Water Absorption Of Recycled Aggregate Research Articles

Study on Reducing Water Absorption of Recycled Aggregates (RAs) by Microbial Mineralization.

Crushing waste concrete and using it directly as RAs has the disadvantages of high porosity and high water absorption. To achieve the reuse of resources, the researchers use microbial mineralization methods to further reinforce RAs. In this paper, the effect of the microbial carbonic anhydrase mineralization method on the water absorption of RAs was investigated, and the macroscopic analysis was performed by determining the indexes of water absorption and apparent density of RAs before and after the modification, and the microscopic analysis of RAs by using the methods of SEM, XRD, DSC, and EDS as well. According to the microscopic analysis, the mineralization products of microorganisms are calcium carbonate crystals, and with the increase in microbial liquid concentration, the water absorption rate of RAs shows a trend of decreasing and then increasing, and it can be found through the microscopic morphology that abundant mineralization products attached to the surface of the aggregate lead to the surface of the aggregate becoming rougher and more porous. The method of soaking the RAs in 3% bacterial solution and 0.1 mol/L calcium acetate solution followed by carbonation with 20% CO2 resulted in a 4.85% reduction in water absorption.

A simple method to address the high water absorption of recycled aggregates in cementitious mixes

The high water absorption of recycled concrete aggregates (RCA) can cause a significant reduction in the flow properties of cement-based materials in relation to mixes produced with natural aggregate. In order to compensate for this adverse effect, two extreme and simple approaches based on the water absorption of RCA in pure water are often used: using RCA in a saturated surface-dry (SSD) state or adding extra water based on the 24-hour water absorption of RCA in pure water (WA24h). However, these approaches can result in a significant drop in mix’s strength due to excessive water being used. Therefore, this study proposes a simple method to accurately evaluate the water absorption of RCA in cement paste (WAP) based on the variation in the mortar’s apparent density resulting from the RCA absorption of water from the cement paste. This study also investigates the effects of the aforementioned two approaches on the mortars’ performance. Recycled concrete sand (RCS) was adopted as the object of the study. The results show that, after an initial increase, the WAP of RCS stabilizes at 24 min. WAP also increases with the mortar’s W/C and WA24h, and decreases with the aggregate’s volume fraction. In addition, the maximum water absorption of RCS in cement paste (WAP_30min) is lower than WA24h, and the WAP at 6 min (WAP_6min) can reach 80% of the WAP_30min. A campaign of mortars was produced using the WAP_30min as previously assessed. It is found that the flowability of mortar prepared with RCS is poorer than that of mortar prepared with natural sand at the same effective W/C, but their strength is higher. Using RCS in a SSD state or adding extra water based on the WA24h of RCS can compensate for the flowability loss of the mortars to some extent, but at the expense of an excessive reduction in mechanical performance. Based on the results of this study, the authors suggest that the mix design of mortars prepared with RCA can be improved using WAP, namely adjusting the initial W/C to balance flowability and mechanical properties.

Study of Surface Modification of Recycled Aggregate and Interfacial Zone Microstructure of the Concrete

Due to most of recycled aggregate (RA) possesses higher water absorption, therefore, the process of mixing recycled aggregate concrete (RAC) is more complex than that of normal aggregate concrete (NAC). In this paper, an organosilicone modifier was used to modify RA to reduce its water absorption. Four mixes of concrete were made up with modified RA at a concentration of 1:1, modified RA at a concentration of 1:2, unmodified RA, and NA, respectively. The water absorption unmodified RA and modified RA was compared, and effect of RA surface modification on the microstructure of the interfacial transition zone (ITZ) between the RA and the hardened cement paste was examined. The investigation results show that a higher concentration of modifier is more effective in reducing the water absorption of RA; and there is an optimal value of modifier concentration, and the use of in high concentrations modifier will adversely affect the integrity of interfacial zone.