International Concrete Abstracts Portal

This investigation experimentally documents mitigation of alkali-silica-reaction (ASR) expansion through partial replacement of cement with coal-biomass cofired fly ash. ASTM C 227 and 441 guide the experimental techniques. Biomass and Class F fly ash reduce ASR expansion within 0.1% at the 6 month with 35% replacement ratio, although biomass fly ash has much higher available Na2Oequiv (by ASTM C 33). Further analysis of pore solution by high pressure extrusion illustrates that biomass fly ash mixes have similar alkali metal concentrations to Class C mixes, but biomass fly ash is at least equal or much better than Class C in mitigating ASR expansion; this implied that biomass fly ash may absorb alkalis from the pore solution and may form non-expansive products instead of the expansive alkali silica gel.

This study reports the suppressing effect of fly ash on the expansion of mortar due to alkali-silica reaction. In this study, 5 types of fly ash were used. As a result, it became clear that the larger the replacement ratio of fly ash, the less the expansion of mortar became. However, the effectiveness of fly ash to suppress the expansion of mortar was different when the chemical composition or physical property of fly ash was different. It is shown that the suppressing effect of the fly ash was affected by the content of amorphous silica and the specific surface area of fly ash. Suppressing mechanism of fly ash for expansion due to ASR can be explained with electrical double layer theory. It is hypothesized that silanol sites on fly ash surface adsorb alkalis as counter ions and alkali concentration in pore fluid becomes low. This theory corresponds to the proposed index in this study.

Alkali-silica reaction known as ASR is series of complex reactions between alkali hydroxides in pore solution and active forms of silica. The product is alkali-silica gel that absorbs water and swells and causes cracks within the fabric of concrete. Using pozzolans as preventative method is very useful to protect against ASR. Zeolite, a new mineral admixture seems to be very efficient in controlling this reaction. In this paper, the effect of replacing portland cement with zeolite coming from Semnan, Iran, and fly ash has been investigated. Results show that zeolite and fly ash can reduce the potential of ASR. By taking economic considerations into account, using zeolite seems is an efficient alternative to be used in concrete for controlling ASR in Iran.

The mixtures of normal portland cement with fly ash from black coal combustion in classic installation and fly ash from brown coal combustion in fluidized bed were examined; the silica fume additive was used as a hydration process modifier. The hydration kinetics was followed by means of calorimetry. Then the calcium hydroxide content was determined by TG method and the electric conductivity of hydrating suspensions was measured. The course of so-called pozzolanic reaction between the additives and calcium ions was investigated and a selective consumption of calcium ions by silica fume was deduced from the data thus obtained. The use of silica fume as an additive brings about the acceleration of hydrolysis of cement components and acceleration of the formation of hydration products, usually by intensive consumption of Ca ions from the liquid phase. It has been found that silica fume, by quick consumption of calcium ions has a retarding effect in the beginning, but soon after, within a few hours, an intensive hydration, with the formation of dense C-S-H, takes place.

This paper investigates the influence of two silica rich fine materials - a fine glass powder used as an inert filler, and nano-silica used as a supplementary cementing material - on the behavior of cement pastes at early ages using electrical impedance. The effective electrical conductivities of the pastes as a function of time, and their derivatives are used to identify the different stages in the microstructure development of the pastes. The incorporation of fine glass powder in the cement pastes in the proportions used in this study does not significantly influence the setting and hardening behavior of the pastes. The increased normalized conductivity of glass powder modified pastes at very early ages even when the local pore volume is reduced by the addition of glass powder filler, and the near equivalence with plain pastes at relatively later times is explained using the pore connectivity factor. The nano-silica modified pastes show significantly different conductivity-time response. It is seen that the setting and hardening phases of cement hydration are accelerated by the presence of nano-silica in the mixture. The nano-silica modified pastes exhibit a significant degree of matrix densification at early ages as could be observed from their conductivity responses as well as compressive strength results.