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Author Topic: Pigmenting concrete  (Read 10874 times)

chrismahon

  • Joined Dec 2011
  • Gascony, France
Pigmenting concrete
« on: September 04, 2023, 11:11:01 am »
I'm in the process of repairing a stone step. The stone has split away due to lack of support underneath. I have the original pieces. It's a long way from that bottom step to the floor, so the intention is to add a cast reinforced concrete step beneath it to underpin the repair. Our stone here is yellow, so a grey concrete step is going to look horrible. Putting a new stone step in, or adding a stone step below isn't practical- the weight of each will be about 150Kg.


I want to add pigment to the concrete mix to make it a better colour. Doing a bit of research and I realise it may not be as simple as that. Does anyone have any experience of this to offer advice?

Fleecewife

  • Joined May 2010
  • South Lanarkshire
    • ScotHebs
Re: Pigmenting concrete
« Reply #1 on: September 04, 2023, 10:05:32 pm »
I don't, sorry, but I'm very interested in the reply.
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arobwk

  • Joined Nov 2015
  • Kernow: where 2nd-home owners rule !
Re: Pigmenting concrete
« Reply #2 on: September 05, 2023, 02:26:21 pm »
If a very close colour match is desired, I would suggest white cement (or hydraulic lime) and a light coloured sand. If your natural stone is grainy/flecked then a good mix of sand for lime-work might be worth seeking out. For both pigments and sand mixes, my local lime supplier has a colour chart to match up with the existing stone (for stone repairs) or render/mortar and then mixes up the appropriate pigment formula to go with one's chosen sand mix:  the range of pigment colours going into the mix might be a surprise !  Otherwise I guess it would be a question of getting some yellowish pigment and trialling in small batches (cubes - well cured and with well-scrubbed/finished face) to see how close you can get. I suspect you will need darker pigments than might seem obvious, but I'm no expert. The "Cornish Lime" web-site might provide some additional insight.

Another option might be to use stone "paving" slabs (re-cut as necessary and layered or applied as a facing/topping) matching your existing stone.  Yes, they will look like an obvious addition, but potentially not an eyesore.  And If you have any brickwork on your property, a lower step built to include matching bricks might also look OK.


3rd option - buy a local stone, cut to match/fit, and hire a tripod crane.  I used a tripod crane (frequently used to lift vehicle engine blocks) when I found a cut-granite 'step' buried in my front garden which I then re-used to restore the front steps (correcting some crass DIY mod's by previous owners).  It was not possible to place that "step" in original position so I positioned to make the lowest step a longer "platform/landing" with an ever so slight fall for casting-off rain and infilled with concrete capped with natural stone paving slabs which closely matched my yellow sandstone house façade. As tripod would not travel well, I used the tripod crane to "drag" the old granite block towards it's new position - it just needed crane to be repositioned several times so that the lifted block didn't end up swinging massively in the "direction of travel" !  You'd have to think about before recognising my handiwork as non-original especially as I could achieve a similar rise, ref the other original steps, for that newly formed 1st step/platform/landing


If you cannot achieve the same rise for your new 1st step, then the extended step ("platform/landing") is a particularly good option as it is less confusing to the senses leading up to a series of steps that do actually have an equal rise.


Good luck !
« Last Edit: September 05, 2023, 08:08:49 pm by arobwk »

chrismahon

  • Joined Dec 2011
  • Gascony, France
Re: Pigmenting concrete
« Reply #3 on: September 05, 2023, 07:01:53 pm »
Thank you for your comprehensive reply [member=152775]arobwk[/member] . Good point about the cement colour because that affects the pigmented result a lot. As you say, darker pigmentation will be necessary to get the final result. I had intended to use premix bags, but the formula has changed since the last lot and the one bag I bought for another job was black when wet and very dark grey when dry, so no good.


We have a concrete floor in the barn which has been coloured. I think it was mineral dyed. That has the problem of surface porosity causing variations. It's slightly patchy, but that's possibly due to scrubbing off barn owl poo?


The steps are inside (from the kitchen) and very difficult to get to. We use the room for storage and put the chickens in there when it's over 35C outside. The original colour stone was from a small totally overgrown quarry at the end of the garden (also full of ancient chemical containers and oil cans- best left undisturbed). Later additions/ extensions are much lighter stone.


