[DUKE ENERGY LETTERHEAD]

Exhibit 10.21

Certain confidential information has been omitted from this Exhibit 10.21 pursuant to a request for confidential treatment filed separately with the Securities and Exchange Commission. The omitted information is indicated by the symbol “* * *” at each place in this Exhibit 10.21 where the omitted information appeared in the original.

[DUKE ENERGY LETTERHEAD]

January 4, 2010

Mr. John Dickman

Director of Purchasing

Lafarge North American Inc.

12950 Worldgate Drive, Suite 400

Herndon, VA 20170

Subject: Letter of Understanding - Between Duke Energy (Miami Fort Station) and Lafarge

Dear Mr. Dickman,

In reference to the Amended and Restated Gypsum Contract dated June 8, 2005 between The Cincinnati Gas & Electric Company, an Ohio corporation and operating owner of the Miami Fort Generating Station (“Miami Fort Station”) located in North Bend, Ohio (CG&E), and Lafarge North America, Inc., a Maryland corporation (“Lafarge”), this Letter of Understanding revises the “test method” for the testing of * * * of the contract.

Below is the new agreed upon test method for testing * * * effective per this Letter of Understanding:

* * *.

We appreciate the cooperation between the Silver Grove and Miami Fort personnel to investigate and resolve this test method issue.

Mr. Scott Walton (Lafarge)

Mr. Ben G. Ruggiero (Duke Energy)

Mr. Steven Chipman (Duke Energy)

Mr. Richard T. Wilburn (Duke Energy)