@ash ock..these are hollow blocks made of clay or concrete that are laid in suspended slabs so as to reduce the volume of reinforced concrete hence making the slab cheaper.
The ones being advertised are good as long as you involve your independent structural engineer to design for you.
The sellers also offer structural engineering services.
Their design is often rejected by my engineers. Coincidentally, both the langata building and kasarani building that recently collapsed had used these blocks.
The reason why both collapsed is coz both did not employ services of registered architects and engineers and not coz of the quality of the blocks .
As Iron Sharpens Iron, So one Man Sharpens Another.