The leaching mechanism of hydraulic mortars as part of autogenic self-healing process

Abstract

Historic mortars have exhibited long term durability which has been largely attributed to their self-healing properties. The aim of this work is the study of the autogenic self-healing mechanism as observed in historic mortars as well as the simulation of the phenomenon in laboratory prepared lime-pozzolan and natural hydraulic lime mortars. One of the most significant parameters in autogenous self-healing phenomenon is the source of the calcium ions, which controls the formation of new healing phases inside cracks. In this study, an electrochemically accelerated leaching procedure was designed to examine the influence of mineralogy and curing time on the dissolution of Ca-bearing phases of lime-pozzolan and NHL mortars, which are characterized by high portlandite content. It was found that, apart from portlandite, Ca+2 are also leached from C2S and C–S–H phases and it was shown that this phenomenon takes place in two separate time-frames: an early period (t < 28 d), which is controlled by the portlandite content of the binder and, a later period (t > 28 d) where decalcification of C–S–H and dissolution of secondary portlandite are dominant. The phenomenon was further studied on mortar specimens where healing of micro cracks was observed by precipitation of secondary calcite inside cracks and voids, highlighting the role of leached Ca+2 on the autogenic self-healing mechanism. It is proved that, contrary to what is believed about the autogenic healing, besides portlandite, the contribution of hydraulic phases is also significant. Overall, this study confirms the significant role of the Ca-leaching process in the autogenic self-healing mechanism and highlights the different sources of calcium ions, as well as the different time frames of the events. Finally, the findings elucidate the high self-healing potential of traditional mortars which contribute to their prolonged service life.