Difference between revisions of "Cement"
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* [[:Category:Mortar|All mortar related articles]]
* [[:Category:Mortar|All mortar related articles]]
* [[Mortar Mixes]]
* [[Mortar Mixes]]
* [[Cement Mixing]]
* [[Cement Mixing]]
* [http:www.//Youtube.com Youtube videos] demonstrate a wide range of cement & concrete techniques
* [http:www.//Youtube.com Youtube videos] demonstrate a wide range of cement & concrete techniques
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Revision as of 23:25, 12 September 2010
- a grey powder that sets hard after mixing with water
- cement plus sand (or other fine aggregate)
- Mortar plus stone
Types of cement
Most cement used in DIY is Ordinary Portland Cement. Other types are also used for various tasks.
Sulphate resisting portland cement. Sulphates in groundwater attack OPC, but not SRPC, so its used where groundwater contact can occur.
Used where high temps are to be expected, such as flues & fireplaces. Its very weak when set, and is prone to falling out under its own weight.
How cement works
In the classic mixture of sand & cement, most of the load is borne by the sand particles. The cement acts as a glue, preventing the sand particles moving out of the way.
Concrete also incorporates large particles of stone, a cheap hard filler that replaces much of the sand/cement mix.
Set & Cure
Cement hardens in 2 phases.
- The first one is set, typically happening after 40 minutes or so. This changes the mix from liquid to a very weak solid.
- The second phase is cure, which takes a month or so. This produces massive strength gain. Optimum gain is achieved by keeping the cement damp.
Quick guide to strength
- 40 minutes: only just solid
- 1 day: rubs off readily
- 2 days: rubs off
- 3 days: now hardening
- 28 days: near max strength achieved
Cement mix strength specs are as measured at 28 days.
Below about 3°C cement ceases to set or harden. Note however that ground temperature is not the same as air temp, and the former is usually a little higher in freezing conditions.
Methods to set concrete in the cold include:
- Accelerating admixes
- Covering with sheeting
- Use of heated water in the mix
Compression & tension
Mortar has great strength in compression, but is weak in tension. So concrete is always designed to remain in compression during its service life. If for some reason it goes into tension, failure is on the cards.
One way to keep mortar in compression when used to span over window and door openings is to use an arch. The arch shape keeps it all in compression. Lots of old houses use this approach.
Another way to keep concrete over doors & windows in compression is to cast a straight beam containing prestressed reinforcing steel. The steel pulls the beam ends together with great force, keeping the beam all in compression.
Cement has little strength when first set. Even a high strength mix can usually be rubbed off with a finger for the first 2 days. It takes a month for cement to reach a high percentage of its final strength. A lot of newsgroup enquiries result from not being aware of this.
Chemically there are two different sets happening. The first happens typically in 4-24 hours, and the second over about 6 weeks.
Mortar that starts to harden can be remixed or 'knocked up' to enable longer use. However doing this reduces its final strength significantly, and its not good practice. Best practice is to dispose of any mortar not used within 40 minutes after mixing.
Mortar in thin layers tends to break up.
There are many mixes for mortar, concrete, and other mixes.
The strongest straight mix is 3:1. This is the ratio at which cement fills all the spaces between the sand granules. More cement than that results in microcracking as it sets, plus some of the packed hard sand being replaced by cement. Both these result in less strength rather than more. There are ways to make stronger mixes, but they're non-trivial and not used in DIY.
Quoted mix ratios refer to volumes, not weight. Sand, cement, stone, lime etc have differing densities so the wrong mix will result if mixed by weight ratio (unless the required ratio is recalculated of course).
Any type of sand works with cement. A percentage of sharp sand gives better strength.
Most types of stone are fine in concrete, but limestone should be avoided, its porous.
All in one mixes are often used rather than separate sand & stone.
Lime acts as a plasticiser, making the mixture stickier, and much more able to hold its position. Adding lime to cement mixes does not give it the properties of lime mortar.
Cement to lime ratio should be 1:1, other ratios have been found to suffer premature failure sometimes. The 1:2:9 mix is no longer considered best practice for this reason.
