Abstract
The binding of alkalis and calcium to calcium silicate hydrate (C-S-H) can be measured by cation exchange using cobalt hexamine solution with adapted pH. The effective cation exchange capacity (CEC) decreased at higher Ca/Si in C-S-H. A maximum of chargeexch/Si of 0.08–0.10 was measured at Ca/Si = 0.8 decreasing to 0.01–0.02 at Ca/Si = 1.6. At high Ca/Si, only a minor fraction of Ca was present at exchangeable sites, while most of the Ca2+ present in interlayer or surface was specifically bound and thus not exchangeable. In contrast, sodium was present only at exchangeable sites in the C-S-H, more at low than at high Ca/Si. An increase of pH in the presence of 100 mmol/L NaOH increased the effective CEC of C-S-H by 5–10% indicating a limited influence of pH on the deprotonation of the silanol groups.
Highlights
Calcium silicate hydrate (C-S-H) is the main solid phase present in hydrated Portland cements
The alkalis contained in Portland cements and supplementary cementitious materials (SCM) or used as activa tors sorb partially on C-S-H and remain partially in the pore solution, where they contribute to the high pH values present in the pore solution of Portland cements and blended cements [2,3]
The characteristic bands of C-S-H centered at ~940–970 cm− 1 were assigned to Si–O stretching vibrations and those at 800–825 cm− 1 to Si–O stretching vibrations of Q1 silica [34]
Summary
Calcium silicate hydrate (C-S-H) is the main solid phase present in hydrated Portland cements. The composition of C-S-H may change depending on the availability of calcium and silicon. Hydrated Portland cements usually contain C-S-H with a Ca/Si of ~1.5 to 2 [1]. The alkalis contained in Portland cements and SCMs or used as activa tors sorb partially on C-S-H and remain partially in the pore solution, where they contribute to the high pH values present in the pore solution of Portland cements and blended cements [2,3]. The pH value can strongly affect the stability of the phase assemblage on the cement paste, its integrity and its mechanical properties
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