Due to the high increase in pollution levels, the use of biomass ashes become a fundamental need. These ashes are considered to be environmentally friendly since they emit less CO2 to the atmosphere than other cementitious materials. This paper is a part of a wide investigation for the use of phragmites australis ashes (PAA) as a cementitious material in mortar mixes. It examines mechanical, durability and length change properties. Length change include drying, autogenous and expansion tests. The test results for total and capillary water absorption at 24 h, at all curing ages 1, 7, 28 and 90 days, show an increase as the percentage of replacing cement by PAA increases from 0 to 30% in increment of 10. Correlations between mechanical and length change properties are also examined.