Long-term analysis of the physical properties of the mixed recycled aggregate and their effect on the properties of mortars

Abstract

The use of mixed recycled aggregates (MRA) for the production of cementitious materials is still limited due to the high heterogeneity of the rubble sources and the lack of specific regulations, which contributes to maintaining the problems associated with the management of construction and demolition waste (CDW) in recycling plants. Therefore, this paper investigates the influence and effects of the use of MRA, collected over time, on the physical and mechanical properties of cement and lime-based mortars, based on a statistical modelling using Machine Learning algorithms. For this purpose, an extensive experimental programme with three stages was developed, intending to evaluate the variability of the MRA produced. The first phase consisted of the physical characterization tests of 36 samples of MRA. The second phase intended to perform technological tests to select the best volumetric ratio of cement: hydrated-lime: MRA (1:1:6, 1:1:7 or 1:2:9) to be used with 100% MRA (modified mortars). Finally, the third experimental phase investigated the physical and mechanical properties of the modified mortars produced with the volume ratio selected in the previous phase and using MRA collected at different periods of time. The results obtained were analysed using a T hypothesis test, a joint analysis of all variables using a Robust Principal Components Analysis (ROBPCA) and a partner recognition model based on data driven and ROBPCA (Data Driven Soft Independent Modelling of Class Analogy - DD-SIMCA). From the results, it was noticed that MRA 1:1:6 mortars presented a better performance in terms of mechanical strengths and water absorption by capillary, due to the filler effect of the MRA. As a matter of fact, the statistical tests have proven that no statistically significant differences were found in the physical properties of the 36 MRA samples and among the physical and mechanical properties of mortars in these periods investigated, which demonstrates the potentially reproducibility of CDW. Therefore, it was found that a proper selection and processing of CDW, as well as a definition of efficient mixing ratios can minimize the effects from the high heterogeneity of these wastes and enable the effective use of MRA in mortars.