How to add glyphicon icons in volt input field

i want to add glyphicon-envelope image to my input field written in volt.... i have tried this but this is not working...Please help!!!!

<label for="email">Email</label>
    <div class="controls">
    {{ text_field('email', 'class': "form-control  glyphicon glyphicon-envelope")  }}



<div class="input-group">
  <span class="input-group-addon"><i class="glyphicon glyphicon-envelope"></i></span>
  { text_field('email', 'class': "form-control ")  }}

you should stop using glyphicons and using fontawesome, or google material font