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.
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)
|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.