Abstract
AbstractThe effect of microencapsulated phase‐change materials (MPCM) on the rheological properties of pre‐set geopolymer and Portland cement mortars was examined. Microcapsules with hydrophilic and hydrophobic shells were compared. The shear rate dependency of the viscosities fitted well to a double Carreau model. The zero shear viscosities are higher for geopolymer mortar, illustrating poorer workability. The time evolution of the viscosities was explored at shear rates of 1 and 10 s−1. New empirical equations were developed to quantify the time‐dependent viscosity changes. The highest shear rate disrupted the buildup of the mortar structures much more than the lower shear rate. Microcapsules with a hydrophobic shell affect the rheological properties much less than the microcapsules with a hydrophilic shell, due to the higher water adsorption onto the hydrophilic microcapsules. Shear forces was found to break down the initial structures within geopolymer mortars more easily than for Portland cement mortars, while the geopolymer reaction products are able to withstand shear forces better than Portland cement hydration products. Initially, the viscosity of geopolymer mortars increases relatively slowly during due to formation of geopolymer precursors; at longer times, there is a steeper viscosity rise caused by the development of a 3D‐geopolymer network. Disruption of agglomerates causes the viscosities of portland cement mortars to decrease during the first few minutes, after which the hydration process (increasing viscosities) competes with shear‐induced disruption of the structures (decreasing viscosities), resulting in a complex viscosity behavior.
Highlights
Geopolymers are synthesized by reaction of amorphous aluminosilicates, for example, fly ash or slag, with a concentrated alkaline solution such as sodium hydroxide or sodium silicate.[1]
There are very few studies examining how the rheology of geopolymers or Portland cement composites is affected by microencapsulated phase-change materials (MPCM) addition.[15,18,34,35,36]
Shear rate dependencies of Portland cement paste containing MPCM has been examined, and it was found that the pastes went from shear thinning to shear thickening behavior when the amount of superplasticizer was increased.[36]
Summary
Geopolymers are synthesized by reaction of amorphous aluminosilicates, for example, fly ash or slag, with a concentrated alkaline solution such as sodium hydroxide or sodium silicate.[1]. There are very few studies examining how the rheology of geopolymers or Portland cement composites is affected by MPCM addition.[15,18,34,35,36] Most previous studies only measure slump, which generally show that MPCM addition increase the water demand to obtain a constant slump value,[34,36] or analogously that adding microcapsules decrease the slump.[15,18,36] More thorough rheological investigations utilizing rheometers or viscosimeters are scarce.[35,36] Shear rate dependencies of Portland cement paste containing MPCM has been examined, and it was found that the pastes went from shear thinning to shear thickening behavior when the amount of superplasticizer was increased.[36] The time-dependent viscosities of geopolymer pastes containing three different MPCMs have been studied at a constant shear rate.[35] It was found that at the start of the reaction, there was a slow viscosity increase due to the formation of geopolymer precursors, and that these precursors formed faster in the presence of MPCM. The MPCM concentration was varied (replacing 0-20 vol% of the sand) Both the shear rate dependency of the viscosities of mortars shortly after mixing and the development of the viscosities over time has been explored. New empirical equations have been developed to quantify the time-dependent changes of the viscosities and the reaction kinetics
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