The stone has red flecks in it so perhaps add coarse red sand into the mix?


I'm trying not to spend too much time on it because the clay mortaring outside is more important, but this step seems to be taking over. Good idea to cast samples of the coloured mix. I think that's the way to go because we don't want any nasty surprises when it's finished and dried. Might be able to use the trial castings somewhere else?


To make it even more complicated the steps are damp, so they look much darker anyway. They are below ground level and dry out somewhat in Summer and are wetter in Winter. There will be an element of luck in the final result I think?

arobwk

  • Joined Nov 2015
  • Kernow: where 2nd-home owners rule !
Re: Pigmenting concrete
« Reply #4 on: September 05, 2023, 08:20:05 pm »
Depends on what you consider to be "damp", but a lime mix will cure when a cement mix might not.  There are hydraulic lime variants for use in damp conditions.


[And I'm intrigued by mention of external "clay" mortaring.  What are "clay" mortars and what do they involve please, for my education ?]
« Last Edit: September 05, 2023, 08:47:14 pm by arobwk »

chrismahon

  • Joined Dec 2011
  • Gascony, France
Re: Pigmenting concrete
« Reply #5 on: September 06, 2023, 10:21:32 am »
I've just been to our proper builders yard to see what they have to work with taking a piece of the stone step with me. Their premix concrete is too dark as well. Can't use lime because of the steel reinforcement.


They have a range of colours to mix into the water, none of which match the stone for the simple reason that the 'stone' is actually a mix of small pebbles in a hard matrix of clay/sand. So I'm thinking along the lines of adding more fine white sand to the concrete (which is small pebbles and sand about 12mm maximum) plus some coarse red sand and colourant. The stone of the step isn't yellow at all, more a beige colour. All the walls are a more yellow evenly coloured stone.


Then casting a test piece (or pieces) as suggested. Wire brushing the surface after 3 days to reveal the aggregate may work and will certainly improve the anti-slip. The whole reason for this repair is to make the steps safer. After 6 years we haven't fallen down them and our luck may be running out?


Regards the clay mortar [member=152775]arobwk[/member] . It's a mix dating to pre-1790 which I have rediscovered. Used now only for restoration work on historic buildings because it is so difficult to make I suppose. The house is 1769, so this material is period correct, although never previously used on it. I'll make a new post on the subject because it's pretty complicated- clay is used for a binder normally, not a finish.

arobwk

  • Joined Nov 2015
  • Kernow: where 2nd-home owners rule !
Re: Pigmenting concrete
« Reply #6 on: September 06, 2023, 03:06:34 pm »
For lime renders, [member=23925]chrismahon[/member] , plastic mesh is used these days for additional stability:  not sure what sort of pins are used to hold it in place (?? stainless steel ?? - I've used stainless wall ties with lime mortar with no ill effect after 12 years despite the wall concerned being blasted by every storm).  I would have thought a plastic mesh bedded into the new step in various layers would keep everything very stable (if needed at all !?), but a smattering of stainless steel or plastic cavity-wall ties (which I've since come across) within the mass of your new step will almost certainly do the trick as-well.
Because the new slap/step will receive foot wear, an NHL5 grade lime might be appropriate.  I believe it should adhere well to a concrete floor, but roughen-up the concrete floor under the new step if at all concerned.


(Not wishing to sound like I'm trying to teach granny.  Once again - good luck and I look forward to hearing how it all turns out.)
« Last Edit: September 06, 2023, 04:45:30 pm by arobwk »

chrismahon

  • Joined Dec 2011
  • Gascony, France
Re: Pigmenting concrete
« Reply #7 on: September 06, 2023, 06:23:29 pm »
I would never use plastic for anything important [member=152775]arobwk[/member] . The question isn't "will it fail?" but "when will it fail?" because fail it will. You have to consider how hot things get here as well. Plastic clothes pegs last a year, so we've gone back to wood. Stainless steel stuff is special order only. Lime is sold in two grades 'hydraulic' or 'live'. I checked out the cement and there were two Spanish bags, so I asked the difference. The answer was they are exactly the same but one is cheaper because it doesn't carry the certification marks! Believe me, we live in different worlds.



