Add all category as classes in foreach loop

by

Please replace:

$new->category_slug = wp_get_post_terms($post->ID, 'portfolio_category')[0]->slug;

with

$category_slug = wp_get_post_terms($post->ID, 'portfolio_category');
$cat_cls="";
foreach($category_slug as $cat_classes){
    $cat_cls .= " " . $cat_classes->slug;
}
$new->category_slug = $cat_cls;

Because the code you have written above will store only first category it will get.