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The microstructure of recycled aggregate prepared from the crushing of old concrete was studied. It was found that the recycled aggregate is covered with loose particles that may prevent good bonding between the new cement matrix and the recycled aggregate. The old cement paste that remained on the natural aggregate was porous and cracked, leading to weak mechanical properties of the recycled aggregate. Treatment of the recycled aggregate by impregnation of silica fume solution and by ultrasonic cleaning was studied with the objective of overcoming the above-mentioned limitations. An increase of ∼30 and ∼15% in the compressive strength at ages 7 and 28days was observed after the silica fume treatment. Ultrasonic treatment led to an improvement of ∼7%.
Amnon Katz (Tue,) studied this question.