Modern chemical plasticisers do a similar task to lime, using much smaller amounts. The result is slightly cheaper.
Some cement powders already contain plasticiser. (Some Mastercrete products were mentioned.)
Washing up liquid
Basic washing up liquid acts as a plasticiser, but reduces the final set strength of the mortar.
Many washing up liquids also contain soluble salts, which cause temporary efflorescence and are hygroscopic.
Superplasticisers allow reduced water use, producing a stronger end result. Semi-dry mixes need vigorous compacting.
Accelerator gives quicker set and cure.
Retardant gives a slower set, prolonging working times.
Added to the water before mixing. Leaves a water repelling film around cavities in the mortar which prevents moisture wicking through by capillary action. Probably won't prevent water under significant pressure wetting the mortar, for which tanking would be required.
A combined waterproofer with plasticiser is commonly available.
Replacing some or all the sand with coal ash gives black mortar. Black mortar is known for its shorter life than cement & sand.
Adding PVA increases adhesion. This is used when fixing coping stones to cap a wall, to reduce the chance of them coming loose. The PVA should be mixed into the water before its added to the cement mix, otherwise its unlikely to end up mixed in properly.
Used to prevent the water freezing, allowing sub-zero setting.
Fibres provide crack control and improve tensile strength.
- Plastic fibres are cheap and effective
- Glass fibre provides higher strength
- Various other fibre types are less often used.
Fibres aren't a replacement for steel reinforcement.
Earth gets used as an additive in concrete ornaments where a natural (green or brown) biofilm finish is desired. The biofilm soon develops in use. This eliminates the grey cement like appearance. Earth is less stable than sand.
The quickest simplest finish to produce is tamped. Lightly whacking the concrete down with a stick of wood creates a level slightly rippled finish. This is the usual finish for driveways, paths etc. The slight rippling helps with grip in icy conditions.
Trowelled cement mixes give a smooth finish, with or without trowelling lines. Polishing is a way to maximise smoothness & evenness, and is a good option for domestic floors where they won't be covered over.
Cement mixes are sometimes simply thrown onto a wall. This may be done from a trowel, with a tyrolean gun, or the mix can be sprayed on (its then called shotcrete). The result is a very rough finish that traps dirt. Sprayed mixes are used mainly to minimise time & cost.
Pebbledash was often used on exterior house walls, but has fallen out of fashion. Cement mortar is applied to the wall, and pebbles are then thrown into it. The result is lots of exposed pebbles in the mortar.
There is also another deprecated way to do it. The pebbles are mixed in with the cement mix, its applied in one layer, and the wet mix is gently rinsed to expose the pebbles. This method leaches cement from the surface, reducing strength.
Most cement is grey. White cement is also available. Other colours are obtainable by using various additives.
Commercial colouring powders:
- Red is iron oxide, and is permanent and stable.
- Other colours tend to fade or become streaky over time. Strong dark colours can look bad when this happens.
- Coloured water based paints (emulsion, acrylic) will colour cement mixes
- Red brick dust is high in iron oxide, a permanent stable pigment.
- Coal ash blackens cement, but can cause earlier failure
- Stone waste can colour cement mixes to a limited degree.
Concrete stains produce a mottled coloured effect on polished concrete. Results can be impressive if done well. Youtube has videos showing how to do it.
Terrazzo is concrete flooring into which stone pieces, usually marble, are pressed. After setting, the surface is cut and polished. Metal strips are also incorporated to control cracking and for decorative effect. The result is very pretty and hard wearing, but vulnerable to staining unless coated. Correct choice of cleaning chemicals is important.
With the right moulding technique and finish, concrete can get a smooth high gloss finish.
Someone famously said there are 2 types of concrete, that which has cracked, and that which is about to crack. Crack control is an important aspect of the design of concrete structures, floors & bases.
Cement mortars consist of hard sand particles glued together with much weaker cement. Consequently they are very strong in compression, but weak in tension. The first rule of crack prevention is to design so that all mortar or concrete stays in compression. This is a basic necessity for all successful concrete use.
