Effects of using waste glass with different particle sizes to replace sand in pre-cast concrete blocks

Abstract

In Hong Kong, majority of waste glass is not being recycled and disposed to landfills directly which causes a serious environmental problem to the society. It has been proved that using recycled glass as a material in concrete is a feasible way to solve the problem. Previous studies found that the particle size of recycled glass cullet would affect the properties and ASR resistance of concrete. Moreover, most of the previous studies focused on wet-mixed concrete but few studies focused on dry-mixed concrete. For these reasons, this study aims to investigate the effects of different replacement percentages and particle size distribution of recycled fine glass (FG) aggregates on the properties of dry-mixed and wet-mixed concrete blocks, and the alkali-silica reaction (ASR) resistance of dry-mixed and wet-mixed mortar bars. All the block mixtures were proportioned with a fixed total aggregate/cement ratio of 4 and 50% of the total aggregate was fine aggregate. A total of 17 concrete block mixes, including a control (0% of glass) mix, were produced using four different particle sizes of FG (Un-sieved, <2.36mm, <1.18mm and <600μm) as replacements of sand. The replacement ratios were 25%, 50%, 75% and 100%. Properties such as packing volume ratio, hardened density and water absorption, as well as the effects of air and water curing upon 7 and 28-day compressive strength were studied. All concrete blocks (dry-mixed and wet-mixed) containing FG showed higher water absorption and lower hardened density than the control. The effect was more pronounced for FG with particle size less than 600μm. Slight reductions in compressive strength were observed with the use of coarser FG, while significant increases in compressive strength occurred when the particle size of FG was reduced to less than 600μm. This indicates that finer FG exhibited appreciable pozzolanic reactivity. For dry-mixed concrete blocks, the results show that the water demand for mixing and fabrication increased with decreasing fineness modulus of the fine aggregate. The effects of different particle size of FG on the properties of dry-mixed and wet-mixed concrete blocks were similar. The effect of water curing increase on compressive strength of the wet-mixed concrete was larger than that of the dry-mixed concrete blocks. The ASR expansion test results reveal that ASR expansion is reduced with reducing particle size of glass used. For a given mix proportions, the dry-mixed method led to lower ASR expansion (by up to 44% in average) as compared with the wet-mixed method. SEM images reflected that no cracks were observed in the dry-mixed mortar bars but cracks were observed in the wet mixed mortar bars after the ASR expansion test. The addition of PFA and MK (5-15% by mass of aggregate) led to the lower ASR expansion of the mortars. The positive effects of PFA and MK in reducing ASR expansion make these materials as potential ASR suppressants.