Abstract
This study investigated the use of concrete sludge, a by-product of the ready-mix concrete industry, in combination with high-calcium fly ash in binary cementless binders. Concrete sludge was used in substitution rates ranging from 0% to 60% in test fly ash-based mortars to determine potential synergy. The mortars were tested for fresh and hardened properties; workability, viscosity, strength development, open porosity, early-age shrinkage, and analytical tests were carried out. A mortar with 50% fly ash and 50% limestone filler as binders was used for comparison purposes. Furthermore, a series of mortars with fly ash and concrete sludge were alkali-activated in order to determine potential strength gain. In the activated mortars, two fractions of concrete sludge were used, under 75 μm and 200 μm, due to different silicon oxide contents, while one mortar was cured at 40°C to investigate the effect of heating on alkali activation. Results show that sludge contributes to the formation of C-S-H and strength development when used in combination with high-calcium fly ash even at high replacement rates. The alkali activation of fly ash-concrete sludge system contributed to early-age strength development and to early-age shrinkage reduction.
Highlights
Today, in Europe, it is estimated that fresh concrete waste, at the production stage or the cleaning/washing stage of concrete trucks, is approximately 1–4% by weight of the total concrete produced [1]
Xi et al [6] have studied the use of marble and concrete sludge as supplementary cementitious material by replacing cement in mortars. e researchers have found that both materials could be used as fillers, while the use of concrete sludge showed higher compressive strength than marble sludge, probably due to its higher alkalinity
Binary mortars based on fly ash and concrete sludge were tested for strength development for an increasing rate of fly ash replacement with concrete sludge
Summary
In Europe, it is estimated that fresh concrete waste, at the production stage or the cleaning/washing stage of concrete trucks, is approximately 1–4% by weight of the total concrete produced [1]. E process of alkali-activation is a field of study that focuses on the use of by-products like fly ash and slag It includes a chemical reaction by polymerization of aluminosilicate oxides under the influence of silicon solutions in highly alkaline conditions by heating [17]. Binary mortars based on fly ash and concrete sludge were tested for strength development for an increasing rate of fly ash replacement with concrete sludge Fresh properties such as plastic viscosity and workability of the mortars were measured and flexural and compressive strength were tested at the ages of 7, 28, and 90 days. Selected fly ash-concrete sludge binary mortars were prepared with a combination of sodium hydroxide and waterglass solutions as activator and tested for strength development
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