<?php echo $this->Html->link($post['Post']['total_comments'].' comments', array('controller' => 'posts', 'action' => 'view', $post['Post']['slug'].'#comments'),array('class' => 'comments-icon')); ?>

echo $this->Html->link($post['Post']['total_comments'].' comments', array(
       'controller' => 'posts',
       'action' => 'view',
       $post['Post']['slug'],
       '#' => 'comments' //Or #comments only without key
      ), array('class' => 'comments-icon'));