A borehole designated ERDA-10 was filled with portland cement based grout in October 1977. The borehole is located in Eddy Co., N. Mex. The hole was filled with grout of four compositions, three of which were similar and made with salt water as mixing water; the upper portion of the hole was filled with a grout made using fresh water as mixing water. During the filling operation, test specimens were cast using these grouts. The deepest plug (plug 1) was cored 48 hours after placement. During the filling operation, test specimens were cast using all four grout compositions. The core from plug 1 and the test specimens were sent to the Structures Laboratory of the U.S. Army Engineer Waterways Experiment Station for physical tests and petrographic examination. They were stored in the laboratory at temperatures ranging from about 23 C to 54 C. Tests and examinations were made at three ages: (a) 23 + or - 8 days, (b) approximately 1 year, and (c) approximately 3 years. This report presents the results of the petrographic examination which included use of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The grouts made with salt water formed tetracalcium aluminate dichloride-10-hydrate (C3A(CaCl2)H10) at the expense of ettringite during continued hydration. The C3A(CaCl2)H10 was already present when the grouts were examined at the early ages (23 + or - 8 days).