Scottie's right - no salt based product is allowed airside. Not even on the airside roads.
The grit used airside has, allegedly, been shown to be digestible by engines - well, in small amounts, presumably.
GK430 hit several nails squarely on the head.
Also, LHRs problems were compounded by a/c that had been going out to depart when the bad weather hit having to return for extra de-icing treatment, so suddenly stands that were being held for incoming flights fill up and there's nowhere to put them in two shakes of a wotsit. That creates knock-on problems for ages.
Once a stand is occupied it is impossible for the machinery to go round them to treat them effectively.
However, as with anything, I'm sure there are lessons to be learned from this and HAL were very fortunate that the next day was clear blue sky which helped melt everything - eventually.