Abstract
The fly ash microspheres (FAMs) formed during the mineral transformation stage in coal combustion are hollow spherical particles with a density less than water. This paper presents the results of X-ray micro-computed tomography and an automatic image analysis system of the porosity in the structure of hardened concrete with microspheres. Concrete mixtures with ordinary Portland cement and two substitution rates of cement by microspheres—5% and 10%—are investigated. For all considered mixes, a constant water/binder ratio (w/b) equal to 0.50 was used. The distribution of the air voids and the compressive strength of the concrete were tested after 28 days. With the increasing mass of cement replacement by FAMs, the compressive strength decreases after 28 days. The total volume of the air voids in hardened concrete with fly ash microspheres tested by X-ray varies from 5.1% to 7.4%. The closed pores constitute more than 80% of the total content of air pores. The study proves that the use of microspheres grains with specific dimensions has a significant impact on concrete porosity. Their application in concrete technology can be an alternative aeration solution for fresh concrete mixes and an effective method for utilization.
Highlights
A number of studies have proven that it is difficult to preserve appropriate frost resistance of concrete and reinforced concrete structures [1,2]
A clear improvement in frost resistance is obtained by reducing the water/cement ratio (w/c) with an appropriate aeration of concrete mixture
The frost resistance of concrete depends on the content and structure of air voids
Summary
A number of studies have proven that it is difficult to preserve appropriate frost resistance of concrete and reinforced concrete structures [1,2]. To ensure frost resistance of concrete in exposure classes XF2–XF4, the European standard EN 206–1 [3] requires an air content in fresh mixture of at least. Dosing a larger amount of air usually increases the concrete porosity and reduces its strength. A clear improvement in frost resistance is obtained by reducing the water/cement ratio (w/c) with an appropriate aeration of concrete mixture. The frost resistance of concrete depends on the content and structure of air voids. The achieved resistance is the result of a specific balance between capillary pores filled with water, capable of freezing, and pores, which are not fully filled with water. There is an opinion that during water freezing in concrete structures, the ice pressure in pores is transferred to the inside of pores, including air
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