Effects of the old cement mortar attached to the recycled aggregate surface on the bond characteristics between aggregate and cement mortar

• We evaluated the bond characteristics between recycled aggregate and cement mortar matrix.
• Adhered ratio of old mortar is different according to the recycled aggregate size.
• Adequate amounts of old cement mortar attached on the surface of recycled aggregates can improve the bond quality.
• Recycled aggregate should be classified and replaced depending on the size, along with the production of the smaller sizes.

A simple but interesting experiment was carried out to evaluate the bond characteristics between recycled aggregate and cement mortar matrix. The growth shape of mortar generated on the aggregate surface was investigated using image analysis, with the number of uses of the aggregate. The quantity of attached old mortar to the surface depending on the recycled aggregate sizes was analysed. The attached old mortar is a parameter determining the quality between the recycled aggregate and old mortar matrix, but this study showed that adequate amounts of old cement mortar can improve the bond quality, due to providing a fair degree of angularity, contrary to common opinion. Moreover, recycled aggregates of around size 10 mm had old mortar of the proper amount to contribute to the enhancement of the bond quality as aggregate for concrete. This paper suggested that recycled aggregate should be classified and replaced depending on the size, along with the production of smaller sizes than about 20 mm.