From text to icon

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This was a great post and it helped me to replace the default icons. One more thing if I may…

In the following, how can I put a FontAwsome icon inside content?

th.topic-list-data.posts.num > span {
    visibility: hidden;
}

th.topic-list-data.posts.num > span:after {
    content:'a FontAwesome Icon here'; 
    visibility: visible;
    display: inline-block;
    padding-bottom: 1.2em;
}