Dry premixed plasters and smooth finishes

Cement-based dry premixed plasters


Permanently introduced in the current use they substitutedon the italian market around 90% of traditional plasters. They are now favourite to traditional plasters thanks to their easy application, their versatility and the rapidity they allow in finishing works.

In a more and more quick world this last aspect gains great importance for labour costs impossible to reach in other ways.

If we consider that a skill bricklayer can apply in 8 hours work around 15 m2 of a traditional plaster against around 65 m2 the same operator can apply in the same time with a premixed plaster, it becomes evident that the higher cost of the premixed plaster compared to the traditional plaster is widely compensated by lower cost of labour.

These aspects are obviously valid only when the application of a premixed plaster is made through the spray machine.

It is the application technique that needs a composition for the dry premixed plaster a little bit different from the composition of a traditional plaster.


Technical advantages and applying problems of the premixed plaster


Being an industrial product, the dry premixed plaster requires a careful study and set up in order to always obtain good reults and repeatability of the final features of the product.

For these reasons it needs the selection of the basic componenets of the mixture and their constancy, reliability of the plant together with frequency and care in controls on the finished product.

This all allows to ensure an easy and fast application with reliable and constant final results.


The difference between the composition of a traditional plaqster and a premixed one lies in the different use of the raw materials.

The traditional plaster mortar is normally prepared in the building site by loading in the cement mixer a fixed number of shovelfuls of wet sand, a proportional number of shovelfuls of cement and hydrated or hydraulic lime depending on the use and final features.

In this case it is clear that if the prepqaration of the mortar is followed by a skill and conscientious operator, the plaster will accomplish its functions even with the unavoidable dishomogeneity between the different mixtures. On the contrary, as more and more frequently happens, if the cement mixer operator is the more “absent-minded” of the team, the final results will be those you can find daily on every building site.

The spray premixed plaster does not allow these differences, as it arrives in the building site already perfectly mixed in the dry state, so reducing man operations to a mechanical sequence and, if the spray machine is perfectly setted, the final result is guaranteed.

It is obvious that if the “absent-minded operator” forgets the main applicative rules of the premixed plaster, a good result is not granted.


Conditions to be followed in order to correctly apply a premixed plaster are:

  1. substrate clean and slighty wet during the dry season (not soaked)
  2. make the absorption constant when substrates with different porosity are present
  3. avoid applicatoin under direct sun or in strongly ventilated places
  4. apply the plaster using the lower water amount in accordance with the right workability on the wall
  5. do not apply the plaster when the temperature is lower than +5°C or when the temperature in the hours following the application is predictable to be near 0°C
  6. apply the plaster in layers up to 2 cm thick in order to avoid sliding and to improve substrate adhesion (while compressing it)
  7. do not apply the second layer over the first until the first layer gained a consistency sufficient to bear the weight of the second layer
  8. work over the plaster surface only when its consistency is still plastic
  9. smooth the plaster surface in the dry state (rabotatura) only after the hardening stage has begun
  10. if there is an excessive evaporation protect and eventually wet the surfaces where the plaster is applied


Paying attention to the mentioned rules it is evident that they are the same necessary for traditional hand-applied plasters.


The advantages of a spray premixed plaster are:

  1. easy application
  2. quick application
  3. homogeneous results
  4. higher physical and mechanical features
  5. higher final economy (a skill bricklayer can apply in 8 hours work not more than 15 m2 of a traditional plaster against around 65 m2 the same operator can apply in the same time with a premixed plaster)

Condition necessary to gain the advantages of a spray premixed plaster is that the same plaster is well engineered and devoid from defect.


The main defects a premixed plaster can present are normally due to a non correct choice of the raw materials, to a wrong dosage of the same raw materials and to a lacking in controls on the finished product.

All these deficiences show themselves with:

-          short working time

-          sliding appearance during the application

-          formation of cracks in the plastic stage

-          formation of cracks in the hadened state

-          low mechanical strenght after hardening


Engineering and composition of a premixed plaster

In order to avoid the mentioned defects a perfect knowledge of the raw materials in use and/or alternative is necessary. It is also essential to have all technical knowledge to correctly engineer the plaster to make it always have a behaviour that fits to the applying conditions.

