Fly Ash Cement Systems Research Articles

Compressive strength and microstructure of carbon nanotubes–fly ash cement composites

In this work, carbon nanotubes of 0.5 and 1% by weight were added for the first time in a fly ash cement system to produce carbon nanotubes–fly ash composites in the form of pastes and mortars. Compressive strengths of the composites were then investigated. It was found that the use of carbon nanotubes resulted in higher strength of fly ash mortars. The highest strength obtained for 20% fly ash cement mortars was found at 1% carbon nanotubes where the compressive strength at 28 days was 51.8MPa. This benefit can clearly be seen in fly ash cement with fly ash of 20% where the importance of the addition of carbon nanotubes means that the relative strength to that of Portland cement became almost 100% at 28 days. In addition, scanning electron micrographs also showed that good interaction between carbon nanotubes and the fly ash cement matrix is seen with carbon nanotubes acting as a filler resulting in a denser microstructure and higher strength when compared to the reference fly ash mix without CNTs.

Self-healing ability of fly ash–cement systems

Abstract Concrete is susceptible to cracking due to both autogenous and drying shrinkage. Nevertheless, most of these types of cracks occur before 28 days. Because fly ash continues to hydrate after 28 days, it is likely that hydrated products from fly ash may modify microstructure, seal these cracks, and prolong the service life. This research investigates the self-healing ability of fly ash–cement paste. Compressive strength, porosity, chloride diffusion coefficients, hydration reactions and hydrated products were studied. The research focuses on behavior after 28 days. According to the experimental results, the fly ash–cement system has the self-healing ability for cracks that occur from shrinkage. The self-healing ability increased when the fraction of fly ash increased.

A strength model for no-slump concrete with fly ash

The current paper presents a model for predicting compressive strength of no-slump concrete with and without fly ash by considering the effect of elevated curing temperature. The model was constructed based on the current authors' test results of compressive strength of no-slump concrete. Additionally, the influences of other parameters, such as mix proportion, chemical composition and physical properties of binders, were also taken into account. The model was established based on a relationship between gel/space ratio of paste and compressive strength development ratio of concrete. In the present study, the gel/space ratio concept for cement paste proposed by Powers and Brownyard in 1946 was adopted and extended for cement–fly-ash system application. The gel/space ratio was found to have a good correlation with compressive strength development ratio of concrete. The proposed model was calibrated and verified by the data obtained from the studies of the current authors and other researchers. The verification of the model showed satisfactory accuracy.

Improving the performance of ternary blended cements by mixing different types of fly ashes

For overcoming certain drawbacks characterizing both basic types of fly ashes (of high and low calcium content), different ash intermixtures consisting of two types of fly ashes were prepared. The principal idea lying beneath the effort presented herein is that beneficial assets of the one type of ash could compensate for the shortcomings of the other. Compressive strength development, pozzolanic activity potential and nature of hydration products of all ternary cements were closely monitored and presented in comparison to the respective properties of the initial binary blends. Moreover, efficiency factors were calculated for all new systems and were further used to validate previously reported expressions describing Binary Fly ash-Cement (BFC) systems. In accordance with previous works, ternary fly ash systems examined here outperformed the respective binary systems almost throughout the curing period. Synergy between the different types of fly ashes was considered the main reason for the excellent performance of the ternary mixtures. Results obtained indicate that previously developed analytical expressions, correlating active silica of SCMs and k-values, can be applied in the case of multicomponent ash systems as well.

Assessment of the mechanical stability and chemical leachability of immobilized electroplating waste

The effectiveness of cement based treatment technology, in immobilizing chromium laden electroplating sludge was assessed by conducting toxicity characteristic leaching procedure (TCLP). The mechanical stability of the blocks was tested by measuring the compressive strength. Other leaching tests such as NEN 7341 test, ANS 16.1 and multiple TCLP (MTCLP) test conducted on select solidified blocks showed that chromium was immobilized by the binder studied. A linear relationship was obtained between the cumulative fraction of chromium leached (CFL) and square root of time in the solidified samples proving that diffusion is the controlling mechanism for leaching of chromium. The leachability indices (LI) obtained for the solidified materials using cement and cement-fly ash system (EPC6, EPFC6A and EPFC6B) satisfy the guidance value as per US NRC, which clearly indicates that chromium is well retained within the solid matrix. Chromium concentrations in the TCLP leachates of the above mix ratios were well within the regulatory level of United States Environmental Protection Agency (USEPA). Molecular characterization of the solidified material was carried out using Fourier transformation infra red (FTIR) technique.

Investigating the role of reactive silica in the hydration mechanisms of high-calcium fly ash/cement systems

High-calcium fly ashes (ASTM Class C) are being widely used as a replacement of cement in normal and high strength concrete. In Greece such fly ashes represent the majority of the industrial by-products that possess pozzolanic properties. Even thought the contribution of factors, such as fineness and water/binder ratio, on the performance of fly ash/cement (FC) systems has been a common research topic, little work has been done on examining whether and to what extent reactive silica of fly ashes affects the mechanisms occurring during their hydration. The work presented herein describes a laboratory scale study on the influence of active silica of two high-lime fly ashes on their behavior during hydration. Volumes up to 30% of Greek high-calcium fly ashes, diversified both on their reactive silica content and silicon/calcium oxides ratio, were used to prepare mixes with Portland cement. The new blends were examined in terms of compressive strength, remaining calcium hydroxide, generation of hydration products and microstructural development. It was found that soluble silica of fly ashes holds a predominant role especially after the first month of the hardening process. At this stage, silica is increasingly dissolved in the matrix forming additional cementitious compounds with binding properties, principally a second generation C–S–H. The rate however, that fly ashes react in FC systems seems to be independent of their active silica content, indicating that additional factors such as glass content and fineness should be taken into account for predicting the contribution of fly ashes in the final performance of pozzolanic cementitious systems.

