During the production of manufactured sand, a large amount of stone powder was sieved and buried, leading to resource waste and environmental pollution. To improve the utilization rate of manufactured sand stone powder, this study explores the high-value application of waste stone powder in concrete. By treating the stone powder in manufactured sand as a cementitious component to partially replace cement, the effects of granite stone powder on the microstructural evolution of hardened cement paste were investigated using XRD, TG, SEM, and other characterization methods, leading to the identification of the optimal cement replacement range. Furthermore, by adjusting the stone powder content in manufactured sand, coarse aggregate gradation, sand ratio, and water-to-binder ratio, the workability and mechanical properties of concrete were optimized. The study reveals the mechanism by which paste volume fraction influences concrete’s workability and mechanical performance, and successfully produced low-cost concrete with acceptable workability and mechanical strength using manufactured sand with a high stone powder content. The results show that cement paste with 10% stone powder retained a denser micro-structure, as the amount of hydration products showed negligible reduction compared to that of pure cement paste after 7-day and 28-day curing. However, when the substitution of cement with stone powder exceeded 20%, the amount of hydration products decreased significantly by more than 20%, leading to a porous microstructure and lower compressive strength compared to that of pure cement paste. When manufactured sand (MS) with high stone powder content is used in concrete production, the dosage of superplasticizer needs to be increased slightly under the same slump requirement. Additionally, the optimal workability and mechanical properties of MS concrete were achieved when the volume fraction of paste lay in the range of 31~32%. Consequently, C30, C40, and C50 concretes meeting target property requirements were prepared using MS with 15.1%, 16.5%, and 18.7% stone powder content, respectively, resulting in cement consumption reductions of 54, 63, and 92 kg/m3, and thereby significant reductions in cost and carbon emissions.
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In recent years, Portland cement production in China has exceeded 1.8 billion tonnes per year, while CO2 emissions from limestone decarbonation and fuel combustion remain high (i.e., ≈1.1 billion tonnes), accounting for ~8% of the national total. Replacing limestone with Ca–rich, non–carbonate solid wastes (e.g., recycled concrete powder, RCP) represents a promising pathway toward low–carbon clinker production. The RCP mainly consists of quartz and hardened cement paste. The RCP with higher paste fractions is reported to have lower clinker formation temperatures. Previous studies indicate that Ca(OH)2, unhydrated clinker particles, and certain minor elements (e.g., S) can facilitate clinkering. C–S–H is a dominant phase in hardened cement paste (>55%). Upon heating (≈500–700 ℃), C–S–H undergoes dehydration and structural depolymerization/rearrangement, potentially generating reactive Ca–Si intermediates that may affect clinkering reactions. However, most previous work treated a hardened cement paste as an integrated entity, and the specific role of C–S–H in governing clinker mineral formation, reaction kinetics, and microstructural evolution remains insufficiently understood.
To decouple the multiphase effects of RCP, this study used synthetic C–S–H to replace quartz, thereby isolating the contribution of C–S–H to clinkering reactions, phase evolution, and kinetics.
C–S–H was hydrothermally synthesized using Ca(OH)2 and SiO2 nanoparticles at a CaO/SiO2 molar ratio of 1.2. Its thermal evolution was preliminarily characterized by X-ray diffraction (XRD). C–S–H lost a long-range order at ~800 ℃ and recrystallized into C2S and wollastonite (CS) after heating to 900 ℃. At 1000 ℃, the crystallite sizes were ~15 nm for C2S and ~50 nm for CS (the Williamson–Hall method).
Five raw meals were designed with a silica modulus (SM) of 2.8, an alumina modulus (IM) of 1.8, and a lime saturation factor (KH) of 0.91. The reference mix used analytical–grade SiO2 together with CaCO3, Al2O3·2SiO2·2H2O, and Fe2O3. In the remaining mixes, C–S–H progressively replaced SiO2 (and the corresponding fraction of CaCO3), with increasing the silicon substitution ratio of C–S–H to SiO2 from 25% to 100% (on a Si basis). Pellets (Φ 30 mm × 7 mm) were pressed at 40 MPa, dried at 105 ℃, heated in a muffle furnace at 10 ℃/min to selected temperatures, and followed by air quenching.
