Synergistic effect of combining low kaolinite grade calcined clay with conventional cementitious materials

Utilizing amorphous aluminosilicate as an alternative cementitious material has been found to enhance the properties of Portland cement (PC) when exposed to normal and aggressive media. However, the pozzolanic reactivity of supplementary cementitious materials (SCMs) varies, with some SCMs exhibiting high efficiency in early-age hydration and others in later ages. This trade-off often leads to a compromise in the early or later performance of hardened materials. Therefore, the main goal of this paper is to fabricate ternary, quaternary, and quinary mixtures containing commercially available amorphous silicate/aluminosilicate materials with different reactivities. These materials include low-grade metakaolin produced using Fanja (FNJ) calcined clay, silica fume (SF), fly ash (FA), and blast-furnace slag (BFS). The aim is to optimize the early and later performance of hardened PC mortars. The resistance of the hardened mortars to different aggressive attacks, such as sulfuric acid, nitric acid, and sodium sulfate/NaCl solution, was evaluated. The results revealed that replacing PC with FNJ, FNJ-SF, and FNJ-BFS, as well as FNJ-SF-FA, resulted in a significant improvement in the performance of cement mortars at both early and later ages. The ternary, quaternary, and quinary mortars demonstrated higher 7-day compressive strength than that of PC-FNJ blend. The sample with 10 wt.% FNJ and 40 wt.% BFS showed the highest 7-day compressive strength with a value higher than that of the PC-FNJ blend by 78%. Although the ternary PC-FNJ-FA, quaternary PC-FNJ-BFS-SF, and quinary PC-FNJ-BFS-FA-SF mixtures showed lower later performance compared to the PC-FNJ blend, their performance was still higher overall. Additionally, all blended mortars showed high resistivity to chloride diffusion, as simulated by the rapid chloride permeability test (RCPT). The composite materials exhibited different performances in acid and salt attacks; however, they recorded higher resistance than that of the reference sample. Overall, the findings of this study suggest that utilizing amorphous silicate/aluminosilicate materials in combination with PC can lead to improved performance in cement mortars, in terms of both early and later ages, as well as resistance to aggressive attacks such as acid and salt exposure.