Final answer:
The cement that emits an exothermic reaction when mixed with water is slaked lime, produced from the reaction of quicklime with water. It's mixed on a cool, thick glass slab to dissipate heat.
Step-by-step explanation:
The cement referred to in the question that gives off an exothermic reaction and must be spatulated over a wide area of a cool, dry, thick glass slab is slaked lime or calcium hydroxide, Ca(OH)2. When quicklime, CaO, is mixed with water, an exothermic reaction occurs, producing slaked lime. This reaction releases a significant amount of heat, hence the need to carry out the mixing process on a wide area to dissipate the heat and prevent the glass from breaking due to thermal shock.
In the construction industry, slaked lime is essential for making plaster and mortar. The wide area of a cool, dry, thick glass slab not only allows for efficient mixing but also serves to absorb the heat generated by the reaction. This practice ensures that the mixture does not become too hot, which might lead to the rapid setting of the mixture, and it helps in preventing the glass slab from damage.