Using Prompt In Concrete Repairs

PROMPT is an ideal cement to make mortars or concrete to repair existing concrete structures. They will not be subject to carbonisation, and will effectively prevent further deterioration of any exposed reinforcing bars. They usually form a smooth, hard, waterproof surface that is resistant to Seawater, Road Salt and attack by many other chemicals. They link well with Portland based concrete exhibiting very good bond characteristics, setting hard very quickly, with a low shrinkage value.

PROMPT can be used with the same aggregates and in the same way as a Portland cement, but because it is generally being used in difficult circumstances, and because of its special properties, it is recommended that, in comparison with O.P.C. mixes, the following points are noted:-

  • The proportion of cement used IN THE MIX is greater than with O.P.C.
  • The water content is A LITTLE LESS than with O.P.C.
  • The mixing is more vigorous AND PROLONGED than with O.P.C.

A usual setting time for a PROMPT mortar at 21oC would be 21/2 minutes. It is therefore vital that the work surface is prepared in advance, and that very small quantities are mixed at a time. In most circumstances setting will need to be retarded. The example mix given below is retarded to a 25 minute set. Please see the section 'RETARDING THE SET' for guidance in controlling setting times.


Mix proportions can be designed to suit specific requirements. The user may contact THE NATURAL CEMENT Co. Ltd. for assistance. The following is intended as a guide for a general purpose mix.
  • 11 kgs PROMPT 
  • 11 kgs Sand (A 51/51 mix of Soft and Sharp sand will improve the mix)
  • 25 gms Citric Acid (1/4 Bottle of TEMPO)
  • 2.5 LITRES CLEAN Water (Sea or River Water may be used, but must be free of organic material)

Surface Preparation

Any loose or unstable material should be scraped out. All surfaces should be clean, free from dust and soaked with water. Any exposed metal surfaces should be cleaned with a wire brush, and may, if preferred for extra protection, be coated with a layer (+/- 5mm) of PROMPT paste (3 parts Cement, 1 part Water). Primers and Bonding Coats are unnecessary in most situations, but may be used in the same way as for Portland cement. If the sand is wet, the amount of water should be reduced accordingly.

Mixing Method

1) If you are using TEMPO, it may be added to the cement powder. Add Citric Acid into the Water, it will dissolve immediately.
2) To the water in a bucket add first the sand and then the PROMPT a little at a time.
3) Mix very thoroughly by hand or, preferably, with a mixing attachment fitted to an Electric Drill (1111W 811RPM), for at least two minutes. At first the mix will seem too dry. As the mixing proceeds, an abrupt change in fluidity is seen.
4) The final mortar should have the consistency of Putty. The mix may be adjusted by adding VERY SMALL amounts of water or cement. If a more fluid mortar is required, a chemical superplasticiser, of the type based on Naphthalene should be used. 
5) Immediately wash out all equipment.


1) Using a small tool, pointing trowel, or gloved fingers, push the paste firmly onto the substrate, making sure that all pockets and gaps are filled.
2) Build up layers of mortar paste a little at a time, pressing firmly into the previous layer.
3) Work from the outside of the area towards the middle. If it is intended to finish to a smooth surface, finish work 2-3 mm short of the surrounding surface level.
4) If the mix has been correctly prepared, slumping should not occur. If it does, allow the surface to harden, and then continue with a fresh mix.
5) Remember that setting (of this example mix) will start within 25 minutes at 21oC. It will be quicker in warmer weather, and slower in cold. Before setting starts, cease work, and prepare a fresh mix to continue.

Surface Finish 

If a fine surface is required, it is recommended that the work is finished with a skim coat of 2-3 mm of a mortar made in the same way as the repair mortar, but using a fine soft sand. In cases where the surface is liable to exposure to water, or particularly aggressive conditions, we recommend that the finishing coat is made of a very strong mix, i.e. half the quantity of sand. In either case, the mortar should be stiffer than O.P.C. mixes, but the surface may be finally finished with light pressure from a dry trowel. DO NOT attempt to polish or float a surface with a wet trowel.

Once dry, the surface may be painted, or otherwise treated, in the same way as any other concrete or mortar. PROMPT is light beige-grey in colour, which changes to a yellow beige after 2 or 3 days in sunlight. This gradually deepens over a period of years to ochre yellow. If this is not compatible with surrounding finishes, you may add any normal cement colour to PROMPT before mixing. Alternatively, the repaired area may be brushed with an O.P.C slurry, or the surrounding area brushed with a PROMPT slurry.


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