Wikipedia:Reference desk/Archives/Science/2018 April 15

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April 15

Concrete

What is the precise mechanism for the setting of concrete? Do(es) the primary reaction(s) take place in the solid phase, the aqueous phase, or both? 2601:646:8E01:7E0B:0:0:0:9A39 (talk) 01:21, 15 April 2018 (UTC)

"Precise" is hard, as this varies according to composition and conditions. See Concrete#Curing. There are two processes though: the first one (usually a few days) is hydration, then a long term process of carbonation. The hydration is aqueous, the carbonation relies on a gas diffusing through a solid (one reason why it takes decades for thick concrete).

Much of the strength is about the duration of the hydration phase, thus the amount of hydration (and thus the strength increase) which takes place whilst the concrete is still wet enough to permit it. An over-dry mix, a wet mix placed against a dry substrate (which sucks the moisture from it), or air drying the surface are some of the factors which reduce this, thus reduce the concrete strength.

It's a big topic though, and more detail is going to need textbooks, not a simple encyclopedia entry. Look at the effects of pozzolans too, and how different ones make stronger or weaker mixes. Andy Dingley (talk) 12:25, 15 April 2018 (UTC)

It is a big story, I'm afraid you need to be like Daddy pig telling a concrete story to Peppa and George and read up on it to really get into it properly ;-) The article on concrete mentioned above is good though. Dmcq (talk) 17:05, 15 April 2018 (UTC)

So, by what mechanism does hydration proceed? 2601:646:8E01:7E0B:792F:2CDD:A29B:FC67 (talk) 07:32, 16 April 2018 (UTC)

If you really care, then get hold of a copy of Taylor, Peter C. (2013). Curing Concrete. CRC Press. ISBN 0415779529. This is expensive (>£100 locally) but seems to be the only modern, in-depth treatment of the subject. It should be in most college libraries, if they're teaching any sort of architecture, civil engineering or building course.

The problem is that this is a complex topic and it wasn't understood until the '70s (after the 1960s concrete building boom!), partly as a result of lessons learned from serious failures in materials like high alumina cement. As noted, pozzolans are important, and they just weren't properly understood for a very long time - the Romans knew of them two thousand years ago, some textbooks still mis-describe them.

Concrete mixes are well known for using either non-hydraulic or hydraulic cement. Yet some sources (and I don't see Wikipedia wording this terribly clearly) either mis-describe these, or at least are written to be very confusing. It's well known that hydraulic cements (and concretes using them) will cure underwater. Yet what is the curing mechanism? The non-hydraulic cement that does have a different curing mechanism is a lime mortar.

Lime mortars are hydrated (or 'slaked') long before use, to turn quicklime into lime putty. Lime putty is often stored for months to 'mature' it. This putty is then mixed (usually not long before use) with fines (sand) to make a lime mortar. There is endless debate as to how long either the putty or the mortar should be stored before use, and whether putty should be wet or made with only just enough water to hydrate it, so that it's a dry powder instead (this hydrated lime powder is often the form it's sold in). The curing mechanism is then carbonation by carbon dioxide from the air - hydration doesn't cure lime, that was already done when it was slaked. Note that cured lime mortars are much more permeable than cement mortars, so they're breathing more carbon dioxide, more quickly. Curing begins as soon as the hydrated lime is exposed to the air - either applied as mortar, or just stored as wet maturing putty. Maturing it, maybe for months, gives a slightly pre-carbonated and cured mortar which will be harder, stronger and more difficult to work. As the benefit of lime mortar is often that it's less strong than a cement mortar, but may be more workable to allow for artistic effects, then this can either be a good or bad thing.

If lime is mixed with a pozzolan, then you get a cement instead (this is a massive simplification). There are many types, some expensive, some cheap. Some sources will argue as to what a pozzolan is, and whether Portland cement (which uses cheap ground clinker and gypsum) has one at all - I take the modern view, that all such materials are pozzolans (or at least act as one), and it's this which changes lime mortars to hydraulic cements. Hydraulic cement needs to be stored dry (unlike lime putty) and although it can be stored for long periods, it's not improved by this (unlike lime), and so is best used rapidly. Once any water is added, a hydration reaction begins and this is the start of curing. It must be kept moist during curing and the other aspect of a hydraulic cement is that it can even be submerged during curing. Now we're back to my original post - hydration (wet) takes a few days, and then carbonation (probably dry) takes years.

It's not a simple split between lime mortar and hydraulic cement either. There are many inbetween, either hydraulic lime mortars (which cure by hydration, can't be stored wet once mixed, and may be used so wet as to be under water) or non-hydraulic cements, which also cure by hydration but can't be submerged during curing. This is complicated, there are many variants, there are still old books and old views around which argue the definitions. If you want more, read Taylor's book, or something like it, as you're probably beyond what generalists can tell you here. Andy Dingley (talk) 10:30, 16 April 2018 (UTC)