Residual stress on ettringite crystals in mature Type K shrinkage-compensating cement pastes

This study is mainly focused on investigation of the relative content, morphology, chemical composition, microstructural parameters, and the deformation state (mechanical strains and stresses) of ettringite [Ca6Al2(SO4)(3)(OH)(12)<middle dot>26H(2)O] crystals in 1-year-old Type K shrinkage-compensating cement pastes depending on variation of the relative content of ye'elimite admixture (YA) up to 12 wt%. The XRD, SEM-EDS, and optical microscopy techniques were used as characterization methods. Amongst the identified main hydration products in cement pastes: ettringite, calcium-silicate-hydrate (C-S-H), and calcium hydroxide (CH), with increase of the content of YA from 3 wt% to 12 wt% the relative content of the C-S-H and CH phases strongly decreases, whereas the content of ettringite considerably increases. In YA-containing cement pastes, it is revealed formation of near-spherical "ye'elimite-ettringite" clusters (a cluster is composed of a partially hydrated ye'elimite particle/core covered by small-sized, approximate to 1 mu m, radially oriented ettringite crystallites) that exhibit toroid-like agglomerations. For reference sample (prepared from Portland cement with no YA), the average stress components sigma(a) and sigma(c) on etteringite crystals (along the a and c crystallographic axes, respectively) are determined to be -6.6 and -40 MPa, respectively. With increase of the content of YA in cement paste the stress components on etteringite crystals strongly increase reaching maximum values sigma(a)=-163.9 MPa and sigma(c)=-76.9 MPa at 12 wt% of the YA. According to a discussed model, the activation of the shrinkage-compensating effect in YA-containing cement pastes is attributed to formation of the "ye'elimite-ettringite" clusters with a sufficiently high concentration. It is predicted that, amongst the examined samples, the shrinkage-compensating effect is achieved in cement pastes with the YA contents of 9 and 12 wt%.