When limestone (calcium carbonate) and clay (primarily aluminum silicate) are heated together, the resulting substance is cement.

Here's a breakdown of the process:

* Limestone (CaCO3) decomposes when heated, releasing carbon dioxide (CO2) and leaving behind calcium oxide (CaO), also known as quicklime.

* Clay (Al2O3 · 2SiO2 · 2H2O) reacts with the calcium oxide and other compounds present in the mixture.

* The mixture is then ground and mixed with water, forming a paste that hardens over time due to chemical reactions and hydration.

The specific composition of the cement depends on the proportions of limestone and clay, as well as other additives used in the manufacturing process.