Wall connectors are available that consist of a vertical bar with sliding horizontal protrusions. These tie a new wall to an existing one while allowing some vertical movement between them.
The most common method of crack repair is to cut out the broken mortar & fill with new mortar. Removal is generally done with an angle grinder.
On historic brickwork the use of angle grinders is controversial, as its so easy to do a fair bit of damage to soft bricks with them.
The second mortar/concrete repair option is resin. Epoxy, vinylester or polyester resin is injected into the crack. Epoxy is used for structural repairs.
Epoxy mortar is used to repair cracks and damaged areas on floors.
Tile grout can be used to fill floor cracks.
Steel reinforcement should be covered with 2" of mortar all round. If not its likely to fail prematurely. Oxygen reaches the metal, which rusts and expands, breaking apart the concrete.
Rebar is the daddy of concrete reinforcement. It can produce reinforced concrete with enough strength to support medium rise buildings.
Expanded Metal Lath is a cheap galvanised steel sheet mesh. Stainless is also available at several times the price.
When the substrate is soft (when its insulation), EML is applied and rendered over. The result gives a durable layer that can't be obtained without the EML.
4" wide EML is available in roll form. It can be buried in mortar courses in garden walls to minimise damage caused by vandals. It can't replace reinforcing bar. It keeps wall elements in place when brick or mortar is broken, preventing further damage. It should only be used when needed, as rusting will break the wall apart eventually. The top courses of walls are the most vulnerable, and the least work to rebuild after rusting.
Lightweight open nylon mesh available from haberdashers for pence a square foot can be used to strengthen and reduce cracking in thin mortar layers. Compared to EML its less strong, cheaper, lighter, very quick & easy to handle, and doesn't need cutting at window openings etc. When the render is all applied, the mesh over windows etc is removed using a lighter.
Plastic fibres increase the tensile strength of cement composites, but to a much smaller degree than embedded steel. Their prime use is to reduce & control cracking.
Glass fibres provide much greater strength gain. But glass and cement are naturally incompatible, so either the glass fibre must be alkali resistant, or the cement mix modified to reduce alkalinity.
Asbestos cement replacements now use glass fibre. The 2 produce much the same result.
Old asbestos cement is one more area of diy to fall under the control of strict regulation. The cement keeps the fibres very well bound though, resulting in fibre exposure of approximately zero.
The 3 common methods of mixing mortar are on a flat sheet, in a barrow or with a mixer, but there are many more methods too, some of which are sometimes much quicker. See Cement Mixing.
Its worth making the effort to mix the mortar fully, until its all uniform. Otherwise there will be patches of mortar with an uncontrolled and different mix, some of which will perform substandard and may fail early.
The dry mortar materials should be mixed fairly well before any water is added. Additives should be added to the water before its added to the dry mix. Stone for concrete should be added after the mortar is mixed.
Failing to follow this order of work will result in a mortar that's not properly mixed, and make mixing more work.
Machine mixing is a little different, as the machine mixes vigorously without effort. These basic rules are less critical but are still best followed. One possible exception is stone, which could be added at any point in the process, but the mix will be ready quicker if its only added once the mortar is mixed.
Buying cement, mortar & concrete
There are a few ways to buy it.
- You can make up any mix you want, standard or otherwise
- You know exactly what you're getting
- You have to do the mixing
- You have to do the placing
- Mixing large amounts is punishing work
- 25kg bags are heavy, though smaller bags also exist
- Storable for later jobs
Bags or tubs of premixed mortar just need water adding.
- Small bags are handy where only a small amount is needed
- More expensive per kg than bagged components
- You have little control over the mix, and often don't know what mix you're getting
- Due to the above its likely to not match existing mortar
- Tub versions can be mixed in the tub.
You can buy concrete from teams that mix it on-site in barrows.
- Saves a lot of work
- Beware of suppliers adding excess water since you're paying by volume - this leaches cement out as it drains away. Reject any such.
- A useful option for large quantities where a cement truck can't gain access.
- Large quantities can be piped direct to point of use
- Delivery rate is fast, help is needed to spread it fast enough.
