How to merge Excel address columns when some zips begin with zeros

I can successfully merge 3 columns containing a City, State, and Zip code, using a formula of =a1& “, ” &b1& ” ” &c1), where d1 is the merged column.

However, in the merged column, 5 digit zip codes beginning with zero drop the 1st zero. 9-digit zip codes leading with zero print correctly. However, most of my zip codes are 5-digit ones. Can you help me?