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Prompt Natural Cement Technical Information

Prompt is a rapid set cement with a wide range of controlled setting times. It is a widely versatile material and can be used safely with drinking water.

Prompt is a rapid set cement with a wide range of controled setting times
See Prompt Natural Cement in our online store

A Natural Hydraulic Binder

A NATURAL hydraulic binder is a binder manufactured from a single natural raw material, without additives. PROMPT is a NATURAL HYDRAULIC BINDER which results from firing an argillaceous limestone of regular composition extracted from homogeneous rock strata, between 800 and 1200oC, followed by very fine grinding.

It is a fast-setting and fast-hardening binder

Natural Deposit

For more than 150 years, in the Chartreuse mountains, a unique natural deposit has been exploited to manufacture Prompt. This limestone deposit, in the French Alps to the north of Grenoble, consists of folds of sedimentary strata formed during the formation of the Alps. A single geological layer makes up this unique natural deposit. It consists of an argillaceous limestone whose chemical and mineralogical compositions are consistent, precise and ideal for manufacturing cement without external additives.

Prompt is a NATURAL binder


Manufacture of PROMPT

From mining to delivery, the manufacture of PROMPT involves several successive stages


Physical characteristics of PROMPT ( NF P 15-314 )

The tables below show the average values of the physical characteristics of PROMPT obtained after completion of factory quality control inspections.

Physical Characteristics

Specific Gravity2.97 g/cm3
Apparent Density0.7 a 1.0 g/cm3
Surface Area
EN 196-67,000 cm2/g>5,000
Initial Set
(pure paste)
EN 196-31.5 mn<4
Expansion at 80oCEN 196-35mm<15
Shrinkage at 28 days
Mortar 1:1 by weight
NF P 15-4337,000 um/m<1,200
Heat of Hydration
at 1 hour
NF P 15-436120 J/g70 ≤ H ≤ 150

Compressive Strength (MPa)

AgeTest MethodAverage ValueRequirement
15 minutesEN-196-15>4
1 hourEN-196-17>6
3 hoursEN-196-19>8
1 dayEN-196-113.5>10
7 daysEN-196-122>14
28 daysEN-196-131>19
90 daysEN-196-140

Chemical characteristics of Prompt (NF P 15-314)

The table below shows the average values of the chemical characteristics of PROMPT obtained after completion of factory quality control inspections.

Chemical characteristicsTest MethodAverage Value (%)Standard Requirement (%)
Silica (expressed as SiO2)EN 196-218.4≥ 17
Silica (expressed as SiO2)EN 196-23.2≤ 4
Loss on IgnitionEN 196-29.4≤ 14
Insoluble contentEN 196-23.2≤ 6
AI2O3/Fe2O3 ratioEN 196-22.3≥ 2


The mineralogical composition of PROMPT gives it some particular properties:

Very fast setting and hardening
controllable setting time

Compressive strength after 28 and 90 days
Adhesion to all substrates

Strength gain over the long term
Resistance to corrosive water (above/equal pH 4)
Resistance to sea water

Complementary property

Very fast setting and hardening

PROMPT mortar sets very quickly
at 20 oC initial set 2 minutes – Final Set 1 minute later

Hardening is almost instantaneous
PROMPT mortar 1:1 by weight (2 vols PROMPT – 1 vol sand)

The minimum compressive strength is: 4 MPa after 15 minutes at 20 oC
This fast gain in compressive strength of PROMPT mortar is exceptional
PROMPT: a true force of nature !


Very fast setting/Almost instantaneous hardening at 20 ºC

PROMPT mortar 1-1 by weight (2 volumes PROMPT – 1 volume sand) W/C = 0.4
NB: When PROMPT mortar 1:1 by volume is used, the above strengths are reduced by 30% ; 4 MPa is obtained after 30 minutes

To delay setting time

Setting of PROMPT mortar can be delayed by the addition of a retarding agent (Caution! Many retarding agents used for common cements do not work with PROMPT)
The most effective retarding agent is citric acid (E330) sold in 80 grams bottles under the trade name TEMPO To delay setting time Tempo
PROMPT mortar 1-1 by weight (2 vols PROMPT – 1 vol sand) with addition of TEMPO
”TEMPO dosage” = one capful (7 g )/ litre of PROMPT
Setting and hardening are fast
At 20 oC : Start of setting : > 15 minutes Hardening
4 MPa is obtained after 1 hour


NB: When PROMPT mortar 1:1 by volume is used, the above strengths are reduced by 30%
with TEMPO, 4 MPa is obtained after 90 minutes.


NB: When PROMPT mortar 1:1 by volume is used the above strengths are reduced by 5 MPa

Adhesion to all substrates

Due to its high fines content, Prompt develops excellent adhesion to all types of building material ; adhesion to a damp surface is significantly better than that obtained on a dry surface ; it is improved even more if a slurry* is applied beforehand.


* A layer of mortar on the substrate of almost zero thickness used to prime the substrate before laying a full thickness of mortar.

Strength gain over the long term

As far as the development of compressive strengths over time is concerned, the 2 main characteristics of PROMPT are:
Very quick onset in the first few minutes
Continued increase over a very long period (several years)

This continued increase in strength improves the compactness of the mortar as the years progress ; this is an important factor in the DURABILITY of PROMPT-based mortars and concretes.

Hydration of the highly reactive PROMPT aluminates enables development of strength in the very first quarter of an hour ; slow hydration of the C2S generates a strength gain over several years ; this phenomenon, which provides the possibility of autogenous healing over a long period, is another important factor in DURABILITY.


Resistance to corrosive water

Resistance to pure and acidic water
The mineralogical composition of PROMPT means that there is a minimal release of lime during hydration, unlike common cements ; the resistance to pure and acidic water of a Prompt mortar is therefore excellent up to pH =4


Resistance to sulphated water

PROMPT mortar behaves very well in the presence of sulphated water.
Test pieces 4 x 4 x 16 cm were prepared ; mortar 1-2 by weight, W/C = 0.40
They were kept totally immersed in a solution of magnesium sulphate with a concentration fixed at:


This slow increase in expansion is explained by the gradual reduction in permeability of the Prompt mortar, as well as the limited release of lime during hydration; these results demonstrate the excellent resistance of PROMPT mortar to concentrated sulphated water.

Resistance to sea water

PROMPT meets French Standards for sea-water setting cements.
Having demonstrated excellent behaviour during long-term tests, PROMPT has been deemed suitable for work at sea.
It therefore has good resistance to attack from sea air.
These durability tests were performed on cubes of PROMPT mortar submerged for 50 years in the port of La Rochelle (France)
They demonstrated that the cubes were still in a completely satisfactory state of preservation


The permeability of common cement mortars remains constant after 3 months. In contrast, that of PROMPT mortar continues to decrease over time:
In parallel with the strength gain, porosity is red and waterproofing enhanced.


A multifunctional binder for construction

prompt-11PROJECTS with PROMPT mortar PROMPT is used to make fast-setting traditional mortars and renders

Fast masonry work Fixing
Bedding-in Repair work

Decoration and restoration of façades
Works with lime mixes Waterproof rendering.

PROMPT mortar Application


Mix proportions
Preparing PROMPT mortar
Recommendations for use
Some Rules

PROMPT is packed in 25 kg bags



TEMPO gives you the time to : do a good job clean your tools properly
It allows you to mix a larger quantity of mortar at one time
The compressive strength of PROMPT mortar is maintained with TEMPO
PROMPT + ”TEMPO dosage” : Work QUICKLY and WELL

Mix proportions

USUAL APPLICATION of PROMPT mortar (setting time = 15 minutes at 20 oC)
Usual mix : 1 volume PROMPT/1 volume sand.

multiply these quantities according to size of the project to be undertaken and the job time


For the following applications:
high resistance fixings
working in damp, harsh conditions or exposed to sea water
Add the water gradually so as to
obtain the desired consistency
Double the proportions of PROMPT and TEMPO

Preparing PROMPT mortar

Setting time = 15 minutes at 20 oC


Recommendations for use

Mixing Prompt Mortar ready for use.
Prepare site before preparing PROMPT mortar.
Use clean sand.
Do not mix PROMPT and sand in advance : the moisture in the sand hydrates PROMPT and triggers the start of setting.
Avoid excessive water, a main cause of shrinkage and cracking. Mortar with very wet sand will need less water.
Only prepare as much as can be used before the mortar starts to set.

Some Rules

Work on a CLEAN DAMP substrate (for adhesion).

The mortar must be laid completely BEFORE setting starts to have sufficient time to work at your convenience, use the TEMPO admixture.

TEMPERATURE influences the initial setting time : warm weather reduces it, cold weather increases it ; the addition of TEMPO means that in warm weather you still have sufficient time to complete the job.

AFTER setting starts: Do not add water, stir or smooth over: Setting only occurs once (risk of breaking the set).

CLOSE the bag carefully after use, and store it upside down in a dry place.

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