- Delays in unloading are chargeable
- Large trucks can't access all locations, and don't have long delivery hoses.
- Expect minimum orders
- You can order mixes to various standard specs
Cement absorbs moisture from the air, reacting and in time becoming inactive. Cement needs to be stored dry. It lasts much longer in a dry atmosphere than a damp one, and longer in plastic lined bags than paper. Paper bagged lasts longer inside a binbag.
When cement forms lumps in the bag, its lost a high percentage of its strength. Cement in this state is little use.
- If necessary it can be used for concrete for fixing fence posts - these work with no cement so the poor strength is no big issue.
- It can also be used for stabilising earth as long as there is still some active cement left. (If all inactive its fairly harmless.)
- Some can be added to weaker than 3:1 mixes just to dispose of it. Make the desired mix up with good cement, and add some junk cement
Floors & bases
4" minimum is recommended for any kind of concrete base. Thinner layers have much reduced strength.
House floors are done in 2 layers. Concrete is poured and levelled, and when lightly set a screed layer is added. Screed is a sand & cement mix, and is steel trowelled to a smooth flat finish. The 2 layers bond together during setting, forming one cohesive layer, so the screed layer can be thin.
Sprinkling a little cement powder on the surface when trowelling achieves a harder surface.
10% iron oxide in the sprinkled cement powder will give a redder surface colour.
Concrete floors subject to traffic (eg garage, workshop etc) tend to form dust on the surface due to microscopic disintegration. In situations with heavy wear this can also result in patches of surface breaking up.
Concrete paints prevent dusting and improve the surface appearance. Concrete hardeners may be applied to the surface, and soak in to provide a tougher result.
Suspended concrete floors
These are somewhat different to floors on hard ground. Suspended floors must be steel reinforced to keep them in compression.
Concrete floors have occasionally been cast on old wood floors. This gives insufficient support to be safe. As the wood decays, the then unreinforced concrete is likely to give way, the weights involved making this a dangerous event.
When repointing houses with historic soft brickwork, only failed mortar should be removed. The practice of replacing all mortar regardless is prone to causing brick damage that sometimes leads to disintegration of some bricks over a period of decades. Thus the process is one of patchwork, and matching new mortar to old is desirable.
Raking out should be done a patch at a time, as it weakens the area temporarily.
Rubbers are bricks that were rubbed down where necessary to fit accurately, enabling use of very thin joints. Thin joints between rubbers are time consuming to rake out, and less easy to fill. Where appearance matters, some caution is wise when hiring help to repoint such brickwork, as some contractors simply gouge new rough thick joins with an angle grinder, making an eyesore of the wall. In the case of listed buldings this can leave you liable for considerable work.
Mortar used for pointing walls should always be weaker than the bricks or blocks. This enables future maintenance without damage, and ensures its the expendable mortar than breaks in the event of minor movement, rather than the bricks.
1:1:6 is a common repointing mix. The lime content makes it sticky, helping stick it in place while wet.
Mortar for old houses
House bricks from the early 1900s and before tend to be very soft. Such houses are also typically subject to slight movement, since they lack expansion joints, and usually have shallow foundations.
Its thus especially important to use a mortar that's weaker than the brick, as slight movement is normal.
SPAB recommends the use of lime mortar on such walls. This has the advantages of
- lower strength than the brickwork
- minor movement causes spread microcracking rather than a single large split
- the microcracking is self healing, as the lime exposed reacts with CO2 in the air to grow hard crystals across the gaps
- Lime evaporates damp from the wall in preference to the brick, minimising brick spalling
- Lime doesn't pull the surface off the brick when it fails
- Lime's main disadvantage is very slow setting time
1:1:6 is also in common use by DIYers. This doesn't have the advantages of lime mortars.
In the case of listed buildings, matching the new mortar is usually legal requirement, except when the existing mortar is inappropriate. Small trial batches may be mixed up to achieve a match, or the mortar can be analysed. Old mortars tend to use sand similar to a mix of modern sharp sand and building sand.
A fair rule of thumb is to minimise the visible area of mortar as far as is consistent with the job being done properly. Many a wall has been made ugly by use of too much mortar.
