Abstract
Pastes of white Portland cement (wPc) and wPc-pulverized fuel ash (pfa) blends were studied up to 13years. The reaction of wPc with water was initially retarded in the presence of pfa particles but accelerated at intermediate ages. Reaction with KOH solution was rapid with or without pfa. A universal compositional relationship exists for the C-A-S-H in blends of Pc with aluminosilicate-rich SCMs. The average length of aluminosilicate anions increased with age and increasing Al/Ca and Si/Ca; greater lengthening in the blends was due to additional Al3+ at bridging sites. The morphology of outer product C-A-S-H was always foil-like with KOH solution, regardless of chemical composition, but with water it had fibrillar morphology at high Ca/(Si+Al) ratios and foil-like morphology started to appear at Ca/(Si+Al) ≈1.2–1.3, which from the literature appears to coincide with changes in the pore solution. Foil-like morphology cannot be associated with entirely T-based structure.
Highlights
The main silicate phases that are present in Portland cement are a chemically impure tricalcium silicate (C3S3) that is called alite, and an impure β-dicalcium silicate (β-C2S) that is called belite [1]
Whilst AFm-type phases were not detected by XRD in any of the systems studied in this work, a peak assigned to Al[6] in AFm is present on the 27Al nuclear magnetic resonance (NMR) spectra for the water- and KOH-activated pastes, as illustrated in Fig. 2 for the 9-year-old pastes; presumably there was either too little AFm in the pastes or the crystals were not sufficiently large or ordered to be detected by XRD
Since the TEM results from the present study indicate that in such systems the outer product (Op) C-A-S-H would have mixed fibrillar/foil-like morphology, this indicates that the appearance of foil-like morphology in water-activated pastes is associated with a pore solution that is undersaturated with respect to calcium hydroxide (CH) – in agreement with the results of Rodriguez et al [119] for synthetic C-S-H – and that has a concentration of Si in solution above about 0.35 mmol L−1 and Ca/(Si + Al) ratio in solution that is less than about 2.5 [81,92], which is about twice the value for the C-A-S-H
Summary
The main silicate phases that are present in Portland cement are a chemically impure tricalcium silicate (C3S3) that is called alite, and an impure β-dicalcium silicate (β-C2S) that is called belite [1]. The values of x and y in Eqs (1) and (2) can vary and so the calcium silicate hydrate is commonly referred to as C-S-H, the dashes denoting non-stoichiometry. In neat C3S pastes, the value of x – i.e. the molar Ca/Si ratio of the C-S-H – ranges between 1.3 and 2.1 [3] with a mean value of about 1.75 [4] Substitution of this value for x into Eq (1) and a value of 4.0 for y (which corresponds to a relative humidity (RH) of 90% [2]) shows that the mass % of calcium hydroxide in a fully reacted paste at 90% RH is 28.7%, or 40.6% when referred to the mass of the original C3S (i.e. when expressed on the basis of the ‘ignited weight’). The equivalent values for β-C2S are 7.4 and 10.8%
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
