Watch out, hornets can sting more than once!!! Get an assistant with a water hose to back you up & use a full can!!!
I have yellow jackets under my front porch, must be in a ways, after spraying they still come & go. So 5lb of sand and cover entrance with 6" of rock & they still find another entrance. More rock, spray with water to knock down and step on them. Got a trap & took a day to get a bunch more. Wish I could see the nest.
Now for the engineering question, what can be used to fill an air space, we have expansive clay soil so a polyurethane foam would just cause the concrete to heave. Other wise to plan "C", check if th ACME Co has some dynamite.