This study investigates the microhardness and geometric degradation mechanisms of interfacial transition zones (ITZs) in recycled aggregate concrete (RAC) exposed to saline soil attack, focusing on the influence of supplementary cementitious materials (SCMs). Ten RAC mixtures incorporating fly ash (FA), granulated blast furnace slag (GBFS), silica fume (SF), and metakaolin (MK) at 10 %, 15 %, and 20 % replacement ratios were subjected to 180 dry-wet cycles in a 7.5 %MgSO4-7.5 %Na2SO4-5 %NaCl solution. Key results reveal that ITZ's microhardness and geometric degradation decreases with exposure depth but intensifies with prolonged dry-wet cycles. The FAGBFS synergistically enhances ITZ microhardness while minimizing geometric deterioration, with ITZ's width and porosity reduced to 67.6-69.0 mu m and 25.83 %, respectively. In contrast, FA-SF and FA-MK exacerbate microhardness degradation, increasing porosity and amplifying microcrack coalescence. FA-GBFS mitigates the diffusion-leaching of aggressive/original ions and suppresses the formation of corrosion products, thereby inhibiting the initiation and propagation of microcracks. In contrast, FA-SF and FA-MK promote the formation of ettringite/gypsum and crystallization bloedite/glauberite, which facilitates the formation of trunk-limb-twig cracks.

If building loads cannot be transferred into the soil, ground improvements are often used, which require the addition of cement with considerable emissions of CO2. Thermo-mechanically processed crushed concrete fines can partially replace the required cement. This article deals with comprehensive laboratory tests to improve the soil mechanical properties of a typical sand as a building ground and demonstrates the applicability of thermo-mechanically processed concrete fines for the substitution of 25 wt.-% to 50 wt.-% of cement for practical construction purposes. Processing temperatures of 400 degrees C and 600 degrees C proved to be particularly effective, with greater reductions in strength and stiffness occurring outside this temperature range.

There is a huge reservation of loess in the Shanxi mining area in China, which has great potential for preparing supplementary cementitious materials. Loess was modified via mechanical and thermal activation, and the pozzolanic activity was evaluated using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Moreover, the workability of grouting materials prepared using modified loess was assessed. The experimental results revealed that the number of ultrafine particles gradually increased with the grinding time, enhancing the grouting performance. The coordination number of Al decreased upon the breakage of the Al-O-Si bond post-calcination at 400 degrees C, 550 degrees C, 700 degrees C, and 850 degrees C. Moreover, the breaking of the Si-O covalent bond produced Si-phases, and the pozzolanic activity of loess increased. Furthermore, the modified loess was hydrated with different cement proportions. With increasing grinding time, the overall setting time increased until the longest time of 14.5 h and the fluidity of the slurry decreased until the lowest fluidity of 9.7 cm. However, the fluidity and setting time decreased with increasing calcination temperature. The lowest values were 12.03 cm and 10.05 h. With the increase in pozzolanic activity, more ettringite was produced via hydration, which enhanced the mechanical properties. The maximum strength of the hydrated loess after grinding for 20 min reached 16.5 MPa. The strength of the hydrated loess calcined at 850 degrees C reached 21 MPa. These experimental findings provide theoretical support for the practical application of loess in grouting.

Loess, a terrestrial clastic sediment, is formed essentially by the accumulation of wind-blown dust, while stone waste (SW) is an industrial waste produced during stone machining. Utilising loess and SW to prepare environmentally-friendly supplementary cementitious materials can not only address environmental issues caused by solid waste landfills but also meet the demand of reinforcement of coal-seam floor aquifer for grouting materials. In this paper, the effects of the loess/SW mass ratio and calcination temperature on the transformation of calcined products are investigated and their pozzolanic activities are evaluated. The workability, environmental impact and cost of grouting materials based on cement and calcined products are also assessed. Experimental results reveal that higher temperatures favour the formation of free lime and periclase, which tend to be involved in solid-state reactions. Higher temperature and loess/SW mass ratio strengthens the diffraction peaks of dodecalcium hepta-aluminate (C12A7), dicalcium ferrite (C2F) and dicalcium silicate (C2S). The clay minerals in loess become completely dehydroxylated before 825 degrees C, generating amorphous SiO2 and Al2O3. Covalent Si-O bonds are interrupted and that disordered silicate networks are generated in the calcined products, which is confirmed by the increased strength of the Si29 resonance region at -60 ppm to -80 ppm. Although co-calcined loess and SW contain the most four-fold aluminium at 950 degrees C, recrystallisation depresses the pozzolanic activity. Hence, the loess/SW sample designated LS2-825 exhibits the better hydration activity. Additionally, grouting materials composed of cement and LS2-825 exhibit good setting times, fluidity, strength and a low carbon footprint in practical engineering applications, and they also provide the additional benefit of being cost effective.