Sulphoaluminate cement has the characteristic of low alkalinity, and is more sensitive to changes in pH value than ordinary Portland cement. The main product of Sulphoaluminate cement hydration is AFT, which requires a stable alkaline environment with a pH value of about 11. When the pH value decreases to a certain degree, AFT will decompose to generate flaky AFm; when the pH value further decreases, AFT will decompose to generate aluminum gel, CaSO4 and calcium hydroxide. During this process, the decomposed aluminum gel of AFT is absorbed by C-S-H gel, and the magnesium hydroxide generated by the reaction of calcium hydroxide and Mg2+ forms an amorphous crystal, which is easy to agglomerate and has poor dispersion in the material. It will adhere to the surface of C-S-H gel to form a coexisting system of CSH-aluminum gel-Mg(OH)2.
The water-cement ratio has a greater impact on the magnesia resistance of Sulphoaluminate cement. The SAC-1 cement with a lower water-cement ratio is covered with a dense CSH-aluminum gel-Mg(OH)2 coexisting layer at the edge, which blocks the channel for the erosion medium to infiltrate into the cement stone, ensuring the normal hydration of the cement stone without being affected by the erosion medium. The erosion layer mainly consists of CSH-aluminum gel-Mg(OH)2 coexisting substance. It adheres to the surface of the cement stone and has a clear boundary with the internal structure of the cement stone. The SAC-1 cement was analyzed by energy spectrum analysis at the age of 12 months in R1; the SAC-2 cement with a higher water-cement ratio has no obvious erosion layer formed, and the erosion damage has penetrated into the inside of the cement stone. CSH-aluminum gel-Mg(OH)2 coexisting substance is widely distributed throughout the cement stone structure. The SAC-2 cement was analyzed by energy spectrum analysis at the age of 12 months in R1.
Through the observation of the appearance of the corroded specimens, SEM scanning electron microscopy and energy spectrum analysis results, it is found that the corrosion damage of Sulphoaluminate cement is an external-to-internal process, showing the characteristics of obvious gypsum-type sulfate corrosion damage.
During the early stage of corrosion in the edge layer, magnesium sulfate and the hydration product of Sulphoaluminate cement calcium hydroxide react to generate non-bonding hydrogen magnesium and gypsum. The deposition of magnesium hydroxide inside the specimen destroys the alkaline environment that ensures the stable existence of AFT, and AFT decomposes to generate a low-bonding flaky structure AFm. The generation of gypsum promotes the further hydration of Sulphoaluminate cement, and the regenerated AFT generates a large number of expanding cracks in the edge layer of the specimen, providing a channel for the erosion medium to penetrate into the cement.
In the later stage of corrosion in the edge layer, AFT/AFm completely decomposes to generate aluminum gel, gypsum, and calcium hydroxide. The generated calcium hydroxide reacts with magnesium sulfate to generate gypsum and magnesium hydroxide. At this time, the hydration of Sulphoaluminate cement is basically over, and a large amount of gypsum accumulates in the edge layer of the specimen. Its expanding and damaging effect causes the edge layer of the specimen to become loose and fragmented, and the surface of the specimen has sand and corners falling off, showing obvious gypsum-type sulfate corrosion damage characteristics.