Cement

What is the difference between cement and concrete?
A good way to think of this is cement is to flour as concrete is to a cake.  Cement is only one of the constituents of concrete and mortar.  The various basic mixes can be defined as:
• Mortar – cement, fine aggregate (sand) and water.
• Concrete – cement, course aggregate (generally 10-20mm rocks), fine aggregate (sand) and water.
These can also contain various additives (such as lime) and admixtures (such as a waterproofer).

What are admixtures and how do I use them?
Admixtures are used generally by addition to the mixing water to alter the fresh and long term properties of cement based mixtures for a specific purpose.  The most common admixtures are:
• Plasticisers:  Increase the workability of fresh mixes for a given amount of water allowing less water to be used and thus greater long term strength.
• Waterproofers:  Reduce the permeability of the mix.
It is essential that the manufacturers instructions are followed precisely as overdosing can have serious effects.

What are the precautions to be taken when mixing concrete in colder weather?
Generally, whatever the project is, work should not be attempted in temperatures below 5°C.  It is also important not to use frozen aggregates that may have been freshly brought from a colder area.  Care must be taken to ensure that temperatures do not fall below 5°C in subsequent hours as the mix may be vulnerable to a subsequent frost damage for a considerable time. Not only is it extremely difficult working in such conditions, but serious damage is likely to occur from the formulation of ice crystals within the cement matrix.  Also in lower temperatures concrete will gain strength at a much lower rate, thus causing large delays before loads can be applied.

What are the precautions to be taken when mixing concrete in hot weather?
In hot weather it is important to prevent the mix from prematurely drying prior to a desired strength being achieved.  If a mix does dry too quickly, then it may well shrink and crack, be weak, dusty and porous.  It is thus important to damp down, cover and shelter from drying winds and direct sun.  Hot weather also reduces working times significantly.

I don’t want to kill my fish by using a cement based product in my pond.  What can I do to prevent lime entering the water?
Cements contain a small amount of free lime (typically around 1%) which may leech into pond water, potentially slightly increasing pH.  To minimise lime exposure to the fish it is generally recommended that after placement allow the mix to cure and harden prior to refilling.  Then, test with a pH testing kit to monitor and replace with fresh water if required.

I have been asked for a C30 mix.  What is it?
C30 refers to a characteristic 28 day compressive strength of 30 N/mm².  It depends on a number of factors including, among others, workability requirements, aggregate properties, specified minimum cement content, use of admixtures and additives.  Nominal mix proportions for various purposes and requirements can be sourced through the Lafarge Cement Technical Helpdesk.  If a specific strength requirement is specified trial mixes are strongly recommended.

I want to make a mix that is easy to place.  Should I add lots of water to make the mix ‘sloppy’?
The addition of extra water to a mix will indeed make a mix that is far easier to place due to the higher workability.  However, major problems can occur if this is carried out.  Cement based mixes are generally stronger in the long term with the less water that is initially placed within the mix (low water:cement ratio).  With a large water:cement ratio the strength of the mix will be reduced.  Also, segregation (aggregate falling to the bottom of the mix) and bleeding (water rising to the surface) may occur, thus further reducing the mix strength and durability performance.  In general terms use the minimum water content that allows placement and finishing.

Is it true that concrete can be overworked when finishing off to a smooth surface?
Concrete can be overworked when smoothing a surface finish.  If overworking occurs (especially in high water content mixes), water is likely to rise to the surface causing a weaker layer to develop which reduces strength and durability.  It may also cause flaking of the uppermost surface layer.  It is thus important to stop smoothing a surface at the earliest time consistent with a suitable finish and use a relatively low water content in the original mix.

When using Snowcrete my mortar has turned red.  Why is this?
All of the solid constituents in a mortar (cement, lime, sand) will contribute to the final colour of a mortar.  The use of a light, clean sand and lime is thus essential if a white final mortar is required.  Trial mixes should be carried out in small disposable cups and an exact note of the various proportions and sand types marked on the cup.  This will allow consistency and stop mistakes occurring when you finally find the colour you desire.

Can I add extra water to mortar if it starts to harden on the spot board, to make it more workable again, or should I throw it away?
Once the mortar has begun to set, the addition of extra water will not make any difference as the mortar is now non-workable.  Even with or without the addition of water, if the set mix is attempted to be reworked, this will cause major damage to the structure of the mortar.  This will in turn seriously affect both the strength and durability properties and so it is thus vital that the mortar is fully placed prior to the setting time.

What is the difference between hydraulic and non-hydraulic lime?
Hydraulic lime, when mixed with water, gains strength in a similar manner to cement (chemically reacts with the water).  Non-hydraulic lime, when mixed with water, forms a putty that can only slowly gain strength from exposure to air. Hydraulic limes are typically used in soft stonework or historical structural renovations.

What is the most general rule for renders?
The most general rule for renders is that each coat is thinner and weaker than the coat preceding it.  Full mix designs and application methods can be sourced via the Lafarge Cement Technical Help Line.

How thickly and thinly should screeds be applied?
A floor screed is a layer of mortar consisting of cement and clean, sharp sand.  Traditional screeds are NOT intended as a wearing surface and as such tiles, carpets etc are needed to cover the screed.
• Bonded, to a concrete base, 20-40mm thick.
• Unbonded, over a damp proof course, minimum 50mm thick.
• Floating, over insulating material, minimum 65mm thick.

A screed has recently been placed, so when can I lay carpet on it?
Traditional cement and sand based screeds need to dry out slowly (after sufficient strength has been achieved) prior to the application of the surface finish.  Failure to do this may result in bonding and durability problems.  A rough guide is to allow one day drying time for every millimetre depth of screed.  Attempts to reduce this time by prematurely drying out the screed (e.g. heating it) are not recommended as this will cause a loss of ultimate strength and possible durability problems.

What is lime bloom and how can I prevent/remove it?
Lime Bloom is a crystalline or powdery deposit that forms on the surface of cement based products and brickwork. It is normally white in colour. Efflorescence occurs when soluble salts in the concrete, mortar, render or brickwork come into contact with water. As the construction dries out, the water dissolves the soluble salts, carrying them in solution to the surface where they are deposited when the water evaporates.  It tends to form predominantly in cold, damp conditions.

Although efflorescence is unsightly, it is not usually harmful to the concrete or masonry and can normally be removed by stiff brushing or in severe cases a mild brick cleaning acid.

Concrete or mortar is dry so why is it not strong?
The cement in concrete and mortar forms a chemical reaction with water so if there is no water present then essentially no further strength development will occur.  It is thus vital that the mix is kept in a moist condition after it has set.

Common causes of moisture loss are: high ambient temperatures, exposure to direct sunlight, drying winds and excessive suction of moisture by surrounding porous materials.

Common prevention methods are to cover with damp hessian and/or plastic sheet ing (being sure that this does not mark the concrete) to prevent evaporation, prewetting of surrounding porous materials, misting (for use especially with mortars and renders) with light water spray at regular intervals and pooling water on horizontal surfaces.

Removal of stains from slabs?
Obviously, it is better not to stain slabs in the first place, but if staining occurs then there are several solutions.  Ideally the stain should be removed as soon as possible by extensive flushing and scrubbing of the affected area.  If this is not possible to remove immediately, then physical abrasion or a mild brick cleaning acid may be required to remove.  These can do more harm than good to the slab as they may have to be excessively strong to remove the stain and hence a small trial area is recommended.  In general, the longer the stain is left, the harder it will be to remove.