Abstract
The pozzolanic reaction of fly ash in hardened cement-based paste internally activated by a natural injection of saturated Ca(OH)2 solution from the age of 1 month was investigated. This investigation was conducted with the aim of comprehensively evaluating the long-term effectiveness of this technique. The Ca(OH)2 content, degree of the fly-ash reaction, and pore structure were measured using thermal gravimetric analysis, selective dissolution, and mercury intrusion porosimetry, respectively. The Ca(OH)2 consumption was calculated from the Ca(OH)2 content in the pastes with 0 and 40% replacements of cement by fly ash. The cement hydration and pozzolanic reactions of fly ash in the pastes proceeded gradually up to 12 months, but were negligible from 12 to 24 months regardless of whether solution was injected. An injection of alkali solution increased both the Ca(OH)2 consumption and the degree of the fly-ash reaction after aging for 12 months. The relationship between the Ca(OH)2 consumption and degree of the fly-ash reaction indicated that the injection of alkali solution activated the surface of the fly-ash particles at early ages, whereas the injection of water did not activate these particles but accelerated the cement hydration, which promoted the pozzolanic reaction. The refinement of the pore structure in the pastes confirmed the effectiveness of the solution injection. Consequently, the injection of saturated Ca(OH)2 solution from the age of 1 month directly activated fly-ash particles and accelerated the long-term pozzolanic reaction (i.e., up to 24 months) in hardened cement paste with 40% replacement by fly ash, although it was not effective in further enhancing the pozzolanic reaction from 12 to 24 months.
Highlights
The use of fly ash as a partial replacement for Portland cement in cement concrete has been increasing, 144 Page 2 of 14 with the aim of developing sustainable concrete technologies [1]
The present study focused on a comprehensive evaluation of the effect of internal alkali activation (IAA) on the long-term pozzolanic reaction of fly ash in hardened cement-based paste that was internally activated by the natural injection of saturated Ca(OH)2 solution from the age of 1 month
3 Results and discussion 3.1 Variation in Ca(OH)2 content with time 3.1.1 Within a depth of 0–4 mm from the tip of the needle inserted in specimens The Ca(OH)2 contents in hardened cement-based pastes within a depth of 0–4 mm from the tip of the needle for all cases [Reference, internal water curing (IWC), and IAACa(OH)2] with time are given in Fig. 4 to express the effect of IAA-Ca(OH)2 on the cement hydration and pozzolanic reaction of fly ash with time
Summary
The use of fly ash as a partial replacement for Portland cement in cement concrete has been increasing, 144 Page 2 of 14 with the aim of developing sustainable concrete technologies [1]. In addition to reducing the significant carbon dioxide emissions from cement production and lowering construction costs [1,2,3,4], flyash utilization improves the mechanical properties and durability of concrete, especially at later ages [3, 5,6,7]; the low reactivity and extremely slow reaction rate of fly ash at ambient temperature usually leads to a low strength and slow rate of strength development of cement concrete at early ages [3, 8]. Via pre-wetted lightweight aggregate, super-absorbent polymers, and roof-tile waste aggregate is recommended to reduce autogenous shrinkage and cracking at early ages and to improve the mechanical properties of fly-ash concrete with a low w/cm [12,13,14,15]
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