Tag Archives: Kinetics

Kinetics of cement: Degree of hydration or Degree of reaction?

It is so common to see the terms “degree of hydration” and “degree of reaction” in research field of cemetitious material, such as cement or blended cement, especially when we focus on the kinetics.

Here comes the question, what is the difference between the two terms? are they actually the same meaning? I was confused by the two terms when I read a huge number of papers and found not so many researchers make them clear or maybe they do not realize the difference either.

Degree-of-hydration
Degree of hydration, α, as a function of curing time (By Raymond A. Cook).

First, let’s know what cement hydration is? it means chemical combination of cement and water. Then what is hydration degree of cement? “Degree of hydration” of cement is defined as the fraction of Portland clinker (including gypsum addition) that has fully reacted with water.

There is no need to explain the meaning of word “reaction”. However, “Degree of reaction” of cement is defined as the fraction of Portland clinker (including gypsum addition) that has already fully reacted with water relative to the final reacted cement.

From the two definitions for the two terms, the difference is clear. Degree of hydration does not consider the fact that ultimate hydration, i.e. hydration degree is 100%, does not occur in reality. While degree of reaction is defined as the reacted part relative to the whole part that will be finally reacted, in other words, 100% reaction degree is possible.

Because complete hydration of cement does not happen, ultimate hydration degree of cement is introduced. For Portland cement the ultimate degree of hydration can be calculated using Mill’s formula. Reference value of ultimate hydration degree of Portland cement (water-to-cement ratio ≈ 0.5) could be 75%, yes, the corresponding reaction degree is 100%.

Factors affecting the reactivity of slag

The exact composition of slag varies over a range. In general, factors that determine the suitability of slag for usage in composite cement mainly include the fineness of grinding, glass content and the chemical composition.

  • Fineness

Like most of other cement materials, the reactivity of slag is influenced by its surface area. Increased surface area leads to better strength development and more water requirement; however, the fineness of slag is limited from practical aspects, such as economic and performance considerations, setting time and shrinkage. The following table shows typical fineness data of market slag in some countries.

Table: the surface area of slag in some countries (m2/Kg)

UK USA Canada India
Blaine surface area 375-425 450-550 450 350-450
  • Glass content

During the quenching process, the liquid slag forms glassy and crystalline contents. Practical glass content of slag depends on the cooling rate, in general, rapid rate results in high glass content. The main difference between glass content and crystal content of slag is that the former part has a latent hydraulic property that makes the glass content of slag a very important factor affecting the engineering performance of slag cement.

Though some researchers did obtain a roughly linear relationship between glass content and strength, there is no well-defined relationship between the glass content and strength of slag cement.

As for the relationship between hydraulicity and glass content, increasing glass content of slag improves its hydraulicity; however, research data that slag samples with 30-65% glass contents are still suitable has not shown exact correlation between them. Due to this uncertainty, most international standards classify slag reactivity by testing its direct strength rather than requiring minimum glass content. But from a practical standpoint, the glass content of slag should exceeds 90% to guarantee satisfactory properties.

  • Chemical composition

As stated above, the chemical composition of slag is mainly the four components, namely, MgO, Al2O3, SiO2, and CaO. From a metallurgical standpoint, slag can be sorted as either basic or acidic, and the more basic of slag, the greater its hydraulic activity in the presence of alkaline activators, Lea also reported that the hydraulic values of slag increase with the increasing CaO/SiO2 ratio up to a limiting value (not precisely stated). Further, in European Standard EN 197-1:1992 and British Standards, the ratio of the mass MgO plus CaO to SiO2 must exceed 1.0, by which the high alkalinity is guaranteed and otherwise the slag would be hydraulically inactive.

With a constant CaO/SiO2 ratio, the strength of hydrated slag increases with the Al2O3 content, and a large amount of Al2O3 can compensate the deficiency of CaO. Further research, by a regression analysis of compressive strength on composition using a wide range of west European slags, showed that increase in Al2O3 content above 13% tended to increase the early strengths but to decrease late strengths. Moreover, the content of Al2O3 also influences the sulfate resistance of slag concrete.

The influence of MgO as a replacement of CaO seems depending on both the basicity and the MgO content of slag. Variations in the MgO content up to 8-10% may have little effect on strength development, but high content have an adverse effect. It also reported that MgO in amount up to 11% was quantitatively equivalent to CaO. Frearon and Higgins reported that to get a satisfactory sulfate resistance the content of MgO should be about 13%.

Many researchers attempted to quantify the reactivity of slag considering the four major components together. Among these results, ratio (CaO+MgO+Al2O3)/SiO2 is the simplest and most widely used one. It was observed that the hydraulic activity of slags increases with the increasing contents of CaO, MgO and Al2O3 but decreases with the increasing content of SiO2. Furthermore, minimum values for this ratio, such as 1.0 (Germany) and 1.4 (Japan) have already been adopted in some countries’ standard specifications.

Apart from the four major components, there are also some minor components that may have important effect on the properties of slag, such as MnO is always negative, P2O5 and alkalis are more complicated.

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.