It is also important to make frquent controls both on the raw materials and on the finished product in order to prevent formulation and applicative problems.


Aggregates, that represent the main part of the plaster and the structure of all the system, require a great care in the qualitative choice and in the setting up of the granulometric distribution. This care in essential for a correct planning of the final composition of the plaster.

Qualitatively speaking, for the final strenght, it is not so important the purity grade of the starting stone or its hardness. The most important aspect is the water absorption power of the stone, the speed of absorption and the volumetric variation between the inert in the dry state and the same inert in the wet state.

These parameters are foundamental to verify the capacity of the inert to promote the formation of cracks in the plaster both in the plastic and in the hardening stage.

Another important parameter is the granulometric distribution. Normally at least two ranges of graulometry of the inert are used, in order to change their amount depending on the variations found in the sngle granulometry, so maintaining the final granulometry of the plaster in an acceptable range.

The design granulometric distribution can not be too much great as medium particle size in order to avoid sliding an at the same time can not be too much fine in order to avoid stickyness during the application and formation of cracks in the plastic stage. If great granulometric mistakes are present even the best addiction of additives can not avoid the mentioned problems.


Cement, another important component, must be selected with care.

A good cement for premixed plaster must have start of setting sufficiently quick and a range between start and end of setting around 3 hours.

It must be not too much fine not to increase the water demand and it can develop good final strenght. It also has to be constant in the different supplies.


Hydrated lime. It is not essential but it is certainly useful. It improves adhesion in the wet phase, helps activity and solubility of some additives, improves workability of the plaster. As its dosage in the plaster is low it becomes important that its quality is high. Normally a superventilated is used.


Additives. They are usually represented by a cellulose ether, a starch ether and an air entraining agent, and they are essential to control the plaster’s behaviour both in theplastic stage and in the hardened one. As they influence so directly the plaster, they have to be chosen with great care paying attention to the features of the final product.
- Cellulose ether maintains homogeneous and plastic the mixture in the wet phase and keeps water into the mixture all the time necessary for the application and to slow down water loss in order to prevent crack formation in the plastic stage
- Starch ether gives the wet mortar the right thixotropy. Its dosage depends on the other dosages to avoid sliding on the wall
- Air entraining agent gives the mortar the right density and also reduces shrinkage both in the plastic and in the hardening stage.


When all components are correctly dosed the plaster’s behaviour can only be very good from any technical point of view.

Only an “absent-minded” operator can change its behaviour.


PENTACHEM, a company operating from 1977 in the ready-to-use plasters and hydraulic binder field, boasts of great experience in formulating mortars, experience we give our customers. We provide our customers with quick  and efficient setup of all the additives needed in the formulation of the finished products.

Depending on its production capacity, peculiar features of the plant and/or its market, the customer can choose, between the wide range of Pentachem’s additives, both raw materials and specific compounds.

Pentachem’s Research&Development Laboratories and its Technical Service are always in contact with its customers to supply them in solving their problems with quick solutions and great competence.

This allows customers to be more quiet in facing the necessities linked to finished product problems both in the engineering stage and in the applivcative one.


Standard composition of a cement and hydrated lime-based plaster



Recipe 1

Recipe 2

Recipe 3

OPC 42,5 R




Hydrated lime




Calcium carbonate 0/1.2mm









0.14 – 0.17





0.08 – 0.11

0.08 – 0.11













*  See single Technical Data Sheet in the Products area

°  Additive recommended to avoid cracks after long time. Already included in Pentagel I 100




Comparison of technical results between plasters on the market and Pentachem’s formulations.


Technical feature

Plaster A

Plaster B

Recipe 1

Recipe 3

Water demand

l/100 kg











Wet density






Air content

% vol.






very good


very good

very good







Compressive strenght

28d   N/mm2





Cracks in the plastic phase





Cracks after hardening