[jalacu]

Dear chemists

I am  not a chemist or scientist just a lay person with an interest.
I would be grateful for any advice on cleaning up lime wash render from an asphalt pavement/road that has a drain leading directly to a chalkstream river which I would like to avoid polluting. My research so far leads me to Hydrochloric acid/Muriatic acid (8-10%). Would you advise this method with adequate dilution/neutralisation? If so how do you know when the acid is neutralised or adequately diluted? And if it were neutralised with Liquid Caustic Soda how do I avoid using too much Liquid Caustic Soda thus polluting the watercourse by that instead?

Many thanks in advance.

[billnotgatez]

I am not sure we can help you.
But, how big an area are you talking about.

I thought lime was used as a soil stabilizer.

[jalacu]

Hi, thanks for the reply.
The area is approx. couple of square metres.
I think what I am trying to get at is at what quantity (ppm? or %?) does HCl or Liquid Caustic Soda become toxic to aquatic life?

[jalacu]

Also is the reaction (exothermic) strong enough to damage the asphalt or indeed the HCl or Liquid Caustic Soda?

[billnotgatez]

Is this a first coat of lime applied to a brick or stone surface or pavement

Is this the kind of lime you are talking about
http://en.wikipedia.org/wiki/Whitewash
or is this better
http://www.limestuff.co.uk/pages/user-guides/how-to-apply-limewash.html

[jalacu]

The 2nd link to do with limewash render. A lime plaster/render was mixed on the pavement/road outside our house for plastering work inside. They made a mess of the pavement/road with spillage and the plaster mix won't scrub off.

[jalacu]

It is plaster mixed with hydrated lime or calcium hydroxide, Ca(OH)2) if that helps?

[billnotgatez]

I was going to suggest using Muriatic Acid in small amounts with a stiff brush that you get from the hardware store.
I would then only use just what I need to remove the wash.
But, maybe it would be better to ask the hardware store staff what is the recommended method.

[Borek]

Plaster won't dissolve in muriatic acid.

Actually I don't think it will dissolve in anything.

[billnotgatez]

I guess this is wrong then

http://www.ask.com/question/what-dissolves-plaster

[Borek]

This is an equilibrium process - in low pH solubility of calcium sulfate goes up, but we are still talking about salt that is mostly insoluble (or - at best - weakly soluble).

Calcium carbonate - when treated with any acid strong enough - will react, loosing carbon dioxide and leaving well soluble salt. Calcium sulfate will be not converted to the soluble salt, it will be temporarily dissolved, but it can precipitate back once the pH goes up high enough.

OTOH its solubility is high enough that several weeks of rain can wash it out (depending on how much of the plaster was present).

[jalacu]

Thank you for your responses.
I am confident that HCl will dissolve the lime but my principle concern is it's toxcity to aquatic life. Essentially how much HCl can you put in a river before it has negative impact on the environment? I'm imagining it wouldn't take much. This question is probably inappropriate for this forum?

[Borek]

Essentially how much HCl can you put in a river before it has negative impact on the environment?

I doubt a simple answer exists. It depends on how fast the water runs in the river and what are its buffering properties and initial pH. Every river is different.

Lime (which is most likely already converted into carbonate) will dissolve easily, it is the plaster part that sound doubtful.

[jalacu]

The plaster amount is minimal and mostly hydraulic lime. If I am careful adding small amounts of HCl and doing small patches at a time I will minimize any excess HCl. Any excess will be safely diluted by washing down the area and then further in the river. If I used 500ml of HCl at 10% and half of it was neutralized by the lime then 250ml would be diluted in the river. Is that a safe amount? I have no idea.

[Borek]

Define "river".

If you are so concerned about the aquatic life, why don't you wait several moths to see if the problem doesn't disappear on its own?