<button class="btn-floating btn-large btn-floating-primary" aria-label="Toevoegen">
    <span class="mdi mdi-plus" aria-hidden="true"></span>
</button>

Button Floating

Based on the version from MDB:
https://mdbootstrap.com/components/buttons/#floating-buttons

Accessibility

Be sure to add textual content to the button using aria-label so this will be available to assistive technologies. Icons should be hidden from screen-readers by applying aria-hidden="true" to the child span mdi element. If the button is a link, then use an anchor tag instead <a class="btn-floating btn-large btn-floating-primary" href="/add" aria-label="Toevoegen">...</a>.

Using

No additional actions, aside from the markup, are needed to use this component.