Do all agree that sometimes the workability of plasters are compromised by adding even small amount of redispersible powders?
They becomes sticky and the spread, tough.
This seems to be a common belief by most dry-mix chemists, do anyone has any idea to counter such problems?

this phenomenon depends on your formulation and also on the type of redispersible powder you are using, Some powders even improve the workability. You can your supplier which powder he can recommend for good workability, but you can also overcome such problems by an admix of a suitable starch ether. All big producer of celluloseethers are also offering such additives. Another possibility is to incorporate an admix of a suitable bentonite (Optibent from Suedchemie ), and reduce somewhat the level of celluloseether. Also high dispersed silica like Wacker HDK H 15 (~0.5%) will help.
regards
Adler

rookie - stickiness of wet mortar depends mostly upon the combination of additives inside your formulation. One major influence comes from the MC used - choose the ríght grade (the ones for CTAs mostly DONT work; get assistance from your supplier). I do agree with Dr. Adler on all the other influences, additional fillers and/or fibres should not be too fine (makes sticky, too).

often, especially in renders, the molecular weight of the MC used has an impact on stickiness / workability. Next to that type and level of "modification" play a role as well.
A comparably high molecular weight can increase stickiness and decrease leveling properties.
There are many differnt products on the market and you carefully need to select the right MC as well as the type and degree of modification (which is often tailor made for a specific application). Your supplier will be of help.

in my experience, too high viscosity MCs do not only influence the workability (renders become sticky etc.) but also retard the cement. Therefore these renders have a tendency to give a hard surface but underneath it a soft, powdery interior. My best experience is with a moderate viscosity type MC (10.000 ?) in low dosage (0.05-0.1%) and some starcgether.

To rookie:as I konw,RDP can really improve the workability of plasters.
If they becomes sticky,I think it is related with the W/C,RDP dosage and also the MC dosage.

There are lots of additives in drymix.
1 Redispersible powders(RDP)
Enhanced workability,better yield distinct improvement of water retention,increased bond and surface strength, reduced shrinkage and improved flexibility,longer open time.
2 Cellulose ethers(MC) or Starch ether
Better workability, improved sag resistance, less tackiness, longer open time, less skinning.
3 Air-entraining agents
Better workability and plasticity, increased yield (more volume by introduced air volume),reduced shrinkage, improved water retention and homogeneity, prevent water separation.
4 Plasticizing agents
Reduced water demand and increased compressive strength (very important for concrete!), improved workability, homogeneity and pumpability.
5 Fibres (Cellulose, Polyacrylnitrile, Polyethylene, Polypropylene)
Improved tensile strength, strong reduction of cracking given by their reinforcing effects to mortars and concrete, consistency regulating, improved sag resistance and structure build up.
6 Setting retarders and accelerators
Regulation of setting/hydration of mineral binders in order to obtain most suitable and required working time, hydration promoting agents for improved early strength, mainly for cement based systems.
7 Hydrophobing agents (Stearates, Oleates, Silicones)
Reduced water absorbence, improved water proofing and repellent effect, distinctly better weather proof properties (rain, frost).

But customers,they tend to use less additives in drymix.how to optimize the types of drymix additives become the great problem.
So,here is my questions:commonly，RDP and MC are widely used in drymix.What is their great distinction in drymix?how to choose?
Yours sincerely
ganbo zeng
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nice to meet you! Yours ganbo