From article: “In his report to the public inquiry, Professor Luke Bisby said evidence ‘strongly supports’ the theory that the polyethylene material in the cladding was the primary cause of the fire's spread.” and “The ACM (aluminium composite material) product on Grenfell Tower incorporates a highly combustible polyethylene polymer filler which melts, drips, and flows at elevated temperature.”.

I am not a materials engineer, but it looks to me that bricks with more than 10% of polyethylene are likely to be flammable in the case of a house fire.

I don’t know whether the Grenfell tower cladding had filler, but it is common for the polyethylene core to be mixed with filler within the cladding to help protect against fire, for example the ALPOLIC A2 product mentioned in [1] has an image showing a table with:

ALPOLIC fr: Combustible ingredients < 30%. < 13 MJ/kg

ALPOLIC A2: Combustible ingredients < 10%. < 3 MJ/kg

And [1] says “Its calorific value is given as something less than 13 MJ/kg, more than four times the value considered to be of ‘limited combustibility’ in the UK”.

Above facts are why I think even a large percentage of sand might still leave flammable bricks (polyethylene is like solid petroleum, until it melts and behaves more like a liquid or wax with a viscosity depending on multiple factors).

[1] http://theriveroflife.com/2018/02/08/grenfell-tower-is-the-c...