Hemp-lime aggregate is good insulation, but cement-fiber aggregates do not have the characteristic properties of more famous composite materials. Additionally, Portland cement is highly basic and tends to weaken wood, so the hemp is not really acting as a reinforcement in this case, but a filler which reduces density. Hempseed shells (the indigestible portion of hempseed) are often preferred here. See:
http://en.wikipedia.org/wiki/Fiber-reinforced_concrete