International Concrete Abstracts Portal

This paper describes the effect of fly ash on composition of C-S-H gel. The CaO/SiO2 ratio and chemical bonding water of C-S-H gel were estimated by the combination of the Rietveld analysis by XRD, the selective dissolution analysis and loss of ignition measurement. The results indicated that the CaO/SiO2 ratio of hydrate gel in fly ash-portland cement paste decreased as the hydration of fly ash proceeds, while the CaO/SiO2 ratio of C-S-H gel in portland cement paste did not vary with the progress of hydration. At the same amount of produced C-S-H gel, bonding water in C-S-H gel of paste prepared with 50% fly ash was higher than those of pastes prepared with of fly ash 25% and 0%. The corresponded results can be seen from decreasing of density of C-S-H gel when replacement ratio of fly ash increases. The bonding water in C-S-H gel decreases as its CaO/SiO2 ratio increases. The effect of chemical bonding water in C-S-H gel can be applied to modify the gel/space-strength model.

In this paper experimental evaluation on the effect of high volume fly ash as partial replacement of cement on fracture toughness of cement mortar are presented. The fly ash replacement level was 50%, 60% and 70% by weight of cement. Three-point bend notch beams were used to measure the fracture toughness of mortar. Results show that the use of 50% fly ash as partial replacement of cement reduces the fracture toughness values between 38% and 58% compared to that without fly ash. Reduction of compressive strength and Young’s modulus in mortar containing 50% fly ash as partial replacement of cement compared to that without fly ash is also observed in this study. The use of 60% and 70% fly ash as partial replacement of cement is found to have negligible effect on the reduction of fracture toughness of cement mortar. Long term effects of high volume fly ash (50% cement replacement) on fracture toughness, compressive strength and Young’s modulus of cement mortar are also evaluated in this study. Tests were conducted at 28, 56 and 91 days and at 5, 7, 10 and 12 months. Results show that the rate of increase of fracture toughness of cement mortar containing 50% fly ash with time is very slow. Compressive strength and Young’s modulus also increase with time.

Finely ground lightweight aggregate (LWA) based on calcined and expanded clay has been proven to be quite an effective pozzolan. When replacing cement with 15% LWA fines the concrete compressive strength becomes more sensitive to temperature and time, and 28 day strength is typically 10-15% lower when cured at 5 °C, equal when cured at 20 °C and 15% higher when cured at 35 °C. However, cement replacement by such a mineral increases the concrete resistance to chloride ingress, reduces alkali aggregate reactions and increases the sulfate resistance. Carbonation rate, on the other hand, is somewhat increased as for most pozzolana. Freezing and thawing resistant concrete can also be made with this additive. Thus, finely ground LWA can be utilized to make high performance concrete.

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.