Utilization of solid waste from tunnel excavation as manufactured sand with different lithology and pre-washing process for preparation of eco-friendly ultra-high performance concretes: Properties and microstructural analysis

Abstract

Although manufactured sand is becoming increasingly popular to prepare the cleaner and sustainable construction production in face of the depletion of natural river sand, it needs to be further promoted in some advanced concretes like ultra-high performance concrete (UHPC). Different lithological manufactured sands with the same particle gradation were used as fine aggregates to prepare UHPC in this study. Meanwhile, the UHPC mixtures prepared with washed or unwashed sands were also compared. The following aspects were evaluated: flowability, rheology, setting time, mechanical strength, volume stability, thermogravimetric behavior, pore distribution, microstructure characteristics, environmental and economical benefits. Results indicated that sand washing process greatly lowered the apparent viscosity and yield stress, and improved the flowability of UHPC. Increments of 16.7 %, 19.6 % and 15.7 % in flowability were observed for washed granite, tuff and limestone sand UHPC. Pre-washing process significantly delayed the setting times. Tuff sand UHPC demonstrated the optimal mechanical properties (compressive, flexural and tensile strengths), with a compressive strength exceeding 150 MPa at 28 days. In most cases, sand washing process had a slightly beneficial effect on mechanical strengths. The use of washed sand reduced the autogenous shrinkage of UHPC by 5.1%–10.6 %. Tuff sand UHPC possessed the largest cumulative pore volume, while limestone one had the fewest macropores (>100 nm). The cumulative pore volume and total porosity decreased with the addition of washed sands. Compared with traditional UHPC, the utilization of manufactured sand with different lithology and pre-washing process in UHPC could not only illustrate good workability and mechanical strengths, it also demonstrated lower carbon emissions and production costs. The prepared eco-friendly UHPC in this study effectively utilized the solid waste and lessened the environmental burden, which proposed an emerging approach for the application of manufactured sands in green advanced concretes in future work.