Sagging of premixed plasters



The phenomenon of sagging of premixed plasters, more frequent during the cold period, is referable to the loss of initial thixotropy under the mechanical action of striking off or of the type of processing after spraying.

Of course, the mechanical action is only the means revealing the phenomenon, whilst its cause has to be searched for in the composition of the plaster and/or in the ambient situation and, not infrequently, in a bad or inaccurate application.

It is obvious that, if the cause is due to an incorrect application, the problem can be solved by giving the applicators the necessary leads.

However, in that case, the problem may be caused by:
-  supports soaked with water
-  frozen supports
-  excessive thickness of the plaster in one coat
-  smooth, not absorbing supports; resin-based concrete and building materials etc.
-  supports treated with grease, wax, products to take the shuttering down, etc.
-  bad regulation or malfunctioning of the plaster-coating machine.
-  damaged bag or worm
-  uneven supply of water
-  uneven supply of plaster

After excluding the causes mentioned above, it is necessary to investigate the origin of the problems in the environmental situation and in the composition of the premixed mortar

If the applications are carried out at temperatures above 15°C, the cause has to be sought only in the composition of the plaster

The shortcomings of the composition that bring on the sliding down may be either structural or in the formulation of the additive

Structural causes concern the type of aggregate and its particle-size distribution; the type and the quantity of cement and, when it is included, the quantity and the type of lime. The causes due to the additive concern the type and the dosage of the cellulose ether, the quantity and the type of the air-entraining agent and, when it is included, the type and the quantity of the starch ether and of the possible wetting agent.


Structural defects

When the quantity of total fine components (cement, lime and fine part of the aggregate below 0.180 mm) is less than 43 %, the plaster is structurally at risk of sliding down. And the more water is required in the batch, the higher the risk.

That’s why the total of the fine components included in a plaster usually ranges between 45 (for a morphologically semiamorphous aggregate) and 50% (for a crystalline aggregate)

Anyway, it is necessary to remember that an excess of fine components, exceeding 50%, must be supported by a proper addition of additive not to incur the formation of cracks during the plastic phase.

Also the quantity and the quality of the cement have an important role in the behaviour of the plaster. In fact, it is important to remember that the less reactive the cement is, the greater the risks of an exposure to environmental inclemency: ventilation, too hot or too cold temperatures, frost, insulation, etc. The ideal cement for ready-to-use plasters must be able to absorb and keep the quantity of water needed for its hydration and the formation of gels right from the moment when the plaster starts releasing water to the support and by evaporation. This way only, it is possible to reduce the loss of the water in the cement following the mechanical action of striking off and the consequent risk of sagging. 

Defects in the formulation of the additive 

Of course, the addition of additive is another factor having a crucial role in the behaviour of the plaster, not only in its general context, but also for its possible sagging.

A bad balance of its three basic components: cellulose ether,  starch ether and air-entraining agent may cause a sliding down of the plaster. But even an incorrect choice of the three components brings to the same outcome.

When measured out in the right quantity, cellulose ether must keep the viscosity it produces and release the absorbed water very slowly. On the contrary, if the type of cellulose ether used releases water under the pressure of the striking off, the plaster will slide down.

The same thing can be said for starch ether, in particular during the cold period, if it is not of a good quality, it is subject to a sudden drop of thixotropy with an immediate sliding down of the plaster

Always in order to avoid the sliding down, it is important that the chosen air-entraining agent produces high- stability micro-bubbles able to have a binding strength on the fine particles while reducing the density of the plaster.  

If one or more components of the added additives do not have the necessary features to keep the right thixotropy during the whole time of the laying, it takes very little to upset the balance of the plaster and to trigger its sliding down.


What Pentachem proposes to solve the problem

The descriptions above draw up a list of the possible causes.  All producers know them well and have always tried to find a solution by working on known parameters.

In spite of that, the problem occurs again, fatally, on the arrival of the cold season. In a more or less substantial way, but enough to cause several problems and forced interventions on the building site, with direct and indirect costs.

The technicians of Pentachem, who have always known the problem, have developed a new type of polymer, insensitive both to low temperatures and to the pH of the cement-hydrated lime–water system.


PENTAMIX  PA” , this is its name, develops high thixotropy in contact with water and keeps it for a long time without jeopardizing its striking off.

It allows laying the plaster and striking it off on ceilings and on (clean) concrete without any problems and laying coats thicker than usual without the risk of sliding down.

Besides, it allows and, within limits, also adjusts size-particle shortcomings as well as clear defects of the dosage or of the quality of the other additives.

Last winter was a severe “bench test” for PENTAMIX  PA, but it enabled us to assess its qualities in depth and to get satisfaction from the fact that it passed all the industrial tests.

The chart below synthesizes some of the most significant data showing the effectiveness of the product.


The data collection was carried out by using a premixed industrial plaster that had already shown clear problems of sliding down at an ambient temperature of  +5°C. It had the following percentage composition:
- cement 42,5 R                                                      11,0
- hydrated lime                                                          4,0
- calcareous aggregate 0/1,2 mm                      85,0
- cellulose ether (viscosity 20.000 mPas)           0,0900,090-----------
- starch ether                                                             0.018                                                    00,018
- sodium alkyl sulphate air entraining agent      0,01800,018


All tested formula variations showed to be pejorative for applications at temperatures ranging between +10°C and +3°C , whilst the addition of 0,003% of PENTAMIX  PA allowed spraying the same plaster, in the same conditions, at coats exceeding 3 cm without any problems.

The chart shows that, right from an addition of 0,001%, PENTAMIX  PA gives the plaster a capacity of spreading below 75%, that means beneath the critical limit, beyond which sagging starts.