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Pozzolanic activity of natural clay minerals with respect to environmental and civil engineering applications
Clays and cements are among the most widely-used materials in many geotechnical and environmental engineering applications and are often applied as a composite material. In the field of geotechnical engineering, injection grouting, slurry wall and soil stabilisation techniques have to be mentioned. In the environmental geotechnical field, applications include engineered barrier systems for landfill, contaminated land and nuclear repositories. The extent of chemical reaction of the two materials in the composite and the implications for strength development and long term performance of engineered barriers or stabilised soils is still not well-understood.
Hydrated Portland cement contains up to 25 % calcium hydroxide (CH) which might react slowly with so called pozzolanic materials to form cementitious products. Even though natural clays are excellent pozzolanic materials according to ASTM C618-89a (He et al. 2000), little is known about their pozzolanic activity or their reaction tendency with CH. The pozzolanic reaction is a CH consuming process in contrast to the CH producing process of cement hydration.
Generally in cementitious composites, CH might have several negative effects on strength, permeability, hydraulic conductivity and thus durability of cementitious composites. CH buffers both pH and calcium concentration at high values, leading to high chemical gradients with the surrounding soils. This potentially leads to high leaching rates of CH from the composite accompanied by an increase in permeability. In contrast, the pozzolanic reaction usually leads to a more durable material as a result of the consumption of CH and its substitution by hydrated phases with higher molar volumes which fill up capillary space. In the composite, pore space and permeability decrease and strength slightly increases during this pore refinement process (Mehta & Monteiro 2002). Additionally, the complete consumption of CH substantially increases the chemical durability in acidic and sulphate environments even in the presence of reactive alumina or silica.
The general goal of the present project is to determine the time-dependent extent of pozzolanic reaction of three different clay minerals and their resulting reaction products. Therefore typical constituents of various clayey materials, a kaolinite, an illite and a montmorillonite were chosen. Work is performed in the relatively simple system of "pure" clay minerals mixed with CH. This initial model provides the basis for understanding the behaviour in the more complex system of clays mixed with cement (additional amounts of sodium, potassium or sulphate complicate the chemical interaction of the two materials).
First results confirmed that reaction products are hydrated minerals similar to products known from cement hydration. Two main products have been identified, strength-constituting calcium silicate hydrates (C-S-H) and hydrated aluminate phases (mainly Afm, here shortly termed LDH for Layered Double Hydroxides). LDH is one of the few known inorganic anion adsorbers. Therefore, increasing interest in environmental applications is focussed on LDH.
Knowing the amounts of consumed CH and clay mineral, the C-S-H to LDH ratio in the reaction products for each clay mineral is calculated from their Si to Al ratio (with some assumptions and simplifications). This knowledge enables a control of the increase in strength by C-S-H and anion sorptive capacity by LDH in the composite through pozzolanic reaction. Mixture design of clays with CH / cements might be optimised in such a way that CH is completely consumed, thus improving the chemical durability of the resulting material. Future work will focus on the prediction of porosity and strength from mineralogical data for clay / CH composites of a given age.
Authors:Müller, Christian and Kahr, Günter and Hermanns Stengele, Rita
Index Terms:pozzolanic; Clay; ClayGroup; environmental geotechnics
Further Information:Date published: 17.09.2003