Abstract
This work deals with the comparison of micromechanical properties and microstructure of cement pastes with additives used in concrete to reduce the lateral pressure on the formwork. This work is a steppingstone for broader investigation of the lateral pressures on different cement mixtures. The work focused on two additives used for the purpose, namely calcined clay (metakaolin) and a type of nanoclay sepiolite. A scanning electron microscope was used to describe their microstructure. Micromechanical properties of both cement composites were investigated by nanoindentation. Large statistical grids of indents were performed on three sample types: plain cement paste and two mixtures containing the enhancing additive of metakaolin and nanoclay. From the evaluated results in the form of property histograms, the modulus of elasticity, hardness and creep parameter were derived. It was found that in the cement paste with metakaolin the amount of C-S-H gel increased compared to the control mixture. Increased portlandite and the amount of unhydrous clinker was found in the cement paste with nanoclay. Nanomechanical response of individual phases was derived from overall property histograms by statistical deconvolution. The results were confirmed by electron microscopy. The micromechanical research was supplemented with the measurement of the compressive strength on cubes at the macroscopic level.
Highlights
In the construction industry, concretes in their fresh state are cast into formworks
Values were calculated from merged results of nanoindentation
The results show that the wrapping curve of elastic modulus for CM has shifted to the left compared to the control mixture
Summary
Concretes in their fresh state are cast into formworks. The casting process of tall structures is done in steps to allow mixture compaction and stepwise vibration. Lateral pressures introduced by the ordinary concretes on formwork diminish relatively soon after casting due to the very high mixture viscosity and prolonged casting times. Self-compacting concretes (SCC) are characterized by low viscosity with no need of additional vibration to fill the formwork [1,2,3,4]. SCC are usually poured into formwork in one step filling the whole height of the structure (e.g. a column or a wall). The long-lasting mixture flowability is usually maintained by additives (plasticizers) and hydration retarders and even after casting the mixture behaves like a fluid that introduces high lateral pressures on the formwork
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