Difference between revisions of "Cement"
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The strongest mix is 3:1. More cement than that results in microcracking as it sets, resulting in less strength rather than more. There are ways to make stronger mixes, but they're non-trivial and not used in [[Special:Allpages|DIY]].
Revision as of 13:05, 20 November 2008
- a grey powder that sets hard after mixing with water
- cement plus sand (or other fine aggregate)
- Mortar plus stone
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.
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 harmless.)
There are many mixes for mortar, concrete, and other mixes.
The strongest DIYable mix is 3:1. More cement than that results in microcracking as it sets, resulting 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).
Adding lime to cement mixes does not give it the properties of lime mortar. Lime acts as a plasticiser, making the mixture stickier, and much more able to hold its position.
Cement to lime ratio should be 1:1, as 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.
There are some cement powders that 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.
EVA (sold as waterproof PVA) should be used where exposed to the weather. (EVA is only waterproof when used in cement/mortar mixes. It's not a waterproof glue by itself.)
Used to prevent the water freezing, allowing sub-zero setting.
Upto 30% of cement can be replaced with a pozzolan to modify its properties. The main issue with this for DIY use is that pozzolan cements can corrode copper pipes. They are also slower curing.
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.
Polypropylene fibres are available at builder's merchants.
Earth gets used as an additive in 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.
Someone once 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 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.
Large horizontal areas of concrete need expansion joints to avoid cracking. These are thin strips of plastic or wood dividing the concrete area. They allow slight movement without damage.
Wall ties are available that consist of a vertical bar with sliding horizontal protrusions. These connect 2 tied walls 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 suitable for structural repairs.
Epoxy mortar is used to repair cracks and damaged areas on floors.
Tile grout can be used to fill floor cracks.
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 used 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 considered 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.
Unmodified mortar has very little stickiness, and a percentage always falls off during application. Its best to let it dry in place rather than wipe it off, since 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 will harden and become a problem to remove.
Any remaining staining can be removed with a strong acid (hydrochloric, sulphamic, etc), as long as the workpiece is acid resistant. Weaker acids (such as citric, found in electrical appliance descalers) can also work, but are slow. These can be useful for a workpiece not resistant to strong acids.
Most cement is grey. White cement is also available at higher price. 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.
- 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.
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.
4" wide EML can be buried in mortar courses in garden walls to reduce the damage caused by vandals. It can not 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.
Types of cement
Most cement used in DIY is OPC, ordinary portland cement. Several other types are also used for various tasks.
The other one used in DIY is SRPC, sulphate resisting portland cement. Sulphates in groundwater attack OPC, but not SRPC, so its used where groundwater contact can occur.
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 any effort. These basic rules are less critical but should still be 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.
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, and some will never use it without wearing gloves. Barrier cream is also fairly popular.
There was a uk.d-i-y thread in 2008 on the best skin treatments for cement use, maybe someone can find it.
Buying cement, mortar & concrete
There are a few ways to buy it, each with various pros & cons.
- You can make up any mix you want
- You know exactly what you're getting
- You have to do the mixing
- You have to do the placing
- The bags are heavy
- 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 epxensive 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
- Some suppliers add excess water since you're paying by volume - and this leaches cement out as it drains away
- 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, and 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.
Floors & bases
4" is recommended for any kind of concrete base. Thinner layers have much reduced strength.
Concrete floors for houses 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 commercial 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.
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.
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
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. There's no need to water it.
Earthcrete is earth plus cement, tamped and set hard. Earth is less stable and 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. Suitable for non-critical 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 paper and cement, with several other possible additives. Papercrete recipes can make anything from soft light insulation to hard mortar.
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.
Terrazzo is concrete flooring into which stone pieces, usually marble, are pressed. After setting the surface is cut and polished. The result is pretty and hard wearing, but vulnerable to staining unless coated. Correct choice of cleaning chemicals is important.
Alternatives to cement
- Lime: lime mortar, limecrete
- In a few cases resins (eg terrazzo)
Several substitutes for concrete also exist. See Wall Materials.
Other Cement FAQs