Wiki: Cement

Another one for the group's perusal...
==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.
==rth==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.
==rtiliser==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.
==rthcrete==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]]
[[Category:Mortar]] [[Category:Basics]]
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.
NT
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snipped-for-privacy@care2.com wrote: <big snip>
Suggest you mention that some cement has plasticiser included (I think <some/all> Mastercrete does).
Perhaps some mention of mixing processes? E.g. mix dry stuff before water.
*ALWAYS* wear gloves - alkalinity and skin damage, even through clothing.
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Rod wrote:

The alternative to expanion joints in large runs of brick or block is ties:
These really do make a huge difference.You mention reinforcing rods, but ties are slightly different in their action. In that they reinforce a mortar joint, rather then a long section of concrete.
I didn't see a mention of teh porosty of mortar either, and the need to use additives or stronger mixes when dealing with areas that are both wet and likely to be exposed to frost.
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Rod wrote:

The plastic bagged version has far more additives than the paper bagged version AIUI. It seems its a bit of a holy war issue as to which people prefer!
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John Rumm wrote:

Do they have E numbers?
Didn't know that - which bit is the war about? The bags or the additives?
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Rod

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Rod wrote:

The additives... I have a friend who is a builder, and he always insists on the paper bag version as he says the plastic bag version one is too... "puddingy" I think his choice word is.
Not sure I have ever quite followed what the difference is supposed to be...
According to the Lafarge site though it does say of the plastic bag version "It has the following benefits compared with ordinary Portland cement:
* Lower water demand and more cohesive mix * Less tendency for water to segregate and bleed * Easier to compact, spread and finish * Enhanced resistance of hardened concrete or mortar to freeze/thaw attack"
Perhaps its the "more cohesive" he is not so keen on.
Odd they refer to this product as "Blue Circle Mastercrete *Original*"
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snipped-for-privacy@care2.com wrote:

If you buy a bag of 'mortar' in Wickes, you get a bag of sand with a small bag of cement inside & you mix the powders.. In B&Q you can buy the Hanson brand where the powders are pre mixed - I find this much more convenient for the sort of small jobs I do.
What mix ratio would this be?

Would 'aggregate' be a better word?
Could you give info on which mix ratio is best for which task?
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My main complaint with this stuff is it dries sort of pink so stands out against any existing mortar - here at least. Suppose it must be 'local' sand against something that comes from a factory elsewhere.
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It's worth turning the bag over several times before opening.

It's a strong mix -- I would guess something like 4:1. I've used it with new (hard) bricks. I wouldn't use it with soft bricks.
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On Jul 21, 8:33am, "The Medway Handyman"

I'll incorporate the various suggestions when I've got time, will just answer this one here.

I know its called aggregate, but a) a newbie isnt going to know what that means b) aggregate doesnt differentiate between sand and stone, where in this instance its necessary to differentiate. Hence stone explains it, aggregate doesnt.

There's already an article on that which is linked to. http://www.wiki.diyfaq.org.uk/index.php?title=Mortar_Mixes
NT
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On Jul 21, 8:33am, "The Medway Handyman"

For small jobs, Wickes also do pre-mixed in 10kg tubs. They look only half full but the idea is there's enough space in the tub for the water and it's designed to be mixed in the tub. They don't tell you to do this, but I took the handle off and rolled it around the floor. The lid stayed on!
MBQ
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On Mon, 21 Jul 2008 05:43:38 -0700 (PDT), "Man at B&Q"

... to an optimist they look only half empty.
:-)
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snipped-for-privacy@care2.com wrote:

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     snipped-for-privacy@care2.com writes:

Many washing up liquids also contain soluable salts, which you really don't want in any brickwork which could be subject to moisture.
I suspect it also has the opposite effect of a waterproofer.

Added to the water before mixing. Leaves a water repelling film around cavities in the mortar which prevents moisture wicking through by capilliary 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.

