That's perfectly normal, expected behaviour. SQL will use as much memory as it can to improve performance.
If it's using too much, lower max server memory, and inform the windows team that SQL should be using lots of memory, and for SQL Server machines, lots of free memory would be a concern
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability