Relationships Between Permeability and Microstructural Characteristics of Fly Ash Mortars
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RELATIONSHIPS BETWEEN PERMEABILITY AND MICROSTRUCTURAL CHARACTERISTICS OF FLY ASH MORTARS ROBERT L. DAYa and LADISLAV KONECNYb a. Department of Civil Engineering, University of Calgary, Alberta, Canada, T2N IN4 b. Ontario Hydro, 800 Kipling Avenue, Toronto, Ontario, Canada, M8Z 5S4. Received 30 November, 1989; refereed ABSTRACT The paper describes research to evaluate permeability and the microstructure of mortars containing fly ash as a partial replacement for cement. The replacement levels of cement by fly ash were 35 and 50% by volume and 50% by weight; effectiveness of mortars made with four types of fly ash at two water/cement ratios, 0.47 and 0.65, were assessed. Three types of permeability measurements were made: (a) water permeability, (b) oxygen permeability and (c) permeability to chloride ion by the 6-hour rapid permeability test. Measurements were performed at 7, 28 and 160 days age. Strength gain in the mortar mixes was also monitored. In an attempt to explain trends observed in permeability, the pore structures of oven-dried and solventexchanged specimens were examined by mercury porosimetry. Mortars manufactured with fly ash displayed superior engineering properties when compared to the control mortars. Significant correlations were found among measurements of permeability by water, oxygen and chloride ion. Relationships were also observed between permeability and some microstructural indicators, especially the volume of mercury intruded up to a pressure of 41 MPa (6000 psi). INTRODUCTION In many instances the pore structure of concrete is the most important factor which governs the service life of structures attacked by chlorides, sulphates, acids and various other aggressive agents. Normally, attack will not proceed at an appreciable rate unless aggressive solutions, vapours or gases can permeate or diffuse into the pore structure. Permeability or diffusivity, both of which are intimately associated with porosity and pore structure, are therefore engineering properties which must be recognized as having equal, if not more, importance than strength. Over the years many methods have been devised to measure the permeability of concrete, mortar or hardened cement paste [1-6]. Nevertheless, it is very difficult to obtain accurate and reproducible results [1]; no single method is recognized as being the best. It has been proven particularly difficult to obtain consistent results when materials such as fly ash, silica fume and blast furnace slag are used because these tend to produce specimens which, at later ages, have very low permeabilities [7,8]. Mercury intrusion porosimetry (MIP) has produced useful indicators of pore structure [913] despite difficulties with quantitative interpretation of results [14-16]. Although quantitative interpretation is difficult, we now have a better understanding of the practical factors which affect the permeability of concrete, mortar and cement paste. Moreover, microstructural examination techniques such as MIP have also allowed a clearer picture of the mechanisms by w
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