Prompt Cement and Sea Water

Three key factors in the durability of concretes and mortars in sea-water are the final density of the material, the proportion of cement in the material and the chemical resistance to sea-water of the cement itself. 


PROMPT is nearly half as dense again as ordinary Portland Cement. Mortars and Concrete should contain at least 451kg/M3 of PROMPT in the mix design to be effective.


As the size of the PROMPT cement particles is less than half that of O.P.C. it forms a finer, less porous surface (i.e. is waterproof).


In sea-water, ordinary Portland cement is attacked by sodium chloride (NaCl) and, in particular by Magnesium Sulphate (MgSO4). This causes chemical reactions; the tri-calcium aluminate is converted to ettringite, while the residual lime is dissolved by the magnesium Sulphate. 

Since PROMPT does not contain any free lime, has a stable chemical composition and has a hydraulic index [(SCO2 + Al2O3)/CaO] greater than 1, it resists sea-water chemically far better than other cements.


PROMPT : A Natural Cement will hold fast against storms and tides.


  • Very fast setting and hardening.
  • Very fine particle size (6511 Blaine).


  • High resistance to the principal constituents of sea-water during the hydration process. PROMPT releases very little lime. It thus remains stable in the very long term.
  • PROMPT forms extremely compact concrete and mortars. The very fine particle size, and the nature of the hydrates formed, produce very dense material, especially in cement-rich mixes.
These two essential aspects of PROMPT make it the ideal cement for any work requiring  impermeability.


The fast-setting and hardening properties of PROMPT means that you can beat the tide, it will set and harden in the short time between ebb and flow.

PROMPT performs just as well when mixed with sea-water and sea-sand, provided it is free of organic matter and crushed shells.

When working with metal re-inforcements, it is advisable to treat the metal for corrosion resistance before covering with  concrete.

Because they are subject to the action of water, salt and wind, exposed to the sun and the rigours of winter, over a period of years, maritime structures undergo tremendous stress due to the changes in pressure, temperature and humidity. Durability tests carried out at La Rochelle in Brittany by the Highways and Bridges department of the French Ministry of Transport,  on 41cm PROMPT concrete cubes immersed in sea water for over 51 years showed that they were in a completely satisfactory state of preservation.


  • RE-POINTING Sea Walls and Defences.
  • CONCRETING Revetments.
  • REPAIR of Lighthouses and Buoys, sealing and rendering.
  • CONSTRUCTION and repair of slipways.
  • STOPPING and DAMMING waterways, plugging leaks in ship hulls, erecting structures in running water.
  • SHELLFISH FARMS: fixing seed shells.


1. Traditional. Using a trowel.

2. With a concrete mixer. A mixer can be used for larger quantities either by:- (a) Slowing the set of PROMPT by adding up to 11% O.P.C. do not exceed 11% if protection from sea-water is required.Or (b) Retarding the set of PROMPT by adding up to 1% Citric Acid, usually to the water before mixing. Please see the sheet 'RETARDING THE SET, The Use of Citric Acid'.In both cases, note the effect of temperature. Cold water or ambient temperature slows the set.

3. With a Spray type Rendering or Plastering machine. No particular preparation of the PROMPT mortar is required, other than slowing or retarding the set as above.

4. With a DRY shotcreting machine (when the water is added through a pre-wetting ring close to the nozzle). Please see the separate sheet 'SHOTCRETING'. By carefully adjusting the water content, it is possible to have a virtually instant set, making it possible to shotcrete UNDER WATER.


Please see the separate sheet 'MIX DESIGN'. The same principals and proportions apply when using seawater. As you are normally using PROMPT because it is resistant to sea-water, the mixes are richer, i.e. contain more cement, than is usual with O.P.C., some examples are given below:-

 POINTING 1 1  1/3rd
 CONCRETE 4 2 3 1 1/3rd
 RENDERS 2 1  2/3rds

The setting of all mixes can be retarded by up to one hour by the use of up to 1% Citric Acid, OR up to 11% Ordinary Portland Cement, AND/OR by controlling the temperature of the water.

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