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We need a chemist: Portland cement and Lime.


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On another tread, I typed up a bit about Portland Cement.

Now I am not a chemist at all, in fact I am rather dismissive of them.  Having said that, 5 Chemists that I have chatted to informally have been very good at explaining what happens without relying on using memory test in Latin.

One Chemist, a recent PhD graduate was working in sales and explained long chain polymers to me, they are really quite simple, repeating 'units' that attach to each other.

Another one, again a PhD in chemicals, explained the electron orbits and how they are NOT 2D as drawn on paper, this is what governs, to a certain extent, the shape of molecules.

Another, PhD again, sat me down in a lecture theatre and explained the polarity of molecules and why that causes CO2 to vibrate at certain frequencies and causes a disproportional amount of movement.  This principle of polarity is why a small amount of some molecules can cause a large amount of heating.

Then there was my old line manager, who also had a PhD in Chemistry who once said "when you design, or change a chemical formula to get the properties you want, you get what you are after, and something else".  It is that something else that is important.

Then there was our own @Jeremy Harris, who initially studied Chemistry then moved onto Physics.  He was brilliant as he was a practical man and understood the limitation of the 'something else'.


Now the above is just a way of asking if we have a decent chemist on here who can help out with a lot of the products used in the building trade, there must be at least one.


So getting back to the title, I had a look at Portland Cement and then Lime because of a question that @saveasteading asked about releasing and absorbing CO2.

After a bit of googling I found a long, but easy to read article explaining how Portland Cement is made and then used.


Basically, from what I understand, is that during the roasting, free water is evaporated and then elevated temperature drives out CO2 that is attached to the calcium.  When water is mixed back in, a reaction takes place that causes a rise in temperature, and the hydrogen and oxygen are split up and combine with the Di and Tri Calcium silicates and aluminate, with some Tetracalcium aluminoferrite and Gypsum thrown in.  Di, Tri and Tetra I understand, the Ates and the Ites I don't, they need to be memorised.

I was going to show the changes in formula when Limestone, Clay and Ferric Oxides are roasted, but I can't find a decent explanation (why we need a Chemist).

The reverse hydration i.e. when water is added, is shown.

Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat

2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ


Dicalcium silicate + Water--->Calcium silicate hydrate + Calcium hydroxide +heat

2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ

If I understand the chemistry correctly, because the CaO is already attached to something the SiO2.4H2O chain and the Ca(OH2) there is nothing for the carbon in CO2 to attach to.


So then I had a look at Lime, wanted to know what happened as way too many people seem to think that it is the bee's knees and will cure all ills in a building.  I have been very dubious of those claims and can never find decent evidence to support it.


So Lime is made in a similar way to Portland Cement.  Get the right rocks, crush and heat them to change them:  CaCO3 to CaO + CO2.

Then the difference from Portland Cement happens, water is added, in the right proportion.  CaO + H2O to Ca(OH2). Calcium Hydroxide.

This forms a dry powder called Slaking.  Also known as Hydrated Lime, add a bit more water and you get Lime Putty.

Now, and this is where it is really different from Portland Cement, it absorbs both H2O and CO2 from the atmosphere as it wants to convert back to limestone CaCO3 (calcium carbonate).


So come on Chemists, put me right and explain it better, maybe with a bit of basic theory as a grounding.


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