Here is a link to a photo of one of the pyramids.
I see two things that tell me that this appears to be limestone, not cement/concrete
1. Differences in stratification between different layers
2. Differential weathering between different layers
3. Differences in grain size between different layers
Stratification is marked by the appearance of horizontal layers in a rock, most commonly sedimentary rocks (rocks formed by sediments/sedimentation).
It results from several causes. Some rocks are naturally stratified such as laminated limestones formed in ancient mudflats or cross-stratified
sandstones formed in stream beds. But some stratification forms from the sediments being covered by large amounts of additional sediments which causes
compaction of the buried sediments, forming a layered structure to form. Most of this is natural layering, but it can also be caused by a process in
carbonates (limestones and dolomites) called pressure solution.
To me, the rocks in the photo look like stratified limestone. I would not expect to see stratification in an artificial stone. An artificial stone is
not going to compact because it is basically already solid once it has formed. And the weight of stones above it is going to be far below what is
necessary for the geological process of compaction to take place. Also, I would expect an artificial stone to be rather homogeneous, not full of fine
layering. Layering in an artificial stone, if that is even possible, would probably be something to avoid because it would probably weaken the
Differential weathering refers to the process where different parts of a rock weather at different rates. In the case of limestones, layers which are
predominantly carbonate will weather slower, whereas layers containing a lot of clay minerals will weather faster. I suppose that could be reversed in
some climates or situations. The point is, some layers often weather faster, producing a layer which is recessed back from the face of the stone.
Cement/concrete, I would assume, would tend to be homogeneous. You're not going to have a compositional difference resulting in different rates of
weathering for different parts of the stone. An inhomogeneous artificial stone would tend to be weaker, so this situation would tend to be avoided by
someone manufacturing the stone. If you had someone build a garage for you out of concrete blocks, would you want to look at it twenty years later and
discover that some portions of each concrete block were now weathering away and are now recessed back from the face of the block?
Now look at the photo in the link above. Look at the stone to the little boy's right (our left). You can easily see both Stratification and
Differential Weathering. The top half of the stone is jutting outward more than the bottom half (=differential weathering) because the lower half of
the stone weathered faster. There is obvious layering in the lower half of the stone (=stratification).
LARGER GRAINS IN THE LOWER LAYER
There is one more difference in the two layers. The upper layer is extremely fine-grained (apparently micrite). The lower layer has some larger grains
which are most likely small fossils.
If you now look above the little boy you will see stones that are similar to the one beside the boy. In all three cases the more weather-resistant
part of the stone was placed at the top. I believe that this was done intentionally for an obvious reason. (I won't explain something so obvious)
These are the two layers in the three rocks under consideration:
More resistant to weathering
Very fine-grained rock with no apparent large grains
Less resistant to weathering
Coarser-grained rock that the above layer
All of this tell me that this is limestone, not artificial stone.