International Concrete Abstracts Portal

The high-volume fly ash concrete (HVFAC) was developed by Malhotra and his associates in the mid 1980s. Typically, this concrete is made with low water-to-cementitious materials ratio, low cement content, and high fly ash content. This type of concrete has all the attributes of high-performance concrete, in addition to being environmentally friendly. In 2002, mainly because of its environmentally friendly aspects, CANMET was awarded a project by the Canadian International Development Agency (CIDA), to implement the HVFAC technology in India in order to reduce the CO2 emissions related to cement production in that country. This project was funded by the Canada Climate Change Development Fund, and was administrated by CIDA. In one of the project activities, undertaken to adapt the HVFAC to Indian materials and conditions, studies were carried out in a number of Indian laboratories. This paper presents the results of one such investigation performed at the Bengal Engineering and Science University, Shibpur, near Kolkata, India. Concrete of grades M20, M40 and M60 (nominally 20, 40 and 60 MPa) using fly ash with ordinary portland cement (OPC) and a fly ash blended cement, called portland-pozzolana cement (PPC) in India, were investigated in this study. For each grade of concrete made with OPC, four concrete mixtures were made, namely a control concrete without fly ash, and concrete incorporating 30, 40 and 50% fly ash. For each grade of concrete made with PPC, three concrete mixtures were made including one made with PPC only, one with cementitious materials incorporating 40% fly ash, and the third with cementitious materials incorporating 50% fly ash. For each concrete mixture, the compressive strength at several ages (up to 91 days), splitting-tensile strength, flexural strength and resistance to chloride-ion penetration at 28 and 91 days were determined. The results illustrate that for the same grade of concrete, with the same total amount of cementitious materials, and the same proportion of fly ash, the use of PPC in combination with fly ash resulted in increased compressive strength at early ages and somewhat lower chloride-ion penetrability at 28 days.

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.