The Brew Cabin

I laid some Mashall heritage buff coloured slabs. I then used some cleaner to clean everything up. The consistency seems to have been too strong and a number of the slabs are now white. I presume that they have been burnt by the acid. Is there a dye or anything that I can do to restore the colour?

Nope - I'm afraid you are what is known in the trade as "buggered".

The acid has reacted with the iron oxide based dyes and adversely affected the colouring, although I've never seen a bleaching effect before - the usual reaction with the yellow/buff dye is to darken it to a rusty brown sort of colour.

This is why I always urge extreme caution when using brick cleaning acids, and, top be fair, many of the 'trade' brands do carry a warning that they can affect coloured concrete, but some of the budget 'patio cleaners' can contain up to 2.5% Hydrochloric acid, which is enough to alter the dyes used in concrete paving, but they make no mention of this on the packaging. :(

Is there a lot of flags affected, or would it be feasible to yank them out and slot in replacements?

I bought slabs, they appear to be concrete base with a "sandstone" coating and a design stamped on. I did not take much notice when they were being loaded but when we were laying the patio, three are badly stained white and most of the rest have white speckles and patches. We tried to power wash them but no luck. What could this be and is there a remedy. Ta.

I bought slabs, they appear to be concrete base with a "sandstone" coating and a design stamped on. I did not take much notice when they were being loaded but when we were laying the patio, three are badly stained white and most of the rest have white speckles and patches. We tried to power wash them but no luck. What could this be and is there a remedy. Ta.

Sounds like efflorescence, Catherine. Is is a white, powdery deposit? If so, there's nowt you can do about it. It's a fairly common occurence with cementitious materials and it does disappear with time. However, the time taken can be anything from a couple of weeks to a couple of years.

It can be temporarily removed by scrubbing with a brush, but it will return until the supply of salts responsible for the bloom have been spent, an event that occurs as part of the weathering process.