Abstract
Alkali-activated calcined clays are promising candidates for playing a prominent role in the future construction industry. These binders may achieve excellent mechanical performance, but one issue deserving attention is the proneness to plastic shrinkage and surface cracking. Tackling this issue requires the deployment of laboratory techniques that allow shrinkage-inducing mechanisms to be quantitatively assessed. Here, we demonstrate that time-lapse X-ray imaging can be used to quantify shrinkage immediately after mixing, when the binder is still in its fresh state, with excellent time and space resolution. The numeric quantification of strain is complemented by the real time visual inspection of the displacing sample interface and of the bleed aqueous solution layer that may form. Implementation of this method to a set of alkali-activated cement pastes, prepared by combining calcined clays having different mineralogical composition with sodium silicate activating solutions having different hbox {SiO}_2/hbox {Na}_2hbox {O} ratios, suggests that two main mechanisms control the early dimensional stability of alkali-activated calcined clays. These mechanisms are: (a) volumetric contraction occurring in response to capillary stress arising from water evaporation and (b) segregation by particle settling, favoured in the water-saturated regime.
Highlights
Much attention is currently being devoted to calcined clays as key raw materials for the development of sustainable binders
Strain is controlled by the following machanisms: (a) autogenous shrinkage, which at early ages corresponds to chemical shrinkage [38]; (b) capillary stresses associated with evaporation, which has been observed to induce the formation of vertical moisture gradients at early ages [39, 40]; (c) gravity-induced particle settling
We have presented a time-lapse X-ray imaging method for the study of shrinkage occurring in the early stage of cement reaction, prior to setting
Summary
Much attention is currently being devoted to calcined clays as key raw materials for the development of sustainable binders. Alkali-activated materials, based on calcined clays or other alumino-silicate reactants, represent one possible class of Portland-free cements that may achieve excellent mechanical properties, their actual environmental footprint is strongly dependent on the specific formulation adopted and choice of chemical activators [2], which represent a significant share of the final cost. Despite the excellent mechanical strength of well formulated alkali-activated calcined clays, the performance of these materials is still lagging behind that of Portland cement, when dimensional stability is considered. Quantification of this property, especially at early stage of reaction, has not so far received the same degree of attention as mechanical properties did
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