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Cement, mortar and concrete FAQ
What started off the Cement, mortar, and concrete FAQ was when Phil Addison
(phil@severn.demon.co.uk) posted a series of questions to uk.d-i-y
in September 1997. I posted a reply, and spooled it into a file, which
I have massaged about, and added to since then. Quite a few people
emailed me with encouragement, so I began.... Any comments, contributions
or other questions are welcome. I have added some stuff I wrote earlier,
thanks to those who asked, if I forgot to mention you by name my apologies.
It goes without saying that Matthew Marks asked some of the questions
answered here. The following is for advice only, and I make no implied
warranties at all.
What is cement?
A powder, which by hydraulic reaction (i.e. with water) forms a solid,
cohesive mass. There are several types of cement;
Ordinary Portland Cement (OPC), which is the standard, grey cement used
for most purposes, Blue Circle do a special cement for mortar use, but
it would appear that it is just a 50/50 blend of ordinary portland cement
and slaked lime. The lime acts as a plasticiser, and reduces the stiffness
of the set mortar, so that the masonry/brickwork as a whole is more tolerant
to movement.
There is also white cement, Blue Circle's tradename for it is Snowcrete.
I don't know about commercial availability but my previous employers
bought it by the tanker-load. It is essentially the same as Ordinary
Portland Cement, but the raw materials are selected for good colour,
and the clinker is ground with ceramic balls, not iron as for OPC.
Rapid Hardening Portland Cement, which is chemically very similar, but
ground finer. It still sets as slowly as OPC, but its strength gain is
more rapid after it sets. For most, if not all, DIY purposes, OPC is
perfectly OK.
Sulphate Resisting Portland Cement (SRPC) is for underground work, particularly
with "aggressive" i.e. sulphate containing groundwater. For
the UK it is best to assume that this is the case.
High Alumina Cement (HAC) is a special cement which develops its strength
very rapidly and has high chemical resistance. It can also be used for
refractory concretes, e.g. in steelworks, using the white version. It
suffers from a phenomenon known as "inversion", in which if
it gets hot and wet it disintegrates. There were a number of 1960s high-rises
which had autoclaved HAC panels and started shedding them in the 1980s.
I do not believe that anyone was killed, but a hundredweight panel falling
20 stories was obviously very dangerous. It is also useful in cold regions
as it can be laid in sub-zero temperatures. Lafarge, a French company
are the principal manufacturer of HAC
There are also "fast set" cements based on mixtures of OPC
and HAC, but these do not achieve the same final strength as either OPC
or HAC alone. There are also a few cements with "weird" chemistries,
but these are not used for DIY.
How does cement set?
Ordinary Portland Cement sets by hydraulic (i.e. water) reaction. It
is a complex mixture of components, probably the most important of
which are dicalcium and tricalcium silicates (C2A and C3A to cement
chemists). The water/cement ratio is of paramount importance to the
final set strength of the concrete, and the cement/aggregate ratio
and aggregate size distribution are also important. I am fairly sure
the optimum w/c ratio is .28, but up to .35 is commonplace. It is important
to avoid water loss, especially in the first 7 days, and the old approach
was wet sacking laid on the surface. Nowadays polythene sheet is used,
and there are thin foam products which will prevent light frosts from
causing damage. The actual mechanism is more or less as follows; When
the cement grains become wet, they absorb the water and various chemical
reactions begin. Acicular (needle shaped) crystals begin to grow from
the grains over the next few hours and each grain begins to look rather
like a microscopic sea-urchin. The crystals from adjacent grains begin
to interlock and intergrow and the concrete at this stage takes on
an initial set. This typically takes between 2 and 6 hours, although
the reaction continues, albeit at a slower rate for some time after
this. Even after 28 days the strength is increasing slightly. If a
cement mix stiffens, it should not be knocked back/up (water added
and a good stir) because this knocks off the acicular crystals, and
the final strength will be reduced. This will not make too much difference
for a patching mortar, though. The hydration reactions of cement are
exothermic (Give out heat) as mentioned elsewhere, and for really large
pours there are a number of ways to prevent overheating of the concrete.
