Mastercrete vs portland?

[macadonald]

Hi guys,

Can anyone tell me if Lafarge mastercrete cement is necessarily better for indoor render mix on my walls. Not sure if standard blue circle cement is good enough as this is what I've ordered, although I can change it. I'm guessing standard portland is fine.

Also, I've ordered 2 jumbo bags of burdens building sand from B&Q. I'm supposed to be ordering brown sand, can I assume building sand is the same thing for my sand/cement mix for render?

 

[r896neo]

You can't render with building sand it is too fine will be very hard to work and skimming over it would be a nightmare.

You need to use plastering sand, which any company supplying building materials should be able to send you the right stuff if you tell them what it's being used for

The blue circle is more than adequate but obviously you need to add plasticiser. Not too much though or it will be falling off the walls. Follow the instructions.

 

[macadonald]

Thanks.

So if the plasterer asked me to get 2 large bags of brown sand for using in the scratch coat render, I shouldn't use building sand? I understood that this was all B&Q sold.

 

[r896neo]

I can't tell you for sure what B&Q sell but building sand and plastering sand are two pretty different things. You can't really lay bricks with plastering sand and you can't really plaster with building sand.

If you ring them and tell them it's for rendering i'm sure they'll send you the right stuff. Alternatively a local builders merchant will most likely be cheaper

 

[macadonald]

I've just gotten off the phone with a builder and he still reckons he renders with building sand.

Just to be clear, I'm talking about the rough scratch coat and I will then multi finish over this. Plastering sand is a finishing coat sand.

What do you suggest if not building sand?

 

[wavetrain]

Ronald, why don't you go to a proper builders merchants and look at the bagged sands.

You will find:

Bulding/soft sand
Sharp sand
Plastering sand

Simple really.

If your builder reckons he renders with building sand, maybe you need a new builder

Had you thought of googling 'plastering sand' and learn yourself a bit?

Back to the origonal question, if you use an OPC you will need to buy some plastersiser.

If available I always go for the Mastercrete type cements (or other makers equivelent), they only cost a little more than OPC. I would not waste my money with B&Q / Wickes prices

 

[macadonald]

Sorry guys for the hasty responses. Was in a bit of a panic as I'd ordered the stuff according to the plasterers recommendations and then was being advised otherwise.

Speaking to him again, it sounds like building sand is only suggested because it dries quicker? Sounds luck b%ll to me. He said that getting plasterers sand is okay as well but I will then need lime. Do lime replace the need for a plasticiser?

Or he suggested maybe mixing 1 bag plastering sand with 1 bag building sand..

The more I hear what you guys are saying, it sounds liek the best bet is only plastering sand, with lime and mastercrete rather than normal portland.

Should I get some sika concentrated waterproofer

Am I making the right decisions here?

Thanks for all your advice.

 

[r896neo]

building sand will dry quicker but when it comes time to skim it you will have to soak it over and over to stop it heating the skim so much that it goes off too fast.

If he is happy to do it that way then do what he wants but be sure he is the one who will be skimming it as well.

As or watreproofer you don't need it for indoors unless you are replastering an area that has had damp treated with a chemical dpc or something.

Lets clarify some terms. These are for general scenarios

Rendering out doors will require two coats of 4/5 : 1 plastering sand to cement. The first (scratch coat) will have waterproofer added. The second float coat will not as it needs to be rubbed up with a float and waterproofer would make this very difficult. Both will have plasticiser added or incorperated into the cement.

Indoors walls are 'Float and set' - A 4/5:1 mix just as outdoors but no waterproofer this is applied approx 10mm thick and then scratched to give a key for the finish. Then gypsum based finish plaster is applied to a final finish of 3mm ish

Lime was traditionally used in mortars and renders to aid workability but modern day plasticiser has replaced this. Some people still prefer to use lime and that is fine but they essentially do the same job.

 

[macadonald]

Thanks for that useful post r896neo.

It all sounds like what I originally understood about rendering, apart from using plasticiser instead of lime. I did some outside rendering last summer and used lime and plasticiser. No wonder it took most of the day before I could float it.

Out of interest would lime have more long term benefits as apposed to plasticiser being good during application but once dried that's it.

What would most of you use now days? Lime or plasticiser?

Thanks again for all the useful advice.

 

[wavetrain]

I thought you were getting a plasterer to do this work?

Ronald you sound like a TW

 

[geraint]

building sand will dry quicker but when it comes time to skim it you will have to soak it over and over to stop it heating the skim so much that it goes off too fast.

If he is happy to do it that way then do what he wants but be sure he is the one who will be skimming it as well.

As or watreproofer you don't need it for indoors unless you are replastering an area that has had damp treated with a chemical dpc or something.

Lets clarify some terms. These are for general scenarios

Rendering out doors will require two coats of 4/5 : 1 plastering sand to cement. The first (scratch coat) will have waterproofer added. The second float coat will not as it needs to be rubbed up with a float and waterproofer would make this very difficult. Both will have plasticiser added or incorperated into the cement.

Indoors walls are 'Float and set' - A 4/5:1 mix just as outdoors but no waterproofer this is applied approx 10mm thick and then scratched to give a key for the finish. Then gypsum based finish plaster is applied to a final finish of 3mm ish

Lime was traditionally used in mortars and renders to aid workability but modern day plasticiser has replaced this. Some people still prefer to use lime and that is fine but they essentially do the same job.

lime was not used in that way... and never has been...

 

[r896neo]

Ok, that was what i had understood it to be used for but if that's not the case then apologies. Out of interest what is it used for then?

 

[macadonald]

Wavetrain, I am getting a plasterer in and have no idea what a tw is. Don't suppose you have an answer to the question do you?

 

[stuart45]

Your builder probably want's building sand as it's much easier to work with.
A TW is a transvestite with a lisp.

 

[wavetrain]

Wavetrain, I am getting a plasterer in and have no idea what a tw is. Don't suppose you have an answer to the question do you?

Ronald, this forum is all about DIY - 'Do it yourself'. I have given you advice for you to follow, but you are not prepeared to take this up.

In the Welcome section of the forum it says:

'DIY can be both challenging and rewarding - this is where DIYnot.com aims to help. Our expert teams have brought together hundreds of information pages. Why not Do It Yourself?'

But you wish to employ a tradesmen instead, you therefore need another forum. TW stands for time waster.