Abstract: The correlation between macroscopic mechanical strength and microstructure of fly ash cement soil subjected to water pollution and frost attack was investigated. The macroscopic mechanical strength was obtained from unconfined compressive strength tests through which the effects of water pollution type,number of freeze-thaw cycles,ratio of curing agent,and curing age were analyzed;the microstructure of the stabilized soil such as porosity and mineral composition was examined by permeability test,microscopic scanning electron microscopy(SEM) test and X-ray diffraction(XRD) test. Research results indicated that frost attack reduced the compressive strength of the stabilized soil and undermined the beneficial effect of extending curing age. Water pollution also resulted in a decreased compressive strength,and in particular,industrial wastewater had a stronger effect than sanitary water. In the polluted water environment,the high strength cementation transferred to the low strength cementation,while non-gelling crystals were produced,reducing the bonding performance of the cementitious material in the stabilized soil. As a result,the change in the microstructure impaired the macroscopic mechanical strength of the stabilized soil. In addition,with the increase of fly ash content,the porosity of the stabilized soil declined slightly while the permeability surged remarkably,which weakened the resistance to frost attack,especially in polluted water environment.

Key words: fly ash cement soil, water pollution, freeze-thaw cycles, unconfined compressive strength, microstructure