<===> input.scss

$query-string: "(min-width: 0) and (max-width: 599px),  (min-width: 600px) and (max-width: 899px)";
@media #{$query-string} {
  .foo {
    content: bar;
  }
}

<===> output.css
@media (min-width: 0) and (max-width: 599px), (min-width: 600px) and (max-width: 899px) {
  .foo {
    content: bar;
  }
}