Here's why:

* Ionic bonds form between metals and nonmetals. Calcium (Ca) is a metal, and sulfur (S) and oxygen (O) are nonmetals.

* Metals tend to lose electrons to form positively charged ions (cations). Calcium loses two electrons to form Ca²⁺.

* Nonmetals tend to gain electrons to form negatively charged ions (anions). Sulfur gains two electrons to form SO₄²⁻ (sulfate ion).

* Opposite charges attract, so the positively charged calcium ion (Ca²⁺) is attracted to the negatively charged sulfate ion (SO₄²⁻), forming an ionic bond.

Therefore, the bond in CaSO₄ is an ionic bond.