Loose floor tiles

Since we've had the house, I've been aware that the ceramic tiles on the kitchen floor were neither the best quality nor very well fixed, as one or two sounded a bit hollow when tapped. No problem, as the master plan says the room will be gutted and refurbed comprehensively, in time.
Today, I found a pair of tiles had lifted to form a ridge where they join. OK, only a few mm but to do that, they had clearly lost all adhesion. I have scraped the grout from their edges and removed some of the tile cement beneath, so I could tape them back down for the moment, as I'm severely short on time this weekend. A brief investigation shows quite a few tiles in this area are loose, so maybe a square metre or so will have to be taken up, cleaned and re-stuck.
These tiles are not new but are certainly not as old as the house and I would estimate they are a decade or two old. They were undoubtedly laid 'professionally' (as in someone took cash to do it, not necessarily someone who knew anything) on a floor described to me by a builder as polished concrete, which seems quite robust (but who can tell?). The adhesive looks more like mortar to me than anything I've used.
So, questions: Why would this happen suddenly? It's not damp and there haven't been any serious temperature changes, neither is it a part of the floor that gets significant traffic. In other words, should I look for a cause or just put it down as one of those things?
How much do I need to clean the tiles up before re-laying? Getting the grout off is easy, as is removing the adhesive from the floor. Although any big chunks of stuff can probably be flipped of their back sides, I'm a bit reluctant to start bashing the tiles too hard to get all the adhesive off them as they would be imposible to match, were I to break one, and they're an odd size (probably 8 inches-ish square) to even source a non-matching replacement, without cutting a larger one down.
Any thoughts gratefully accepted.
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On 29/11/14 14:16, GMM wrote:

brick acid will get mortar odd if you use enough
Id say that the mortar bed was too thin and that's why they flipped up. mechanical stress?
recommend ardurit rapid set for laying floor tiles
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On 29/11/2014 14:21, The Natural Philosopher wrote:

I was wondering about that. I've never had much luck with acid penetrating far enough to do much good but it's easy enough to try.

Certainly, two tiles rising against each other seems like mechanical stress, but I can't work out how it would be imposed to squeeze them together that way, at it would seem to require differential expansion of the tiles or contraction of the floor. They have a fair grout line, but the grout seems quite rigid.

I'll see if I can source some. It's not as readily available as Mapei stuff (which probably means it does a better job!)
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On 29/11/2014 14:16, GMM wrote:

Minimum clean off and a thin film of an MS polymer grab adhesive, Sicks Like Sh*t or similar.
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Dave - The Medway Handyman www.medwayhandyman.co.uk

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On 29/11/2014 15:46, The Medway Handyman wrote:

I've a feeling the 'clean off' is going to be the challenge here, having had a little go with brick acid and got not very far. Having taken another look at what's available and found a few examples of this tile size, I'm starting to wonder whether I should work on the principle of removing a rectangle of a few sq m, cleaning up the floor (easy and reliable) and re-laying a contrasting tile in the middle. Clearly that's a bigger job (but no cuts to be done) and it seems a pity to do on a floor that will be replaced in due course anyway, but it's a more predictable process and it would mean I could deal with the fact that a couple of tiles have historic chips in them. I'm just wondering which approach would involve the least swearing....
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On 30/11/2014 10:18, GMM wrote:

Soaking the tile overnight in detergent may soften the adhesive sufficiently.
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On 30/11/2014 10:18, GMM wrote:

Just in case it's helpful to anyone: On the advice of the blokey in my local prof tiling place, I tried soaking in water. This seems to loosen the adhesive on these at least as well as brick acid does, without the attendant hazards. It still needs to be scraped and scrubbed off with a stiff brush, which takes a good bit of elbow grease per tile, so it's a slow process, but will get there in the end.
It has got me wondering whether the apparent lack of water resistance is the root of the problem, if water can creep past the grout, since we mop the floor regularly.
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On 07/12/2014 12:27, GMM wrote:

I think maybe you've answered your own question there. Tile adhesive/grout probably weakens slightly in prolonged contact with water, but mopping is hardly that.
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