We need a chemist: Portland cement and Lime.

[SteamyTea]

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.

http://matse1.matse.illinois.edu/concrete/prin.html

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

And

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.

 

[SteamyTea]

Prompted by something @tuftythesquirrel said in another thread.

I went looking for the mechanical properties of lime v cement mixes.

 

I found this for concrete.

 

https://eurocodeapplied.com/design/en1992/concrete-design-properties

 

Copy to Clipboard

Concrete Design Properties according to EN1992-1-1 (γ_c = 1.50, f_yk = 500 MPa)

Symbol	Description	C12/15	C16/20	C20/25	C25/30	C30/37	C35/45	C40/50	C45/55	C50/60	C55/67	C60/75	C70/85	C80/95	C90/105
fck (MPa)	Characteristic cylinder compressive strength	12	16	20	25	30	35	40	45	50	55	60	70	80	90
fck,cube (MPa)	Characteristic cube compressive strength	15	20	25	30	37	45	50	55	60	67	75	85	95	105
fcm (MPa)	Mean cylinder compressive strength	20	24	28	33	38	43	48	53	58	63	68	78	88	98
fctm (MPa)	Mean tensile strength	1.57	1.90	2.21	2.56	2.90	3.21	3.51	3.80	4.07	4.21	4.35	4.61	4.84	5.04
Ecm (MPa)	Elastic modulus	27085	28608	29962	31476	32837	34077	35220	36283	37278	38214	39100	40743	42244	43631
fcd (MPa) (for αcc=1.00)	Design compressive strength (for αcc=1.00)	8.00	10.67	13.33	16.67	20.00	23.33	26.67	30.00	33.33	36.67	40.00	46.67	53.33	60.00
fcd (MPa) (for αcc=0.85)	Design compressive strength (for αcc=0.85)	6.80	9.07	11.33	14.17	17.00	19.83	22.67	25.50	28.33	31.17	34.00	39.67	45.33	51.00
fctd (MPa) (for αct=1.00)	Design tensile strength (for αct=1.00)	0.73	0.89	1.03	1.20	1.35	1.50	1.64	1.77	1.90	1.97	2.03	2.15	2.26	2.35
ρmin (%)	Minimum longitudinal tension reinforcement ratio	0.130	0.130	0.130	0.133	0.151	0.167	0.182	0.197	0.212	0.219	0.226	0.240	0.252	0.262
ρw,min (%)	Minimum shear reinforcement ratio	0.055	0.064	0.072	0.080	0.088	0.095	0.101	0.107	0.113	0.119	0.124	0.134	0.143	0.152

 

Has a lot of useful numbers.

 

I would like to see some for lime mixes as it would be really good to give a definitive, or at least well informed, answer to this long standing debate.

 

A bit more digging and I have found this.

 

https://www.intechopen.com/chapters/63429

 

It is a long read so may have to come back to it.

 

Mortar composition		fflex (MPa)	fcpr (MPa)	fccy (MPa)	GF (N/m)	ft (MPa)	Ecy (GPa)	Epr (GPa)	lch (mm)
NHL09C04M	Mean	1.3	3.2	2.0	12	0.39	5.0	5.2	390
	Std. dev.	0.1	0.1	0.2	3	0.02	0.2	0.5
NHL08C04M	Mean	1.3	4.2	2.7	13	0.51	5.4	6.0	260
	Std. dev.	0.1	0.3	0.3	1	0.01	0.6	0.2
NHL11C04M	Mean	0.89	1.7	1.4	4.9	0.24	2.8	3.8	240
	Std. dev.	0.04	0.1	0.1	0.8	0.03	0.7	1.0
NHL09C04W	Mean	1.7	3.5	—	—	0.57	—	—	—
	Std. dev.	0.1	0.1			0.05
NHL09C02M	Mean	1.1	3.2	2.0	12	0.49	4.6	5.1	220
	Std. dev.	0.1	0.2	0.1	1	0.05	0.2	0.6
NHL09R04M	Mean	0.96	2.3	1.5	10	0.38	4.2	4.4	280
	Std. dev.	0.06	0.1	0.1	2	0.03	0.2	0.4
NHL09C04MA	Mean	0.91	2.4	1.5	8	0.34	2.8	3.2	190
	Std. dev.	0.02	0.1	0.1	1	0.03	0.4	0.6

Table 3.

Mechanical properties of NHL mortars at an age of 56 days.