Investigation of sulfate attack on aluminum phases in cement-metakaolin paste

Al-containing phases such as stratlingite, C-A-S-H, and TAH play an essential role in studying the sulfate attack of MK blended cement paste. This paper investigates the sulfate attack of various aluminate hydrate products commonly formed in metakaolin (MK) blended cement paste and the evolution of aluminate hydrate products of the cement-MK paste under sulfate attack. The results show that both the synthesized stratlingite and the stratlingite produced by the cement-MK paste have excellent sulfate attack resistance. Additions of 30% and 50% MK consume all portlandite produced by the cement hydration and result in the formations of C-A-S-H, stratlingite, TAH, and C4AH13. For the 30% MK blended cement paste under sulfate attack, C4AH13 reacts with SO42− to form ettringite at the beginning because of its poor sulfate resistance. Once C4AH13 is consumed, SO42− directly targets the stratlingite, C-A-S-H, and TAH due to no portlandite in the system; C-A-S-H and TAH are the two primary contributors of Ca2+ ions and Al3+ ions to the ettringite formation. The ettringite formation is suppressed due to the absence of CH, and the existence of stratlingite, C-A-S-H, and TAH reduces the content of reactive Al that can react with sulfate.