Cement, mortar and concrete FAQ
What started off the Cement, mortar, and concrete
FAQ was when Phil Addison 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.
A brief History of cement
Cement has a long history. By the first century AD, the Romans and Ancient
Greeks were using a variety of cement to make concrete. This was composed
of lime (made by burning sea-shells, limestone or marble) and pozzolan,
a volcanic ash. The Greeks used volcanic ash from an island we now call
Santorini, and the Romans used material from a place now called Pozzuoli,
near the Bay of Naples. The Romans also discovered that powdered pottery
or tiles would substitute for pozzolan if there was no nearby source. The
word pozzolanic still survives to describe a reactive cement additive.
Some of these concrete structures still survive in part nearly two millennia
after they were first built. During the Dark Ages, it appears that the
technology died out along with most other technology. Cement is a word
which first appears to have been used in the Middle Ages, although it was
the Industrial Revolution which provided the spur needed for the development
of modern cement. It was discovered that if a particular seam of argillaceous
(clayey) limestone was calcined (heated strongly) and ground it made a
hydraulic cement. This "Roman Cement" was popular with civil engineers
between about 1800 and 1850, although it bore no real resemblance to the
genuine Roman composition. A Mr L. J. Vicat appears to be the first to
use seperate sources of lime and clay to make a hydraulic cement, and to
this day there is a deadweight penetrometer used for evaluating the set
of cements which carries his name. In 1811 Edgar Dobbs of Southwark patented
a cement, but it was with the issue of patent No. 5022 that Joseph Aspdin
first used the expression "Portland Cement". This was a clever marketing
ruse, as Portland Stone had (and still has) a high reputation, it being
one of Sir Christopher Wren's favorite materials. The original Portland
Cement was not actually terribly good, but has improved over the years
to be the excellent material it is today.
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 retail 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. It can be tinted with pigments, so, should you want
strawberry pink concrete this is possible.
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 "green" is concrete?
Concrete is made from limestone and clay or shale, with a small amount
of gypsum. These are normally quarried near the cement works to reduce
the amount of haulage required. Energy is needed to calcine and grind the
cement as well. However any building activity has an impact on the environment,
and there is clearly a difference in scale between putting in a patio,
and building the M490 through the last known habitat of the Sozzled Newt.
With large projects Pulverised Fuel Ash (PFA) and Ground Granulated Blast
Furnace Slag (GGBFS) are often included in the concrete mix, and as these
are waste products which would othewise require disposal in landfill, this
is a Good Thing. Building by its very nature changes the environment, and
while the environment needs protecting, as long as the population keeps
rising, more homes will have to be built. In the trade "green" concrete
means concrete which has set, but has not fully cured nor dried out.
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.
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John Schmitt john49@mdx.ac.uk
Last updated 19/12/00