==Terms=; Cement : a grey powder that sets hard after mixing with water
; Mortar : cement plus sand (or other fine aggregate)
; Concrete : Mortar plus stone
==Storage=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.
===Lumpy cement==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.)
==Mixes=There are many [[Mortar Mixes|mixes for mortar]], concrete, and other mixes.
===Strongest mix==The strongest possible 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]].
===Mix ratios==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).
=mixes====Lime==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.
===Plasticiser==Modern chemical plasticisers do a similar task to lime, using much smaller amounts. The result is slightly cheaper.
===Washing up liquid==Basic washing up liquid acts as a plasticiser, but reduces the final set strength of the mortar.
===Superplasticiser==Superplasticisers allow reduced water use, producing a stronger end result. Semi-dry mixes need vigorous compacting.
==¬celerator==Accelerator gives quicker set and cure.
===Retardant==Retardant gives a slower set, prolonging working times.
===Waterproofer== ===Ash==Replacing some or all the sand with ash gives black mortar. [[Mortar Mixes|Black mortar]] is known for its shorter life than cement & sand.
===PVA==Adding [[Adhesive|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.
===Pozzolan==Upto 30% of cement can be replaced with a pozzolan to modify its properties. The main issue with this for [[Special:Allpages|DIY]] use is that pozzolan cements can corrode copper pipes.
===Fibres==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.
==êrth==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 gets rid of the cement like appearance.
==Crack control=Someone once said there are 2 types of concrete, that which has cracked, and that which is about to crack.
===Compression==Cement mixes are very strong in compression, but very weak in tension. So t
The first rule of crack prevention is to design such that all mortar or concrete stays in compression.
===Expansion joints==Large horizontal areas of concrete need expansion joints to avoid cracking. These are thin strips of plastic or wood dividing the concrete area.
==Crack repair====Renew mortar==The most common method of crack repair is to cut out the broken mortar & fill with new mortar. On brick & blockwork this is generally done with an angle grinder.
On historic brickwork the use of angle grinders is controversial, as it is so easy to do a fair bit of damage to soft bricks with them.
===Resin==The second mortar/concrete repair option is resin. [[Adhesive|Epoxy, vinylester or polyester resin]] is injected into the crack. Epoxy is suitable for structural repairs.
===Epoxy mortar==Epoxy mortar is used to repair cracks and damaged areas on floors.
==Strength====Compression & tension==Mortar has great strength in compression, but in tension its very weak. 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.
===Curing time==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.
===Knocking up==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.
===Size==Mortar in thin layers tends to break up.
==Snots=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 hydrochloric acid, as long as the workpiece is acid resistant.
==Colours=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.
Other additions * Coloured water based [[paint]]s (emulsion, acrylic) will colour cement mixes * Red brick dust is high in iron oxide, a permanent stable pigment. * Ash blackens cement, but can cause premature failure * Stone waste can colour cement mixes to a limited degree.
==Reinforcement=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.
==Types of cement====OPC==Most cement used in [[Special:Allpages|DIY]] is OPC, ordinary portland cement. Several other types are also used for various tasks.
===SRPC==The other one used in [[Special:Allpages|DIY]] is SRPC, sulphate resisting portland cement. Sulphates in groundwater attack OPC, but not SRPC, so its used where groundwater contact can occur.
==Other uses for cement====Stabilising ground==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.
==þrtiliser==Cement powder contains a few minerals, and is sometimes used as a mineral specific fertiliser. Naturally its best not to add too much :) Not to be confused with a general purpose balanced fertiliser.
==êrthcrete==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.
===Papercrete==[[Papercrete]] is paper and cement, with several other possible additives.
==See Also=* [[:Category:Mortar|All mortar related articles]] * [[Mortar Mixes]] * [[Cement Mixing]] * [[Special:Allpages|Wiki Contents]] * [[Special:Categories|Wiki Subject Categories]]
I know the wallplug one isnt finished, but the other types of plug I dont really know zip about, so best leave it to someone in future who does. Thats how wikis work.