Hello Robin et al,
why not just triturate directly with the concrete powder instead of mixing it up hard?
Of course.... if you want to make Garden Wall you might first want to consult the meditative proving of Mending Wall by Robert Frost. it has good rubrics on "good neighbors" and "bad neighbors".
Sincerely,
Didi Ananda Ruchira
Director, Abha Light
visit: www.abhalight.org
tel: +254 20 445-0181 / cells: 0733-895466 / 0723-869133