The phase evolution of clinkers was quantified by X-ray diffraction (XRD) coupled with Rietveld refinement using the ICSD structure models and ZnO as an internal standard. To suppress the dominant endotherm from CaCO3 decarbonation and enhance clinkering-related thermal signals at a high temperature, CaO was used instead of CaCO3 in the tests of thermogravimetry-differential scanning calorimetry (TG–DSC), and the derived kinetic parameters were interpreted comparatively as apparent values. The non–isothermal TG–DSC measurements were performed at 5–20 K/min in N2, and apparent activation energies and kinetic models were evaluated by the Flynn–Wall–Ozawa (FWO) isoconversional method.
Introducing C–S–H into raw meals produces a pronounced exothermic peak associated with C2S formation at ~870 ℃, consistent with the low–temperature decomposition and recrystallization behavior of C–S–H. When quartz serves as a silicon source, C2S formation kinetics are well described by a phase-boundary-controlled (R3) model, with an apparent activation energy of 540 kJ/mol. In contrast, when C–S–H serves as a silicon source, the best-fit model shifts to the Avrami–Erofeev (A2), and the higher activation energy (797 kJ/mol) can be attributed to a structural rearrangement coupled with C–S–H recrystallization. The Arrhenius analysis further shows that lnA increases from 54.5 to 84.0; accordingly, the instantaneous reaction rate (dα/dt) at α = 0.5 and T = 900 ℃ increases from 0.43 to 7.60 min–1, indicating a great kinetic compensation effect likely associated with the disordered/nanostructured nature of C–S–H and the enlarged effective reaction interface.
Consistently, the XRD–detectable onset temperature for C2S formation decreases from 1000–1100 ℃ (quartz) to 900 ℃ (C–S–H), and the C2S content is increased by 10–24% at the same sintering temperatures (i.e., 900–1300 ℃). The incorporation of C–S–H also increases the C3S content by 5–15% at 1350–1450 ℃. Moreover, a high defect density of C–S–H and nanocrystal formation during heating contributes to crystallite refinement, reducing the C2S crystallite size from 87 nm to 66 nm and the C3S size from 160 nm to 121 nm.
Mechanistically, C–S–H enhances clinker burnability via (ⅰ) structural depolymerization/rearrangement at a lower temperatures, (ⅱ) a higher defect density and early formation of nanocrystalline products that promote C2S/C3S nucleation, and (ⅲ) a reduced effective CaO transport requirement because C–S–H (at Ca/Si ratio of 1.2) supplies part of the Ca needed for C2S formation, thereby shortening the effective CaO diffusion distance.
Using C–S–H as a silicon source could markedly accelerate solid-state reaction kinetics (0.43 min–1→ 7.60 min–1), lower the XRD-detectable onset temperature of C2S formation (1000–1100 ℃→ 900 ℃), and promote the formation of both C2S and C3S. In addition, the recrystallization products of C–S–H provided abundant nucleation sites and led to refined silicate crystallites. These findings could offer a mechanistic guidance for clinkering with RCP containing high hardened cement paste (C–S–H–rich) fractions toward low–carbon clinker production.
With the advancement of urbanization and the transformation of the old city, the production and stock of construction waste in China are ever increasing, with waste clay bricks accounting for 50%~70%. It has been found that recycled brick powder has the potential as a supplementary cementitious material, but it can lead to a significant reduction in the mechanical properties of cement-based materials. In order to explore the effect of particle size on the activity of recycled brick powder and the hydration kinetics of cement, this study prepared recycled brick powder with different particle sizes by high-energy ball milling. The physical and chemical properties and hydration activity of recycled brick powder were characterized. The effect of particle size of recycled brick powder on the hydration process, microstructure and mechanical properties of recycled brick powder-silicate cement system was analyzed. Based on the Krstulovic-Dabic model, the hydration kinetic parameters of the system were obtained to realize the quantitative evaluation of the hydration process. The results show that with the decrease of the particle size of the recycled brick powder, the lattice distortion of the silicon-aluminum mineral becomes larger and the surface binding energy decreases, resulting in an increase in its hydration activity. The early hydration of recycled brick powder mainly plays a physical filling role, which can accelerate the early hydration of recycled brick powder-cement system and improve the hydration degree of crystallization nucleation and crystal growth→phase boundary reaction→diffusion process. With the decrease of particle size of recycled brick powder, the starting time of pozzolanic reaction is advanced and the degree of pozzolanic reaction is higher. Finally, the later strength of cement mixed with 30% fine-grained recycled brick powder exceeds that of pure cement. This paper lays a foundation for the efficient application of recycled brick powder in cement-based materials.
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