Microstructure and road performance of emulsified asphalt cold recycled mixture containing waste additives and/or cement

Waste additives have been used for emulsified asphalt cold recycled mixture (CRM) in recent years. However, the action mechanism of waste additives in CRM is not clear, the reasonable utilization of waste additives worths to be explored for improving the performance of CRM as well. Therefore, this work investigated the influence of different waste additives and/or cement on the microstructure and basic road performance of CRM. Three types of waste additives (rice husk ash-RH, fly ash-FA, flue dust-FD) with different contents (0.75 %, 1.5 % and 3.0 %) and composite additives (0.75 % cement+0.75 % waste additive) were chosen to prepare CRMs. Optical microscope, environmental scanning electron microscopy and X-ray diffraction tests were employed to characterize microstructures of CRMs, and basic road performance was obtained by air void, indirect tensile strength (ITS) at 15 degrees C, wheel tracking, low-temperature ITS and freeze-thaw ITS tests. The results showed that adding FA can accelerate the breaking of asphalt emulsion to some extent. Meanwhile, some locally continuous and denser microstructures composed of waste additives (dispersed phase) and demulsified asphalt binders (continuous phase) are formed in CRM after 2 days of curing. Besides, when the FA and cement are utilized in combination, a pozzolanic reaction can occur and more hydration products are generated in CRM. Compared with the CRM with 1.5 % cement, the CRM with 1.5 % waste additive shows higher ITS (especially for FA) and low-temperature cracking resistance, whereas the air void, water stability and high-temperature stability are lower. Compared with the CRM with only 1.5 % waste additive, the 0.75 % cement+0.75 % waste additive can effectively improve the high-temperature stability and water stability (particularly for FA), while maintaining the low-temperature cracking resistance essentially unchanged. Additionally, waste additives partially or completely substituting cement in CRM can obviously reduce carbon emissions.