Mineralogical study of the pozzolanic properties of burned clays.

Portland cement-based materials such as concrete are nowadays the most widely used construction materials. However, the expansion of the cement industry might be a cause of concern due to the high amount of CO2 emitted during the production process. Blending cement with supplementary cementitious materials (SCMs) is considered to be one of the most effective ways of reducing the environmental impact of the cement industry. Conventional and high-quality SCMs like granulated blast furnace slag and fly ash are being utilized to exhaustion. Hence, there is a shift and interest to search for alternative SCM sources due to supply-and-demand concerns in the future. One of the most promising alternative sources are calcined clays since they have not yet reached their full potential as cement replacement and clay is an abundant and widespread material resource.

Calcined clays other than metakaolin are hardly used as SCMs due to the complexity of clay minerals and ignorance of the underlying reaction mechanisms. To fill this knowledge gap, this study investigates the potential use of calcined clays from a mineralogical point of view by linking the characteristics of the untreated clays to the pozzolanic reactivity of the calcined clays. The key starting point of this study consists of a detailed chemical, mineralogical and physical characterization of both the raw and calcined material. To increase the reactivity of natural clays, the raw clay is calcined in a fixed-bed electrical furnace at temperatures ranging between 500°C and 900°C. The pozzolanic reactivity of the calcined clay is assessed by investigating the reaction between calcined clay and lime (Ca(OH)2) and by identifying the hydration products that are formed.

The combined results of this study indicate the mineralogical composition of the starting material is one of the primary factors controlling the pozzolanic reactivity of the calcined clay. Especially two mineralogical features are critical: 1) the type of the dominant clay mineral, with its characteristic thermal behavior, and 2) the amount of these clay mineral and presence of impurities.

The tested pure reference clay types, consisting of 4 kaolinitic, 3 smectitic and 1 illitic clay, can be considered as the most common naturally occurring clay minerals. Kaolinite rich clays are highly reactive at a broad range of firing temperatures (500–900 °C), which are influenced by the degree of ordering of kaolinite. The smectitic clays possess a clear optimal calcination temperature of 800 °C. Trans vacant smectites are proven to be somewhat more reactive than cis vacant smectites. However, even at 800 °C, its reactivity is significantly lower compared to kaolinite. concersely, while the reactivity of kaolinite rich clays reaches a plateau value at 28 days, the reactivity of the smectitic clays continues to increase steadily up to 90 days. Hectorite and illite calcined at an optimal temperature of 800 and 900 °C respectively, exhibit poor pozzolanic reactivity.

Besides the clay type also the presence of non-clay minerals can have an effect on the pozzolanic reactivity. Even though inert materials like quartz, feldspar and muscovite have no direct influence on the pozzolanic reactivity, feldspar and muscovite enhance the sintering phenomenon upon calcination resulting in a coarser grain size and consequently a decrease of reactivity. Moreover, several reactivity tests on artificial mixtures with a variable impurity content demonstrated that the quantity of the inert material can be negatively linearly correlated to the pozzolanic reactivity. Additionally the presence of up to 30% of calcite appears to increase the pozzolanic potential of the clay, especially at early stages of the hydration reaction.  

The pozzolanic reaction rate is also influenced by physical parameters. The specific surface area of the calcined clay is an important parameter during the first 7 days of the reaction. Higher specific surface area results in a larger reaction surface and as a result the reaction rate increases significantly. Additionally the grain size of the calcined sample is a controlling parameter of the pozzolanic reactivity over longer periods of the reaction. The grain size of the calcined clay is influenced by the chosen calcination temperature and by the type and amount of both clay and non-clay minerals, since these factors determine the extent of the sintering effect upon calcination.

The observed correlations are combined in a mathematical model allowing to predict the pozzolanic reactivity of a natural clay based on the mineralogy of the raw sample and for the early stage also the specific surface area of the calcined material.

The effect of the addition of calcined clay on the reaction product assemblage could also be related to the mineralogical characteristics of the clays. The main hydration products that are formed in calcined clay-lime pastes are hydrated calcium aluminates and calcium silicate hydrate phases. Typically for Al-rich kaolinites strätlingite is formed from 7 days onwards. Calcite rich clays are marked by the formation of additional mono- and hemicarboaluminate at the expense of strätlingite. The quantity and the formation rate of the reaction products are mainly influenced by the amount of clay that is present and by the Si/Al ratio of the amorphous material.