CLR leaving white paste behind on my brush?

I just cleaned my old cast iron tub for the first time with CLR. The stuff worked amazingly well, but it left this strange white pasty stuff caked in my cleaning brush. Any idea what it might be?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.