Investigation of sandstone by-products as supplementary cementitious materials and fine aggregate

Investigation of sandstone by-products as supplementary cementitious materials and fine aggregate

Annually, a large volume of concrete is produced for infrastructure and housing building all over the world. Fine aggregate and cement as the main components of concrete are facing the problems of resource shortage and serious environmental pollution. It is worth noting that the output and storage of supplementary cementitious materials for preparing green concrete is facing the inability to meet the demand for clinker replacement. Recycling and value-added reutilization of industrial by-products to produce building materials to alleviate resource shortages and pollution has shown feasibility and has gradually attracted more interest.

Sandstone by-products are generated during the mining process. Different from previous studies that applied sandstone by-products as coarse aggregates, this research explored the potential of using sandstone by-products to produce and expand the sources of high-quality fine aggregate (sandstone sand) and supplementary cementitious materials (sandstone clay). One tonne of sandstone clay is produced for every 4 tonnes of sandstone sand. This thesis firstly investigated the chemical composition and physical properties of sandstone sand and thereby its application in mortar was studied. Secondly, the thermal activation mechanism and pozzolanic reactivity of sandstone clay (SC) were systematically investigated. Finally, the thermal stability of activated SC blended limestone calcined clay cement was studied.

Compared with river sand, sandstone sand exhibited better particle size distribution and similar microscopic morphology. Study found that sandstone sand contained around 15.7 % fine quartz particles smaller than 150 microns, which significantly reduced the workability. Compared with the method of increasing the water to binder ratio, adding 0.2% superplasticizer was more effective in improving the workability and achieving higher compressive strength and denser pore structure. It was found that although sandstone sand showed potential alkali silica reaction (ASR) risk, the incorporation of SCMs could significantly inhibit ASR-induced expansion. Additionally, length change caused by ASR showed a good linear relationship with the mass change, which can be used as an index to evaluate the ASR properties of fine aggregate.

On the other hand, sandstone clay contains around 50 wt. % kaolinite, which was the potential source of pozzolanic reactivity. FTIR and NMR results showed that the pozzolanic reactivity of the calcined clay comes not only from the kaolinite dehydroxylation process, but also from the variation of the Al and Si coordination environments resulting from the breakage of the Si-O-Al bond. Isothermal calorimetry results indicated that, compared with active silicon, the active alumina in the metakaolin played a major role in the early hydration. Finally, according to the definition of pozzolans, three methods of pozzolanic reactivity evaluation were designed to evaluate thermally activated sandstone clay with different calcination parameters.

For the application of calcined sandstone clay, the phase evolution and microstructure development of activated sandstone clay blended limestone calcined clay cement (LC3) at moderate temperatures (100 - 400 ºC) was investigated from macroscopic, mesoscopic and microscopic. Compared with PC, LC3 exhibited superior thermal resistance. After moderate temperature treatments, PC products were more sensitive than LC3 in terms of length change and the generation of cracks. It was found that the open porosity of both PC and LC3 samples increased significantly after thermal treatments due to the evaporation of free and chemically bound water and the decomposition of hydration products. Research by NMR analysis found that LC3 samples contained longer mean chain length (MCL) and lower Si/Al ratio in C-(A)-S-H gel, which was attributed to the addition of calcined clay, resulting in producing more C-(A)-S-H. Because of presence of aluminum in C-(A)-S-H gel, LC3 possessed superior heat resistance.

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