Você precisa usar o
ExpiresActive On
directiva onde quer que pretenda aplicar Expires
cabeçalhos, por exemplo
<Location / >
ExpiresActive On
ExpiresByType image/png "access plus 7 days"
ExpiresByType image/jpg "access plus 7 days"
ExpiresByType image/gif "access plus 7 days"
ExpiresByType text/html "modification plus 5 days"
<FilesMatch "\.(gif|jpe?g|png)$">
Header set Cache-Control "public"
</FilesMatch>
<FilesMatch "\.(html)$">
Header set Cache-Control "public"
</FilesMatch>
<FilesMatch "\.(php)$">
Header set Cache-Control "private"
</FilesMatch>
</Location>