Properties of high-volume fly ash concrete incorporating nano-SiO 2

This paper presents a laboratory study on the properties of high-volume fly ash high-strength concrete incorporating nano-SiO 2 (SHFAC). The results were compared with those of control Portland cement concrete (PCC) and of high-volume fly ash high-strength concrete (HFAC). Assessments of these concrete mixes were based on short- and long-term performance. These included compressive strength and pore size distribution. Significant strength increases of SHFAC compared to the high-volume fly ash high-strength were observed as early as after 3 days curing, and improvements in the pore size distribution of SHFAC were also observed. In this work, the hydration heat of nano-SiO 2 fly ash cement systems was also studied in comparison to the fly ash–cement systems and to the pure cement systems. In addition, the weight change of fly ash incorporating nano-SiO 2, fly ash, and nano-SiO 2 alone after immersed in saturated lime solution was also studied.

Effect of particle size distribution of fly ash–cement system on the fluidity of cement pastes

This paper investigates the effect of particle size distribution of fly ash–cement system on the fluidity of the cement pastes using class F fly ash collected from the hopper attached to an electrostatic precipitator when the burning conditions and types of coal are changed at a coal-fired power plant. The unburned carbon content of fly ashes used in the experiment is less than 1.5%. To prevent the unburned carbon in fly ashes from affecting the fluidity of the pastes, polycarboxylic acid plasticizer with more saturation amount is added into the pastes for experimentation. The particle size distribution of fly ash–cement system is analyzed using n value of Rosin–Rammler function and the n value is derived with a nonlinear least squares fitting method. As the result, it is shown that the fluidity increases as the particle size distribution becomes wider, i.e., as n value gets smaller.

Strength and microstructural characteristics of chemically activated fly ash–cement systems

The use of fly ash as a cement replacement material increases the long-term strength and durability of concrete. Despite these great benefits, the use of fly ash is limited due to the low early strength of fly ash concrete. To eliminate this problem, many studies have been conducted on accelerating the pozzolanic properties of fly ash. The study reported below investigated the strength and microstructural characteristics of fly ash–cement systems containing three kinds of activators—Na2SO4, K2SO4, and triethanolamine—to accelerate the early strength of fly ash mortars. Through the use of thermal gravity analysis, it was demonstrated that the activators not only decreased or maintained the amount of Ca(OH)2 products, but also increased the production of ettringite at early ages. X-ray diffraction, scanning electron microcopy, and mercury intrusion porosimetry also confirmed that in the early curing stages of fly ash–cement pastes containing activators, large amounts of ettringite were formed, resulting in a reduction in the pore size ranging from 0.01 to 5 μm. The research results support the supposition that the addition of small amounts of activators is a viable solution for increasing the early-age compressive strength of fly ash concrete.

Activation of fly ash/cement systems using calcium sulfate anhydrite (CaSO 4)

A number of studies had been conducted on the activation of fly ash using gypsum and sodium sulfate. Anhydrite, another form of calcium sulfate, has not been used for this purpose. This paper presents an exploratory study on the effectiveness of anhydrite in activating fly ash cement systems. Anhydrite (10%) was added into cement mortars with up to 55% fly ash replacement. The prepared mortars were allowed to cure in steam at 65°C for 6 h before normal room temperature water curing. Significant strength increases (up to 70%) compared to the control mortars were observed as early as after 3 days curing. Improvements in the pore size distribution of the mortars were also observed due to the activation. The results of scanning electron microscopy (SEM) examination and quantitative X-ray diffraction (XRD) analysis show that, with accelerated curing, a large quantity of ettringite (AFt) was formed during the early stage of hydration of the anhydrite-activated fly ash cement pastes. This might be the main cause of the high early strength of the activated fly ash cement systems. A comparison was made using anhydrite and gypsum as activators. For an equivalent SO 3 content, anhydrite is more effective in increasing the early-age strength of the cement/fly ash mortars, but less effective in increasing the later-age strength than gypsum.

95/01412 Microstructural development in high-volume fly-ash cement system

95/01412 Microstructural development in high-volume fly-ash cement system

The influence of fly ash cenospheres on the details of cracking in flyash-bearing cement pastes

Compact tension specimens were prepared from a cement-flyash paste with an especially-high content of cenospheres, to permit examination of the details of the influence of cenospheres on the cracking pattern obtained on loading such specimens in the SEM. It was found that even after extensive aging, the advancing crack typically went around the cenosphere-paste interface rather than cleaving through the cenosphere itself. Branching of the crack on its path around the cenosphere perimeter was commonly observed. Thus, cenospheres in flyash-cement systems appear to act as energy-dissipating inclusions in fracture and do not necessarily weaken the system.