In simple word thermal conductivity is a measure of ability of a material (here concrete) to conduct flux of heat through concrete. The units used are j/m2s0C/m or Btu/ft2h0F/ft.
In normal concrete, this property depends largely on composition of concrete mix. The typical value of thermal conductivity is about 1.4 to 3.6 j/m2s0C/m when concrete is saturated.
Normally density of concrete doesn’t influence (or influence negligibly) the conductivity of normal concrete. But when lightweight aggregate is concerned, this value varies with density of such concrete.
In lightweight concrete there have more air voids and we know air have low conductivity; so this concrete conducts less heat through them.
Our concern is here no fines concrete, which is also of lightweight and should have less thermal conductivity relative to ordinary concrete. The typical value for this concrete varies between (0.69 to 0.94) j/m2s0C/m when the aggregate is itself normalweight.
Again when lightweight aggregate is used to produce this lightweight concrete the thermal conductivity drop down to 0.22 j/m2s0C/m.
But thermal conductivity is increased considerably when moisture content in concrete is very high. This property is important in determining thermal strain, cracking, warping (at very early age of concrete) and designing thermal insulation. We have discussed about thermal conductivity of normal concrete and some posts about behavior concrete under exposure to fire and explosion. You are invited to read these posts.