To tie another thread in, cement set is accelerated in a number of
ways, The only common DIY method being calcium chloride. In concrete,
calcium formate is often used as the chloride (mistakenly IMHO) was
blamed for the corrosion of reinforcing bars.
The mechanism of this is as follows: New concrete contains abundant free
lime. (which you would know if you followed the link above) This inhibits
the corrosion of steel. However, atmospheric carbon dioxide reacts with
the free lime, in a process called carbonation, to form calcium carbonate.
Calcium carbonate does not inhibit the corrosion of steel, so that when
the carbonation penetrates to the depth at which the reinforcing steel
is, the steel corrodes. Worse still, the rust resulting from this occupies
a bigger volume than the steel it is formed from, and as the concrete
is weak in tension, it spalls away to reveal the reinforcement, thus
accelerating the corrosion. This sort of damage is very expensive to
repair. Carbonation proceeds at a rate more or less inversely proportional
to the square of its depth. Most modern reinforced concrete is designed
to last at least fifty years. The majority of reinforcement corrosion
seems to be promoted by the practise of salting roads in the winter.
Can cement irritate my hands?
Every bag of cement Blue Circle sells has a warning to this effect. Some
people are rather more sensitive to the free lime than others. It can
be especially irritating if you get it in your eyes. I am not aware
of any long- term health problems, but it is possible that cracked
skin will leave you vulnerable to dermatitis. Speaking personally,
I find a bit of quality hand-cream after mortar work stops my hands
going flaky.
What is a mortar?
A material composed of sand and cement (and possibly other additives)
used to build brickwork or blockwork with, can also be applied as a screed
or a render, screed is a mortar applied to a floor, render is a mortar
applied to a wall. A screed is also a term used for batten nailed to
the floor for the purposes of getting a flat application of mortar.
Particularly for rendering (though not for building brick/blockwork)
priming the surface with a PVA bonding agent, diluted 4:1, and using
this mixture instead of pure water as gauging liquid, will make the render
tend to stick to the wall both when wet, and after setting.
You said pure water, how pure does the water have to be?
In the UK, if the water is regarded as potable, then it will be suitable.
What is the mortar to use for - brick-laying above damp-proof course?
6:1:1 mortar. That is by volume;
6 soft sand: 1 OPC : 1 Slaked Lime
You may want to use a plasticiser (see below). You should also point
the wall with the same mortar you use, whichever it happens to be.
What is the mortar to use for - brick-laying below a damp course (e.g.
footings)?
Use Sulphate-Resisting Portland Cement in a 3:1 mortar below ground where
there is a possibility of aggressive groundwater. If my memory serves
this amounts to just about anywhere in the UK. This applies equally to
brickwork or concrete.
What other types of mortar are there?
Cement Mortar; Made with cement, sand and (not necessarily) lime. Still
called compo in the North. Greatly disdained by the Victorians for aesthetic
reasons. Used universally in new building work.
And two older types of mortar pre about 1920 as far as I can tell;
Lime Mortar; Made with lime and sand. Regarded as aesthetically the
nicest as the mortar is lighter than the brick. Old Victorian lime mortars
were 3:1 sand and lime, and the resulting brickwork was more tolerant
of movement.
Black Mortar; Made with cement, ash, and (some) sand. Most common in
the Midlands and Industrial North. Very dark in colour.
Surely neat cement will be really strong?
No, neat cement undergoes shrinkage as it cures, and forms masses of
internal cracks. mixes stronger than 3:1 should be avoided.
Why a plasticiser?
The amount of water required to reach a specific workability will be
reduced. In theory this will make the mortar stronger, so you could increase
the sand content, but this puts you in uncharted territory, so I would
not bother.
What is the truth about Fairy Liquid? My neighbour reckons it is a brilliant
mortar plasticiser.
It gives you hands that do dishes. :-) Fairy liquid plasticises mortar,
but adversely affects its final strength and durability. Lignosulphonate
type plasticisers are much better. If you wander over to Dejanews and
interrogate it for the word "Lignosulphonate", you will probably
find a previous post on the subject. You could apply a logic game type
approach to this as in "If fairy liquid is so good in mortar why
does anyone make a mortar plasticiser at all?" Use quality plasticiser,
Febmix, Cormix and Beaver-Cementone being but three that spring to mind,
though there are at least half-a-dozen. I would suggest the use of a
mortar plasticiser in most cases. These are almost all lignosulphonate
(sulphite lye) preparations and are "green" as they are a by-product
of the paper industry, which would otherwise need treatment prior to
disposal. Lime should be used for older properties mortar repair work,
but unless you want a total retro house use gypsum plaster internally.
