Sub Concat()
Const resultCol = "[COLOR=#ff0000]K[/COLOR]", fRow = [COLOR=#ff0000]2[/COLOR]
Dim lRow As Long, r As Long, x As Long, ws As Worksheet, iStr As String, jStr As String, cel As Range
Set ws = [COLOR=#ff0000]ActiveSheet[/COLOR]
lRow = WorksheetFunction.Max(ws.Range("I" & Rows.Count).End(xlUp).Row, ws.Range("J" & Rows.Count).End(xlUp).Row)
For r = fRow To lRow
Set cel = ws.Cells(r, resultCol)
iStr = ws.Cells(r, "I").Value
jStr = ws.Cells(r, "J").Value
cel.Value = iStr & jStr
cel.ClearFormats
For x = 1 To Len(iStr)
cel.Characters(x, 1).Font.Bold = True
Next x
Next r
End Sub