Waterproof PVA (which is really EVA) should be used where exposed to the weather. (Waterproof PVA is only waterproof when used in cement/mortar mixes. It's not a waterproof glue by itself.)

There are two sets - the first happens typically in 4-24 hours, and the second over about 6 weeks.

Snots, nibs, etc (at least in plastering) have different meanings in different parts of the country.

Weaker acids which you might already have to hand can work too, such as kettle descaler.

Cements usually contain chromium (and possibly other) compounds, which you don't want to spread into the ground.
Gypsom and lime plasters are OK.
===Mortar Anti-freeze==Used to prevent the water freezing, allowing setting at lower temperatures. [never used it myself]
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On Jul 21, 2:33am, snipped-for-privacy@care2.com wrote:

Lots of great input, thanks to everyone. New draft here with several new sections, incorporating most of the suggestions and more....
==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).
==Admixes== ===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.
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 soluable salts, which cause efflorescence.
===Superplasticiser=== Superplasticisers allow reduced water use, producing a stronger end result. Semi-dry mixes need vigorous compacting.
===Accelerator=== Accelerator gives quicker set and cure.
===Retardant=== Retardant gives a slower set, prolonging working times.
===Waterproofer=== Added to the water before mixing. Leaves a water repelling film around cavities in the mortar which prevents moisture wicking through by capilliary 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. ===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.
Waterproof PVA (which is really EVA) should be used where exposed to the weather. (Waterproof PVA is only waterproof when used in cement/ mortar mixes. It's not a waterproof glue by itself.)
===Mortar anti-freeze=== Used to prevent the water freezing, allowing sub-zero setting.
===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.
Polypropylene fibres are available at builder's merchants.
===Earth=== 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.
==Crack control== 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.
===Compression=== Cement mixes are very strong in compression, but very weak in tension. The first rule of crack prevention is to design such that all mortar or concrete stays in compression. This is a basic necessity for all successful concrete use.
===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. They allow slight movement without damage.
===Ties=== 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.
==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.
Chemically there are two different sets happening. The first happens typically in 4-24 hours, and the second over about 6 weeks.
===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. Weaker acids (such as citric, found in electrical appliance descalers) can also work, but are slow. These can be useful for a non acid resistant workpiece.
==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.
===EML=== ====Render==== ====Wall reinforcement==== 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 holds wall elements in place when brick or mortar is broken. It should only be used when needed, as rusting will break the wall apart eventually. The top courses of walls are most vulnerable, and least work to rebuild after rusting.
==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.
==Mixing== 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.
===By hand=== 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=== 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.
==Skin irritation== 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.
===Bagged cement=== * 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
===Premixed bags=== 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 * Tub versions can be mixed in the tub.
===Barrowed on-site=== 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.
==Floors & bases== ===Thickness=== 4" is recommended for any kind of concrete base. Thinner layers have much reduced strength.
===Screed=== 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. 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.
===Dust=== 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 approve the surface appearance. Concrete hardeners may be applied to the surface, and soak in to provide a tougher result.
==Frost damage== 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.
==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.
===Earthcrete=== 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=== 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=== [[Papercrete]] is paper and cement, with several other possible additives.
===Fertiliser=== 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.
==See Also== * [[:Category:Mortar|All mortar related articles]] * [[Mortar Mixes]] * [[Cement Mixing]] * [[Special:Allpages|Wiki Contents]] * [[Special:Categories|Wiki Subject Categories]]
[[Category:Mortar]] [[Category:Basics]]
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     snipped-for-privacy@care2.com writes:

The one I've used successfully is sulphamic acid. Never tried citric.

They're still acids.
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Andrew Gabriel
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snipped-for-privacy@care2.com wrote: <snip>

Trade name, so "Mastercrete"
<snip>

soluble
My spiel chucker suggested capillary - but I think you're right this time!

Expensive, and you don't need "per kg". It's still more expensive whatever you measure it in!
Andy (with pedant's hat)
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Another post has just reminded me.
How much cement to coal dust to make briquettes? :-)
--
Rod

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