Influences of spraying sodium silicate based solution/slurry on recycled coarse aggregate

Abstract

Strengthening recycled coarse aggregate (RCA) by soaking it in a variety of solutions/slurries is being explored by many researchers. However, several problems have appeared, for example, (i) plenty of solution/slurry is needed; (ii) the soaking process is time-consuming; (iii) it is difficult to integrate the soaking process into the recycled aggregate (RA) production line. To effectively enhance RCA and save material and time, a spraying treatment method, using sodium silicate based solution/slurry, was proposed based on the solution/slurry absorption property of RCA. The influences of the spraying method on RCA and recycled aggregate concrete (RAC) were tested, including the apparent density, crushing value, 24 h water absorption, dynamic water absorption, and compressive strength. A comparative study was also conducted between the spraying method and the soaking method. To reveal the influence mechanism, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were adopted. The scheme of the integration of the spraying process was also discussed. Results showed that RCA was basically saturated during 3 spraying cycles when the flow rate was 4 g/s per kilogram RCA, and only about 1 min was needed to treat RCA using the spraying method. The crushing value and 24 h water absorption were decreased. When silica fume was incorporated, the crushing value was further decreased, but the 24 h water absorption was increased. The apparent density was increased slightly when the single sodium silicate solution was used, however, the apparent density was decreased slightly when silica fume was incorporated. Based on the dynamic water absorption, a three-stage model was proposed which could be used to evaluate the evolution of workability of recycled aggregate concrete (RAC) and make a proper water supply scheme. The compressive strength of RAC was enhanced when the treated RCA was used regardless the treatment method. It is worth noting that the strengthening effects of the spraying method and soaking method were basically equivalent, however, less material and time were required when the spraying method was used.