Influences and mechanisms of mixing process on properties of the fresh and hardened Type II cement asphalt (CA) mortar

CA mortar, as a cushion layer widely used in ballastless slab track, is a complex composite material. Apart from the mix proportions, mixing process is also a very important factor affecting the properties. In this paper, the influences and mechanisms of a novel mixing process (NMP), in which premixed cement mortar was mixed with water and superplasticizer firstly then all other liquid components, on properties of the fresh and hardened Type II CA mortar were investigated as compared to the traditional mixing process (TMP), in which the cement mortar was mixed into the mixed liquid components. The results confirmed that the CA mortar prepared by using NMP showed a less water and less superplasticizer requirement than that by using TMP. A much lower air content (below 6.0%) during mixing, no bleeding and segregation during hardening were also interestingly observed while using NMP. However, the air content reached as high as 15.0% and at least 3.0% CSS must be incorporated to avoid bleeding & segregation during hardening while using TMP. Through foaming experiments, it was found that there exists foam-boosting effect in the mixed system of superplasticizer and emulsifier, and that is why a very high air content was observed while using TMP, in which the superplasticizer and asphalt emulsion was mixed together before mixing with cement mortar. For the CA mortar prepared by using NMP, no foam-boosting effect was observed, typically because the superplasticizer molecules were absorbed onto the cement particles before the asphalt emulsion was incorporated.