Contract

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. Xxxx Xxxxxxx

Director of Purchasing

Lafarge North American Inc.

00000 Xxxxxxxxx Xxxxx, Xxxxx 000

Xxxxxxx, XX 00000

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

Dear Xx. Xxxxxxx,

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.

Sincerely,

/s/ Xxxx X. Xxxxxx

Xxxx X. Xxxxxx, P.E.

Director - Byproducts Management

Duke Energy Corporation

cc: Xx. Xxx Xxxxx (Lafarge)

Mr. Xxxxx Xxxxxx (Lafarge)

Xx. Xxx X. Xxxxxxxx (Xxxx Energy)

Xx. Xxxxxx Xxxxxxx (Duke Energy)

Xx. Xxxxxxx X. Xxxxxxx (Xxxx Energy)