White on grey slab mix?

[JohnRoss]

Trying to pour some exterior concrete slabs and am a bit stunned by the difference in price of white cement compared with grey at the local Brico, white about 19 euros a sack and the same quantity in grey like Lafarge about 7.8 euros. Any expert out there who knows if I can make the slab mostly using the grey cement and then pour a mix on top using white cement while the grey mix is still uncured, i.e. as soon as I have finished pouring the grey cement mix. Will the two mixes bond and will the grey discolour the white mix surface?...........JR

[Braco]

Bricoman have white cement @ 10.11 per 25kg bag (under 4 for grey) 

[broy]

I don't see why not, I've laid slabs just like that so it must be possible.

[JohnRoss]

Good prices Braco as the ones I quoted were for 35kg sacks but yours were significantly cheaper, not seen a Bricoman shop around here, Deux Sevres. Thanks for the info broy so I will give it a go and see what happens. I gather you can buy fibres to strengthen the mix rather than use rusty metal rods, rebar, any experiences with that?...................JR 

[Chancer]

I have used the fibres to do my window cills and they havnt cracked but maybe they wouldnt have anyway, they are a complete rip off though and mine came from floor sweepings of a GRP fabrication shop. If you look really carefully you can see wisps of fibre appearing where the corners are eroding, some little bits of concrete mould flash are hanging on by the fibres like tagnuts on a dog!

Will using white cement actually make your concrete any whiter? wont you have to use white sand? In any case wont it just discolour anyway? I always find that standard concrete looks pretty white after a coup de Karcher, that is the bits that remain around the exposed aggregate. 

[Braco]

Hi JohnRoss

I am a highways civil engineer by training - though I have spent the last thirty years on the management side.

What exactly are you building -size/depths/use?

The best way to start is to design your mix/formwork/joints.

We will shortly be concreting an area near the pool - 50m2 x 0.175.

As this area is only to be used for sun beds I will use a C15 mix for the first 100mm and top off with a 75mm mix of white cement/white stone chippings/sand. The slab will have two expansion joints.

 

[Braco]

Another option is the addition of pigments to the final layer.

CONCRETE PIGMENTS
Here is a list of our most popular concrete dyes. Also refered to as cement dye the powdered pigment is packaged in 25kg bags and is used to colour precast concrete by adding it to the concrete as it is being mixed. These Iron Oxide pigment blends are inter mixable and are the professional choice for colouring concrete.
Order total (Any combination of colours)
COLOUR 30+BAGS 10-29 BAGS 1-9 BAGS   £ P £ P £ P YORK BUFF 18.00 21.00 23.00 BUFF 21.50 24.50 26.50 DEEP BUFF 21.50 24.50 26.50 YELLOW 24.00 27.00 29.00 STRONG YELLOW 27.00 32.00 34.00 NUT BROWN 20.00 23.00 25.00 LIGHT BROWN 32.00 35.00 37.00 BROWN 24.00 27.00 29.00 RUSTIC BROWN 31.50 34.50 36.50 DARK BROWN 33.00 36.00 38.00 ORANGE 31.00 34.00 36.00 PLUM 27.00 30.00 32.00 RED 24.00 27.00 29.00 TERRA COTTA 24.50 27.50 29.50 STRONG RED 130 26.50 31.50 33.50 STRONG RED Y101 27.50 32.50 34.50 STRONG BLACK 27.50 30.50 32.50 STRONG WHITE 81.00 84.00 86.00 CREAM 42.00 45.00 47.00 YORK GREEN 30.00 33.00 35.00 SAGE GREEN 26.00 29.00 31.00 OLD YORK GREEN 32.00 35.00 37.00

[JohnRoss]

Ok it is a base for an outside cat run so our boys, house cats, and their adopted auntie can get some fresh air. Not load bearing other than the weight of three cats and me when I go in to clean up. I have tried the two layer approach of grey and a toping of white which seems to work to some extent. The project is a bit of an experiment as I have not used white cement before and if this is successful I want to cover an old concrete base in the garden with slabs cast on top using the white concrete and a very small amount of colouring agent powder which is stocked locally by Bricomarché.

I did notice that when being a bit clumsy removing the form from one of the slabs that one of the corners of the white concrete came away cleanly from the grey concrete underneath which might indicate that the two layers had not fused properly but time will tell I guess. I will keep the slabs damp for the next few days. I am using a 3 to 1 mix for both types of concrete and the slabs are 5 to 6 cms thick and 1 metre square and have rusty iron grids inserted. The ground has not been disturbed for many years and is quite firm and whilst I accept that they are a bit thin I am hoping that in the event that they move cracking will take place in the joints between the slabs and not the slabs themselves. Anyway an interesting experiment and time will tell!.......................JR