@Gaurav Recently a common question that popped in our heads when we were discussing the cleaning of the radiator before putting in new coolant was that how can dirt get in when the radiator is completely sealed and no outside elements can get in. I asked Naeem and he had a very good answer which I think strikes a cord.
The raw form of coolant cannot be used in the radiator as it is very thick, so some mix of water is required, this water if distilled would reduce the problem greatly, but we cannot be sure what water is used in the mix. Generally in Dubai all water including branded water on the shelves have added solid in them, like calcium, minerals and you can read the list on any bottled water. These solids over time would settle at the bottom of the radiator, so it is not dust or sand which accumulates over time, but the solids from the water which is the problem.
Hope this satisfies your curiosity.