I tried wire brushing 3 day old concrete and it works well. Takes out the partially set cement binder to reveal the aggregate without loosening it.


Bad news with the colourant. A €30 tub is sufficient for 20-30Kg so it will need to be in a final 2cm top layer applied whilst the rest is still fluid. Means the bulk of the step will just be concrete and only one mix required of 'fancy stuff'.


Finished the steel reinforcing. Think both the SIP140 welder and the rods are getting rather old- 40 years!

chrismahon

  • Joined Dec 2011
  • Gascony, France
Re: Pigmenting concrete
« Reply #8 on: September 15, 2023, 06:21:48 pm »
Having looked in more detail on French sites they all say that to achieve good colouration you need to use 'white cement'. I found some whilst searching the buildings for shed felt and yellow sand (neither of which they had).


There is only recently a translation for 'shed', because they didn't previously exist. But the new builds in the countryside need to put their cars in the garage (hail) and so need space for garden equipment. They now build very substantial interlocking items to cope with the extreme changes in humidity- tongue-and-groove boarding doesn't work. Arbri de jardin= garden shelter= shed.

chrismahon

  • Joined Dec 2011
  • Gascony, France
Re: Pigmenting concrete
« Reply #9 on: December 09, 2023, 09:48:56 am »
Moving on with this project I have experimented to produce a yellow cement based mortar, with great success. White cement with a 3:1 mix of fine white sand (not the very fine extra white you can also get here) which is similar consistency to washed sand in England and very fine red sand. I'm mixing a half bucket at a time. Water first, then cement, then the red which gives a pink mixture as you would expect. Adding the white and mixing sees it remaining pink until the last few moments of mixing, at which time a miraculous transformation takes place in an instant and it turns yellow! Why I don't know, but this is what the locals have been telling me happens- to get yellow, mix white and red?


So applying this to a concrete mix knowing that the ballast used with a 25Kg bag of cement is 3.5 buckets of fine white sand and 5.5 buckets of gravel. This equates to 7 buckets when mixed. So I just added a bucket of very fine red to achieve the required 1:3 ratio (which being very fine should make no difference to the finished volume) and 'hey presto' we have yellow concrete, so no colourants needed.


A white film developed on the surface of the test piece which was removed with a soft toothbrush after 24 hours. The colour match to the stone step above is perfect, the only difference is the size of aggregate pieces revealed after brushing. The concrete flecks are much smaller, so the next test will be to add coarse red as well, with a little extra cement to compensate. I'm very close now to a perfect result so won't be rushing. As a bonus, the finished volume of one cement mixer full is the same as the new step and so the job will be one mix only.

arobwk

  • Joined Nov 2015
  • Kernow: where 2nd-home owners rule !
Re: Pigmenting concrete
« Reply #10 on: December 20, 2023, 08:50:11 pm »
I seem to have lost the ability to post a "Like", but pleased to hear your formula/s getting there.

chrismahon

  • Joined Dec 2011
  • Gascony, France
Re: Pigmenting concrete
« Reply #11 on: December 21, 2023, 10:20:10 am »
Finally ended up with a workable solution @arowbwk after discovering that, when fully dried, the concrete lost its slight yellow tint. The reason is simple. There is twice as much cement to sand ratio in concrete than mortar, so it just swamps the colour. Thought about a final skim of mortar, but I don't think it will wear well with foot traffic. So back to the colourant which is 3-5% of the weight of binder (liant), being cement or lime (any more and strength is lost). The idea is to mix a very small amount just to apply a tint to the cement, rather than full colouration. So I'm hoping that <1% will be enough in 30Kg. A 500g tub of yellow was €16. Can't trial it accurately, but I will mix the cement with water in the mixer first and then add the colour which will be a bit of 'pot luck'. Once I know the percentage needed it can be repeated on other jobs here.


Have all the other materials but the yard had no cement in stock, so I await their call in the New Year. Then we get to use our bargain mixer- electric in full working order bought from a couple moving to Corfu, €50.

arobwk

  • Joined Nov 2015
  • Kernow: where 2nd-home owners rule !
Re: Pigmenting concrete
« Reply #12 on: December 21, 2023, 02:30:16 pm »
The trials and tribulations of trying to match/replicate existing !! 

 

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