk while walking by to mock him for giving a not only an inaccurate question, but for picking something that's so general and inconsistant that the actual formula is difficult to pin down. >:[ That'll show him!

No, don't do that! I was over reacting. Forget I was here...