Microstructural characteristics of low-cement concrete：From matrix to interfacial transition zone

The extensive use of cement generates substantial CO2 emissions, exacerbating the global greenhouse effect. Low-cement concrete (LCC) incorporates high volumes of supplementary cementitious materials (SCMs), and has been gaining increasing attention. However, the microstructure evolution in LCC, particularly in the interfacial transition zone (ITZ), remains insufficiently understood. This study employed X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, pore-solution pH measurements, and backscattered-electron imaging to investigate the hydration behavior and ITZ pore-structure evolution of LCC. The results show that, although the Ca(OH)2 content in LCC is low, the pore solution remains alkaline, with a minimum pH of 12.25 at 56 d. Thermogravimetric analysis further showed that the Ca(OH)2 content of LCC was only 0.71%-1.62% at 56 d. The maintained alkalinity supported continued secondary hydration of the SCMs, promoting the formation of additional C-(A)-S-H gel and contributing to matrix densification. More importantly, the ITZ in LCC is not a weak zone; rather, it is a strengthened region. At 28 d, the ITZ width of LCI30 was approximately 60 μm, 10 μm and 30 μm smaller than those of CI0 and NCI30, respectively, while its ITZ porosity was only 4.68%, lower than the matrix porosity of 6.11%. Multisized SCMs optimize the particle gradation and spatial distribution within the ITZ, improving the initial packing density and reducing the property gradient from the aggregate to the matrix, thereby reducing the ITZ width. Moreover, the low Ca(OH)2 content limits its excessive accumulation in the ITZ, while providing adequate Ca and alkalinity to promote interfacial secondary hydration reactions. The synergistic optimization of both the matrix and ITZ contributes to the long-term development of compressive strength in LCC. From 28 to 180 d, the compressive strengths of LC20, LC30, LC40, and LC50 increased by 42.6%, 24.7%, 25.8%, and 16.6%, respectively, which were higher than the 8.2% increase observed for the control concrete. This study provides key theoretical support for the application of LCC in green construction.