The chemistry and structure of calcium (alumino) silicate hydrate: A study by XANES, ptychographic imaging, and wide- and small-angle scattering

Abstract

PJM 2018, 'The chemistry and structure of calcium (alumina) silicate hydrate: A study by XANES, ptychographic imaging, and wide-and small-angle scattering', Cement and Concrete Research. https://doi.Abstract 27 28 Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended 29 cement concrete. Understanding the chemistry and structure of C-(A-)S-H is essential to 30 optimizing concrete properties such as compressive strength and durability; yet questions 31 remain around the coordination environments of Ca and Al in its structure with various 32 chemical compositions and equilibration temperatures. C-(A-)S-H with Ca/Si = 0.6-1.6, 33 Al/Si = 0-0.1, and equilibrated at 7-80°C are studied by nanoscale soft X-ray 2 35 the intralayer of C-(A-)S-H irrespective of Ca/Si, Al/Si, and temperature. Zeolitic Ca in 36 the interlayer of C-(A-)S-H are highly distorted from an ideal octahedral coordination.37 Third aluminate hydrate is either not Ca-bearing or its Ca is structurally similar to C-38 (A-)S-H and does not resemble the Ca in AFm-phases. Increasing aluminosilicate chain 39 polymerization in C-(A-)S-H shifts the Si K-edge to higher energies, implying Al uptake 40 in the bridging and/or cross-linked sites, as well as a contraction of Si-O bond lengths. C-41 (A-)S-H exhibits a foil-like morphology, with individual foils comprised of nano-sized 42 platelets with comparable thickness regardless of Ca/Si or Al/Si at 7-50°C. Coarser C-43 (A-)S-H foils occur at 80°C and higher Al/Si ratios relative to lower temperatures and Al 44 content. 45 46 47 Keywords 48 Temperature, calcium-silicate-hydrate, third aluminate hydrate, blended cement, C-A-S-49 H 50 51 1.Introduction 52 53 Calcium (alumino)silicate hydrate (C-(A-)S-H) † is the main binding phase in blended 54 cement concrete[1]. The Ca/Si ratio in calcium silicate hydrate (C-S-H) in hydrated 55 Portland cement (PC) is ~1.7 [2]. The addition of supplementary cementitious materials 56 (SCMs), e.g., fly ash and blast furnace slag, leads to the uptake of Al and a decrease in 57 Ca/(Si+Al) ratio in this phase [3, 4]. C-(A-)S-H equilibrated at room temperature is † Cement chemistry shorthand notation is used throughout the text: A, Al 2 O 3 ; C, CaO; H, H 2 O; S, SiO 2 ; and , SO 3 . S 3 58 structurally analogous to defective tobermorite, which contains calcium oxide polyhedra 59 sheets flanked with "dreierketten" -tetrahedral (alumino)silicate chains -on one side and 60 counter-ions (e.g., Ca and alkalis) and water in an interlayer on the other [5-9]. At 61 Ca/Si<0.6-0.8, long silicate tetrahedral chains occur, which predominantly consist of 62 repeating units of one bridging site (Q 2 B ) ‡ connected to two paired silicate tetrahedral 63 sites (Q 2 P ) on either side. At higher Ca/Si ratios >~1.0 these chains are significantly 64 shorter albeit structurally similar, and have varying degrees of vacant tetrahedra in 65 bridging sites [10]. 66 67 Aluminum incorporated into these chains occur in tetrahedral AlO 4 sites. Five-fold 68 coordinated aluminum (Al V...

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Hybrid binders: A journey from the past to a sustainable future (opus caementicium futurum)

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Microstructure and water absorption of ancient concrete from Pompeii: An integrated synchrotron microtomography and neutron radiography characterization

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