The over-burnt free-CaO(f-CaO) existential state in hot-splashed steel slag is understood by observing the microstructure of steel slag section and analyzing the minerals in the steel slag (SS). In the investigation, a new method of using hydrochloric acid as the solvent to modify the steel slag powder (SSP) is put forward. The effects of hydrochloric acid pretreatment on the surface morphology of SS and the hydration activity and soundness of SSP blended cement are investigated. Furthermore, the failure mechanism of steel slag cement-based materials and the hydration mechanism of acid-activated steel slag (ASS) are discussed in detail. The research indicated that the optimum dosage of modified SSP is 20 wt%. At this dosage, the 7-and 28-day compressive strength of paste is nearly increased 15% compared with the untreated steel slag cementbased mortar specimens. Moreover, the acid activated SS-blended cement pastes show much better soundness performances after autoclaved curing tests when compared with corresponding pastes with SS without treatment. It provides a promising approach for efficient and safe utilization of SS as supplementary cementitious material.
A specially designed cleaning system was adopted to wash away mud but keep coarse particles and stone powders in quarry waste. Then the cleaned quarry waste was used as concrete fine aggregate. Size distributions of all concrete constituents were analyzed, and workability and compressive strength of concrete were determined to evaluate gradation effect of cleaned quarry waste to improve size distributions of concrete constituents, thus improving concrete properties. Results showed that the designed cleaning system can wash away mud and keep stone powders and coarse particles in quarry waste. The particle sizes of stone powders were between 0.075 mm and 0.15 mm, and those of coarse particles were between 0.15 mm and 4.75 mm. When the cleaned quarry waste was used, continuous size distributions of all concrete constituents were formed, which solved discontinuous size distribution problems among cement, fine, and coarse aggregates. Therefore, concrete workability was improved and compressive strengths at different ages were increased.
This study investigated the effect of silicate modulus (M = silicon dioxide (SiO2)/sodium oxide (Na2O) by mass) and activator content S (mass proportion of sodium oxide and silicon dioxide to ground granulated blast-furnace slag (GGBS)) of sodium silicate (SS) solution on the compressive strength of alkali-activated GGBS (AAS) mortars. At a given solid content, the specimens activated by SS with modulus of 1.0 and 1.5 had the highest compressive strength. In the case of the same modulus of SS, the compressive strength of the AAS mortars rose linearly and then remained unchanged or declined slightly with the increase in S. The mechanism of the action of SS on AAS was studied using backscattered electron (BSE) and energy-dispersive spectroscopy (EDS). The BSE image of AAS shows that GGBS particles are tightly bonded to the surrounding hardened, jelly-like slurry, and that the structure of the GGBS particles in the AAS slurry is intact without an apparent hydrating layer around it. EDS analysis revealed that only active materials in the GGBS, mainly calcium, are dissolved to participate in reaction, and that dissolving these materials does not damage the GGBS structure. S is solidified by these active materials, which embed the GGBS particles therein to form a whole.
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