This research is part of an ongoing investigation on the use of Phragmites Australis Ash (PAA) in cementitious systems after the CO2 emissions reached an alarming level. The use of bio-ash materials to replace cement in mortar could benefit the environment by decreasing the dangerous gas emissions since they might be considered as eco-friendly materials. PAA was manufactured in a totally closed container where no CO2 was emitted to the atmosphere. This paper examines the dimensional stability of mortar containing different amounts of PAA as partial cement replacement. Cement was replaced with 0, 10, 20 and 30% PAA (by weight) at a constant water to binder ratio of 0.55. Prismatic specimens were prepared and exposed to different environmental conditions in order to determine the drying shrinkage, autogenous shrinkage and expansion. For the drying shrinkage, specimens were left exposed to ambient conditions. The specimens for autogenous shrinkage were wrapped in order not to allow moisture to leave or penetrate into the specimens. For the expansion measurements, specimens were immersed in water tank. The length change was monitored up to 90 days where its values decreased with the increase in PAA percentage in the mortar mix. The results indicate that PAA can be used to partially replace the cement and recommendations were made for future research.