Enhancement mechanism of early age strength in cement paste induced by oil shale residue

Cement production accounts for approximately 8 % of global carbon dioxide emissions. Reducing the clinker ratio and incorporating supplementary cementitious materials (SCMs) are effective strategies for CO 2 emission reduction. However, high replacement levels of SCMs often lead to a decline in early-age strength . To address this challenge, this study developed a novel SCMs using calcined oil shale residue (OSR) and investigated its effects on the compressive strength of cement, as well as the underlying mechanisms. The results revealed that OSR significantly improved the compressive strength of cement. At an optimal replacement level of 30 %, OSR increased the compressive strength by 29.5 %, 31 %, and 9 % at 3, 7, and 28 days, respectively, compared to the reference sample. This improvement is attributed to OSR’s ability to accelerate the hydration of C 3 S in the early stages, thereby significantly increasing the degree of hydration (DoH) at both 3 and 28 days. Furthermore, the pozzolanic reaction of OSR not only consumed calcium hydroxide (CH) but also promoted the formation and polymerization of C-(A)-S-H gel. Microstructural and compositional analyses further confirmed these findings. However, when the OSR content reached 40 %, excessive replacement limited the availability of CH, thereby showing the pozzolanic reaction and reducing the compressive strength. Therefore, a 30 % replacement level is recommended as optimal for OSR incorporation. This study provides new insights into the development of innovative SCMs and contributes to the low-carbon transition and sustainable development of the cement industry. ● Oil shale residue can address insufficient early-age strength in blended cement. ● The replacement amount of oil shale residue in cement can reach at least 30 %. ● New perspective into utilizing petrochemical industry by-products in cement.