Low-alkaline activated construction and demolition waste-based geopolymers

Abstract

This paper focuses on the development of environmentally friendly, easy to use and cost-effective construction and demolition waste (CDW)-based geopolymer mortars activated solely with low-alkalinity NaOH and along with large share of recycled concrete aggregates (RCAs). The low-activity CDW-based precursors used in this study included brick waste and concrete waste. Additionally, slag and silica fume were utilized as industrial waste-based precursors to provide additional sources of Ca and Si, promoting enhanced geopolymerization. NaOH was utilized at molarity levels of 4, 5, 6, 7, and 8 M. The study employed a comprehensive test methodology, enabling a thorough investigation of the different properties and performance of the geopolymer mixtures, ultimately achieving the goals set for this research. The results indicated that the 4 M NaOH concentration was insufficient for early strength development of CDW-based geopolymer systems, although it eventually provided adequate alkalinity for strength development at later ages (e.g., compressive strength of 7.2 Mpa at the end of 28-day). While higher NaOH concentrations generally improved performance, the optimal choice was found to be 5 M NaOH. Nevertheless, 100% CDW-based geopolymer mixtures activated solely with low molarity NaOH solutions showed lower performance (compressive strength of 10.5 Mpa and flexural strength of 2.3 Mpa at the end of 28-day) and were found to be susceptible to water ingression and unable to withstand freeze-thaw cycling. Incorporation of slag significantly enhanced the mechanical and durability properties of CDW-based geopolymer regardless of alkaline content. The inclusion of coarser RCAs (0–4.75 mm) and a higher amount of finer RCAs (0–2 mm) resulted reduction in the performance of the mixtures. Nevertheless, the observed negative impacts were mitigated through 5% silica fume substitution. The findings of the study indicate that low-alkaline activated CDW-based geopolymer mortars can achieve an impressive compressive strength of 24.9 MPa and flexural strength of 4.7 Mpa along with superior durability performance.