Depends on what kind of water was used to cut the antifreeze mix. The deposits are likely mineral residue.
I used Zerex in my radiator for a long time and found that it corroded the aluminum fitting and left deposits in the hoses.
As a personal preference I now use Volvo compound mixed 50/50. I cleaned out (flushed) the system and left distilled water in the engine for a day or so. I drained and refilled with the Volvo antifreeze. I feel better knowing that the system is protected as it should be.
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89-740 GLE_91-940 Sedan _98-V90 Wagon_2001Mazda626

Dear 94 945T,

Hope you're well. Hoping that a water-savvy chemist would reply, I stood aside.

Here are some possibilities, in no particular order. The sand-like grains might be:
- micro-granules of rubber, released as hoses deteriorated.
- carbonates, present in tap water, used to dilute coolant.
- metal, left-over from a long-ago water pump falure.

Plainly, if you're the car's only owner - so always changed hoses preventively, only used distilled water, and never had a water pump fail - these suggestions may not explain anything at all.

If the granules are rubber, they'll produce black smoke - and a strong smell - when heated strongly. If they are carbonates, they'll dissolve slowly when put into vinegar (acetic acid). If they are steel - even if rust-coated - they'll be attracted to a magnet. If they are aluminum, they should be a dull gray and won't be attracted to a magnet.

Hope this helps.

Yours faithfully,

Spook