I would recommend to use mod_disk_cache as it would be faster than mod_mem_cache. Please try below settings.
<IfModule mod_cache.c>
LoadModule cache_disk_module /usr/lib/apache2/modules/mod_cache_disk.so
<IfModule mod_disk_cache.c>
CacheDefaultExpire 3600
CacheEnable disk /
CacheRoot "/path/to/cache/folder/"
CacheDirLevels 2
CacheDirLength 1
</IfModule>
</IfModule>