Low reactivity and reported reaction extent of limestone in blended cement
The extent of reaction of limestone with cement clinkers is relatively low than the corresponding values of cement clinkers, the probable cause for this as stated above arise from the low solubility and low reactivity of limestone particles, though other factors of limestone, including fineness, particle size distribution, etc., do have influence on its reaction degree in blended system.
In experiments of 5% or 15% calcium carbonate reaction with type II cement hydrated up to one year, Klemm and Adams observed that monocarbonate hydrate is formed slower than that of ettringite, and after 129 days hydration, the amount of unreacted CaCO3 is still up to 80-90%. Ramachandran investigated that in mixtures hydrated with 15% and 5% limestone substitution, the amounts of limestone reacted with the hydrating C3S were reacted within the first 3 days. A typical simulation performed by Bentz showed that for a 20% by mass fraction substitution of ground limestone for cement, only about 5% of the limestone present reacts during the first 180d of hydration.
Taylor summarized that the maximum quantity of CaCO3 that can react appears to be 2-3% with most cements, and value up to 5.8% have also been reported; however, he did not mention the substitution level of limestone in cement. The relatively low reaction degree of limestone in blended cement is probably because of the low reactivity of limestone which is also the reason why limestone is regarded as inert “filler” by many researchers.
It is worth noting that the conclusion limestone being regarded as inert filler always arise from the fact high levels of limestone applied. At low substitution level, e.g. using up to 5% calcite, much or perhaps all of the added calcite is reactive with cement.
In terms of the quantitative relationship between the reactive fractions of limestone and time, it is still a challenge, no fundamental result is available from literature, not to mention limestone is considered as inert filler by many researchers provided that large amount of limestone is applied.