Lime mortar is usually gently brushed once filled to expose the aggregate, and soften the whiteness.
The main pointing styles are:
- Flush - traditional
- Weatherstruck - slightly angled, popular today on old houses
- Round or bucket handle - rounded surface created with a tool
- Ribbon pointing - very prominent raised pointing. Tends to cause more water retention than other types, and not suitable for materials that aren't frostproof, including soft bricks.
Cement mixes are often vulnerable to freeze-thaw cycles. Water enters the gaps in the mixture, and expands as it freezes, gradually breaking the concrete.
Little or no damage occurs if the concrete is not saturated, as the ice has space to expand, but concrete that can become waterlogged is at risk.
A solution to the freeze-thaw risk is to use a 3:1 mix at ground level or below. 3:1 is the mix ratio where the cement exactly fills the voids between the sand particles, leaving the minimum possible porosity, and thus minimal risk of frost damage.
Another approach to frost resistance is air entrainment. For DIY use the easiest way to achieve this is by using a suitable frostproofing additive.
Some mortar always falls off during application to walls. Its best to let it dry in place rather than wipe it off, it sucks the water/cement liquid away from the floor as it dries. Snots are best brushed away the next day, or the day after. If left 3 days they harden and are hard to remove.
Any remaining staining can be removed with a strong hydrochloric acid, as long as the workpiece is acid resistant.
Excess wet mortar or concrete can be disposed of:
- Big dollops are too heavy for binbags, can be taken to the tip
- small dollops are more bin friendly
- a hole in the ground
- pour it into suitable containers to make blocks, or even small cardboard boxes
Broken up old mortar can be disposed of at the tip, used as hardcore, dropped in a hole in the ground, or where the quantity is small, put out in rubble bags.
Cement mixes are prone to leaching out salt when new. As salted water travels to the surface and evaporates, salt build up is left behind. Efflorescence is harmless, and can be brushed off or left to be washed off in its own time.
Cement contains free lime which is a skin irritant. People's susceptibility to cement varies, some people are happy to handle it with bare hands, some get burnt if they do.
There was a uk.d-i-y thread in 2008 on the best skin treatments for cement use, maybe someone can find it. These include
- barrier cream
- gloves - mortar tends to get above the gloves
- long sleeved gloves - good if ordinary gloves don't protect you enough
If you suffer badly with lime burns from cement, you could also use a pozzolan to reduce the free lime content of the mortar.
Other uses for cement
Earth may be stiffened and prevented from turning to mud by incorporating some cement. This may be used under a gravel drive to prevent it turning to mud and the stone sinking into the mud. Cement is sprinkled onto the ground and briefly raked to get a somewhat even spread, and preferably pressed down. No need to water it.
Earthcrete is earth plus cement, tamped and set hard. Earth is less stable and much weaker than sand. On the other hand most of the material used is already in situ, and this saves buying and carting large amounts of sand. Usable for non-critical low strength applications where the cost and labour saving is desirable. Subsoil should be used, not topsoil.
Hypertufa is any of various mixtures containing cement and other ingredients such as peat, perlite, and so on. It comes in a range of appearances and properties, and can be used for various purposes, including ornaments, carving, etc.
Papercrete is pulped paper, cement and usually sand. There are various other possible additives. Papercrete recipes can make anything from soft light insulation to hard mortar. The mineral content stops flame, and the lime content preserves the paper.
Cement powder contains a few minerals, and is occasionally used as a mineral specific fertiliser. Naturally its best not to add too much :) Not to be confused with a general purpose balanced fertiliser.
A little cement & water is sometimes added to coal dust to form burnable chunks. Mixes sometimes go as weak as 1:12.
Concrete is sometimes poured in situ and polished to make heavy duty countertops. It should be suitably coated to prevent staining and acid damage.
Alternatives to cement
- Lime: lime mortar, roman cement
- In a few cases resins (eg terrazzo)
- other building methods such as timber or gabions
Several substitutes for concrete also exist. See Wall Materials.
Other Cement FAQs