Mechanical Properties and Micromorphology of Calcium Oxide Expansion Agent on River Sand/Machine-Made Sand Concrete

Freeze-thaw damage is one of the main factors affecting the service life and durability of concrete. Optimizing the microscopic pore distribution of concrete is an effective measure to improve its frost resistance. This paper takes mixed sand (machine-made sand + river sand) concrete as the research object and analyzes the influence mechanism of different content of CaO expansion agent on the physical and mechanical properties and frost resistance of mixed sand concrete. Concrete samples with different calcium oxide contents (0%, 6%, 8%, and 10%) were prepared by mass substitution with fly ash. Early air content, slump, compressive strength testing, and other analysis methods are used to determine the physical and mechanical properties of river sand-manufactured sand concrete with different calcium oxide expansion agent blending ratios. Freeze-thaw cycles were observed under X-ray computed tomography, SEM, and EDS to explore the mechanical properties, porosity, and microstructures of the samples. The concrete air content increases, the porosity first decreases and then increases, and its compressive strength decreases as the mixing ratio of calcium oxide expansion agent increases, but the mixing ratio has little effect on the slump. When the content of calcium oxide in concrete is higher than 8%, the mass compressive strength and relative dynamic elastic modulus of concrete increase with the increase of substituting calcium oxide expansion agent. The cross section of the concrete sample is a two-dimensional plane before freezing-thawing, and the pore structure densified as CaO content increases. However, new cracks are generated in the sample with 10% expansion agent content. After 400 freezing-thawing cycles, 0% and 6% calcium oxide expansive samples have irregular three-dimensional structures with concave and convex, and many connected holes and penetrating cracks appear. There are a small number of disconnected void clusters in the section of the sample with 8% and 10% content. A large number of calcium hydroxide ((Ca(OH)2) crystals fill the pores and cracks and improve the frost resistance of concrete.

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