The high-performance requirements of special cements put forward more stringent control targets for production process conditions, raw material selection, process quality monitoring, testing methods and other aspects. In view of the individual technical needs of the project, the control range of special cement indexes can be adjusted appropriately to provide better control.
Take magnesium oxide as an example, the standard chemical index of general cement lists it as an index that needs to be controlled to affect the stability, while the Three Gorges project uses medium-heat silicate cement, under the premise of meeting the standard, the magnesium oxide (MgO) content is increased within a certain range to make use of the micro-expansion of magnesite (free magnesium oxide) to supplement the shrinkage, so as to prevent the dam body from cracking.
Activated magnesium oxide cement is a low carbon cementitious material, usually prepared from a large amount of industrial waste, a moderate amount of activated magnesium oxide and a small amount of cement. Unlike conventional cements, activated MgO cements produce better mechanical properties after carbonization. The microstructure of activated MgO cement paste prepared by 20%~40% activated MgO, 0~40% mineral powder and cement was investigated after curing in CO2 at 99.9% concentration at different ages. The microstructure of the carbonized cement paste was characterized by dynamic water vapor adsorption method, secondary scanning electron microanalysis and backscattered electron microanalysis, and the carbonation mechanism was discussed. The results showed that the carbonation of hydrated calcium silicate produced amorphous CaCO3 and SiO2, which slightly increased the 0-11 nm mesopores and the specific surface area of cement paste; after carbonation, the activated MgO cement paste produced a large number of tightly connected round Mg-Ca carbonates, which made the microstructure of cement paste more dense; during the CO2 curing process, the carbonation products of activated MgO cement were mainly produced by The carbonation products of reactive MgO cement are mainly formed by precipitation and crystallization in saturated solution containing Ca2+ and Mg2+.
The cement mixed with calcined magnesium oxide has the performance of delayed expansion, when the water slurry is hydrated at 50℃, the expansion rate of hydration 3 days to 15 days is the largest, which is the temperature drop of the dam concrete is obvious, so it can compensate the volume shrinkage caused by the temperature drop of concrete, simplify the temperature control measures, the expansion performance of magnesium oxide cement is mainly determined by the expansion performance of magnesium oxide expander. The test shows that adjusting the calcination system of magnesium oxide can change the expansion rate, the expansion amount and the termination period of expansion.
The actual application effect proves that it can meet the design demand of engineering construction and realize the \”win-win\” of product quality and resource utilization by effectively using high Mg limestone.