Mortar with the addition of ash, either because a black color is desired for pointing, or to reduce cost.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved
black ash mortar, black mortar
A mixture of high-calcium lime, water, and ashes or clinker which relies on its pozzolanic properties for its hard set.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.