make sure you dig the bottom half of the post down further till it sits flush with the existing concrete as the depth of it atm it'll get pulled out quite easy, thats if your not already planning on doing this, also try to flare it out towards the bottom so it's not a straight cut hole to aid the fixing of the post, then use ballast and a 3 in 1 mix (3 parts ballast 1 part cement) and this should be strong enough