Text with numbers are getting converted to numbers

When you set the .Value property of a cell, ClosedXML will call the .ToString() method of the object and proceed to interpret the value as a number, date, boolean, time stamp, or text.

To insert a number as text you have the following options:

1. Prepend a single quote character: cell.Value = "'123";
2. Use SetValue: cell.SetValue("123"); // SetValue will not try to convert to the appropriate type.
3. Change the cell's data type to text after it has the numeric value: cell.SetValue(123).SetDataType(XLCellValues.Text);
4. Set the cell's format to "@" before setting the value: cell.Style.NumberFormat.Format = "@"; cell.Value = "123";

For more information see Data Types