PREMIXED PLASTERS AND SCRATCHES
Cement-based dry premixed plasters.
Now in current use they have replaced ca. 90% of traditional plasters. They are preferred to traditional ones thanks to their ease of application, their versatility as well as the speed with which the works can be completed.
In a world that is moving ever faster, this last item has great importance for otherwise unattainable manpower savings.
If you consider that a good bricklayer can create in approx. 8 working hours 15 m2 of a traditional plaster versus the 65 m2 obtainable in the same time with a premixed plaster, it becomes clear that the higher cost of the premixed plaster compared to the traditional one is largely compensated by the lower cost of labor.
These considerations obviously apply when the application is done by machine.
And it is precisely the application technique that requires a slightly different composition of the premixed mixture compared to the composition used for the traditional system.
Technical advantages and application problems of the premixed product
Pre-mixed dry plaster, being an industrial product, requires particularly accurate study and development to always ensure good results and repeatability of the final characteristics of the product.
It therefore requires the selection of the basic components of the mixture and their consistency, the reliability of the system and the accuracy and frequency of checking the finished product.
All this serves to ensure a quick and easy application with constant and reliable final results.
The difference between the composition of traditionally prepared plaster and premixed plaster lies in the different use of the raw materials available.
Traditional plaster mortar is normally prepared on site by loading a certain number of shovelfuls of wet sand, a proportional number of shovelfuls of cement and hydrated or hydraulic lime into the concrete mixer depending on the use and final characteristics.
In this case it is clear that if the preparation of the mortar is managed by an expert and conscientious construction site man, the plaster will fulfill its functions despite the inevitable lack of homogeneity between the various mixtures.
If, however, as happens more frequently, the man in charge of the concrete mixer is the most “distracted” of the team, then the results are those that we see every day on all construction sites.
Pre-mixed plaster for mechanical application does not allow for certain digressions as it arrives on site already perfectly dry mixed and reduces human intervention to purely automatic actions and, if the machine is perfectly adjusted, the result is certainly guaranteed.
However, it is clear that if the “distracted plasterer” forgets the main application rules of the premixed product, it is not a given that the result will still be good.
The conditions to be respected to correctly apply a premixed plaster are the following:
1. clean and slightly damp support in the dry period (not wet to waste).
2. regularization of absorption when supports with different porosity are present
3. avoid application in full sun or in highly ventilated places
4. apply the plaster with the lowest quantity of mixing water compatible with the right consistency for the screeding
5. do not apply the plaster at temperatures lower than +5°C and if temperatures close to 0°C are expected in the hours following its application
6. apply layers of no more than 2 cm at a time to avoid slipping and improve adhesion to the support (by compression)
7. do not apply subsequent layers on top of the previous one before this has reached sufficient consistency to support its weight
8. level and smooth the surface until the layer of plaster has a plastic consistency
9. carry out the dry leveling operations (roboting) only after the hardening has begun
10. In case of excessive evaporation, protect and possibly wet the plastered surfaces
By carefully considering the previous precautions it is clear that they are the same as those necessary for traditional plasters.
On the other hand, the advantages that can be obtained with premixed plaster compared to traditional plaster are:
1. ease of application
2. speed of application
3. homogeneity of results
4. greater mechanical and physical characteristics
5. greater final economy (a good bricklayer applies no more than 15m2 of traditional plaster in 8 hours versus 65m2 of a premixed one).
For the conditions of the advantages to occur, it is obviously essential that the premixed plaster is well designed and free from defects.
Recurring defects of premixed plasters The prevalent defects found on premixed plasters are normally attributable to an incorrect choice of raw materials, an incorrect dosage of the same and poor control of the finished product.
These shortcomings are highlighted by:
– a reduced processing time
– the tendency to slip during the application phase
– the formation of widespread cracks in the plastic phase
– widespread cracks after hardening
– poor mechanical resistance after hardening
Study and composition of the premixed product
To avoid the above defects, perfect knowledge of the selected raw materials and alternative ones is essential. It is also essential to have all the technical knowledge to be able to correctly design the plaster so that it always behaves appropriately to the application and environmental conditions.
It is also important to carry out frequent checks on both the raw materials and the finished product in order to anticipate any formulation and application problems.
The aggregates, which are the predominant part of the plaster and which constitute the structure of the system, require great care in their qualitative choice and in the development of the grain size distribution. Fundamental care for the correct setting of the final composition of the plaster.