What are the differences between 'builders' and 'sharp' sand?
Sharp sand is coarser, and the particles are more angular, than Soft
sand. Silver sand is of a nearly white colour, and builders' sand is
not very specific, but normally is synonymous with sharp sand. Soft sand
feels like beach sand and is fine like table salt. Sharp sand feels sharp
and gritty.
What is concrete?
A mixture of a cement (q.v.) gravel, and sand, gauged with water, which
may contain admixtures, and other pozzolanic material(s) normally poured
into formwork to create a specific shape. Metal reinforcement may be
included in the structure to improve its tensile/flexural performance.
Concrete can also be divided into two categories by its intended use,
but the composition will be much the same. Mass concrete e.g. for foundations
does not necessarily contain reinforcement, whereas structural concrete
(e.g. for bridge spans) does. In this case the reinforcing steel is likely
to be tensioned. If this is the case, it will either be pre- or post-tensioned.
Pre-tensioned reinforcement is tensioned before the concrete is poured,
and when the concrete sets, the tension is maintained by the bond between
concrete and reinforcement. Post-tensioned reinforcement is tensioned
in a duct running through the concrete after the concrete has achieved
sufficient strength. Both systems have their advantages and disadvantages.
How long should it take to set?
Dependent on temperature, mix specification and other things 2-4 hours
for initial set, 24 for final set depending on the cement type, and the
presence of retarders or accelerators, approximately 90% of final strength
will be achieved within 28 days.
Tips for mixing, yes I know - get someone else to do it.
The cement must be thoroughly distributed in the dry mix before the water
is added, or it will form into lots of little balls, and be difficult
to disperse. As for getting someone else to do it there are several companies
who do a "we mix - you pour" service at a fairly reasonable
rate. They normally want access to water (i.e. an outside tap) and somewhere
to tip their washings. If you want several cubic metres of concrete,
a delivery by mixer is the easiest, but you may want a team of helpers
to move things along. Many hands make light work.
What about this white stuff on my brickwork?
Someone told me it was efflorescence.
The soluble salts in the bricks (and mortar) are being brought to the
surface by water in the brickwork. The water can only escape the brickwork
by evaporation, so the salts are brought to the surface of the brick
where the solution becomes saturated, and crystals form. Efflorescence
is not harmful to the bricks, but can cause masonry paint to fall off
in sheets. Once a wall has dried out, efflorescence should not occur.
If it does continue, you have a problem with water ingress. Either your
DPC is bridged or breached, or rain is penetrating the brickwork, in
which case one of the silicone solutions will fix the problem. The salts
are IIRC mostly sodium sulphate. New brickwork tends to effloresce for
a few months, but stops eventually.
How dense is wet concrete?
Concrete and mortar both have a density of about 2 to 2.1 tonnes per
cubic metre. This figure applies both to wet and cured material. There
are special lightweight concretes as low as 1.4 t/cu m.
What is this plaster stuff, then?
Plaster is calcium sulphate hemihydrate. Gypsum is the dihydrate, and
is roasted to extract the water of crystallization and ground. Certain
plasters contain additives to improve texture, adhesion etc. Old plaster
(even a couple of months) is best thrown away, as it either sets very
fast, or not at all. Confusingly, in some parts of the world "plaster" is
a term used to describe a sand/cement render.
2CaS04.2H2O (heat)»»» 2(CaSO4).H2O + 3H2O
The reaction operates in reverse when the plaster sets.
Copyright notice: With the growth of the net, copyright law has become
very complicated. You may reproduce this FAQ in any form from stone tablets
to putting it on your homepage. You may distribute it to absolutely anyone.
You may even add your own bits to it. However, if you remove my name
from it, or try to pass it off as your own work, I will be very, very
angry with you.
To the index
John Schmitt john49@mdx.ac.uk
Last updated 18/4/00
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