Looping through an array and placing something every somewhat steps into an array

So I have been busy working on an assignment lately, now I'm programming in Visual Basic, Using Visual Studio 2013 Update 4, and I'm working in the .NET Framework.

Here's my problem/question: What I want to accomplish is that I run through an array of 81 (length) consisting of only characters. I want to iterate through this array and after 9 steps of my for-loop I want to save those 9 characters into a string.

With this I mean I want to save characters 0-8 in a string, then 9-17 in another string, and so on…

(Array will be filled in my program)

Dim charactersArray(81) as character

For intIndex as integer = 0 to 81
'Add 9 characters into a string
Next

I have tried a-lot to accomplish this but have failed to find a solution yet, I have searched all over the internet but I couldn't find a solution. So hopefully anybody here can help me out. :D

(Pretty much I'm asking you to make a little algorithm for me :/ )