Qualitatively, due to the mechanical resistance involved, the purity of the stone of origin or its hardness is not so important. What is important, however, is the degree of water absorption, the speed of absorption and the volumetric variation between the dry aggregate and that in the wet state and vice versa.
These last parameters are fundamental in order to verify the tendency of the aggregate to promote the formation of crazing and cracks in the plaster both in the plastic phase and in the hardened state.
Another important parameter is the grain size curve. Normally there are at least two classes of aggregate used so as to be able to “play” on their proportioning in the event that granulometric variations outside the acceptable ranges occur.
The design grain size curve must not be too coarse to avoid slipping and must not be too fine to avoid excessive stickiness during application and cracks in the plastic phase. When there are strong granulometric imbalances even the best additives cannot avoid the problems just mentioned.
Cement, another important component, must be carefully selected.
A good premixed plaster cement should preferably have a fairly rapid onset of setting and an interval between the beginning and the end of setting of approx. 3 hours. It must not be too fine and therefore require too much water and must develop good final strengths. Furthermore, it must be qualitatively constant over time.
Hydrated lime. Not essential but certainly useful. Improves wet adhesion to supports. Helps the solubilization and activity of some additives. Improves the plasticity and workmanship of the plaster. Since its dosage on the plaster mixture is quite low it is important that it is of good quality. Normally the super-ventilated type or lime flower is used.
The additives. These, normally represented by a cellulose ether, a starch ether and an aerator, are fundamental for managing the behavior of the plaster both in the plastic and hardened state. Since, for better or for worse, they are decisive in the behavior of the plaster, they must be carefully chosen according to the characteristics that you want to give to the finished product and their proportioning must be balanced with great care.
– The cellulose ether has the function of keeping the mixture homogeneous and plastic in the wet state and of retaining the water in the mixture for the time necessary for its spreading and of releasing it slowly to avoid the formation of cracks in the plastic phase.
– The starch ether acts as a thixotropic agent of the mixture in the wet state. It is calibrated to the other components in order to avoid slipping on vertical walls and falling from the ceiling.
– In addition to lightening the plaster, the aerator has the dual function of a dispersant and shrinkage reducer both in the plastic phase and in the hardened state.
When all the components of the plaster are perfectly dosed, reaching the right formulation balance, its behavior can only be excellent from all technical aspects. Only the intervention of a clumsy applicator will be able to reverse the final result of the plaster.
PENTACHEM, which has been operating in the ready-to-use plaster and hydraulic binder chemistry sector since 1977, boasts great formulation experience which it makes available to its customers. It provides them with fast and effective development and all the additives necessary for the formulation of the finished products.
Depending on their production capacity, the characteristics of the plant and/or their market, the customer has the possibility to choose from the vast range of Pentachem additives, both the individual raw materials and the specifically designed compounds.
Pentachem’s research and development laboratories and technical assistance are continuously in contact with customers to support their needs and solve their problems, distinguishing themselves for speed of intervention and competence. This allows customers to experience with greater tranquility the needs related to the problems of the finished product both in the study phase and in the application phase.
Suggested composition for a plaster based on cement and hydrated lime
| Components | Formula 1 | Formula 2 | Formula 3 |
| Cement 42,5 R | 11,0 | 11,0 | 11,0 |
| Superventilated hydrated lime | 4,0 | 4,0 | 4,0 |
| Lime 0/1,2 mm | 85,0 | 85,0 | 85,0 |
| PENTAGEL I 100/N M* | 0,14 – 0,2 | – | – |
| PENTA EC 25S* | – | 0,08 – 0,11 | 0,08 – 0,11 |
| PENTAMIX AER 205* | – | 0,02 | 0,022 |
| PENTAMIX ADX 115* | – | 0,01 | 0,02 |
* See the relevant technical data sheets under the products heading.
Comparison table of some technical data between plasters on the market and those obtainable with Pentachem formulations.
| Technical features | Plaster A | Plaster B | 1 | 3 |
| Mixing water (l /100 kg) | 24 | 22 | 22 | 22 |
| Flow (%) | 72 | 74 | 72 | 73 |
| Density (kg/l) | 1,745 | 1,830 | 1,718 | 1,685 |
| Air content (% Vol.) | 19,5 | 19,0 | 21,5 | 22,0 |
| Ease of application | great | good | great | great |
| Vapor permeability coefficient | 10,11 | 10,33 | 8,89 | 8,73 |
| Compressive strength after 28 days (N/mm2) | 3,8 | 4,1 | 4,2 | 4,0 |
| Cracks in the plastic phase | absent | present | absent | absent |
| Cracks after hardening | absent | present | absent | absent |
