ASR in mortar bars containing silica glass in combination with high alkali and high fly ash contents

In the paper, the problem of ASR in mortar systems with high contents of alkali and fly ash is studied. The results show that the danger of ASR exists in this system which it is different from ordinary plain cement system because in these systems, serious ASR was accompanied by great expansion of the specimens studied.     

Compressive strength and chloride resistance of self-compacting concrete containing high level fly ash and silica fume

The influence of high-calcium fly ash and silica fume as a binary and ternary blended cement on compressive strength and chloride resistance of self-compacting concrete (SCC) were investigated in this study. High-calcium fly ash (40–70%) and silica fume (0–10%) were used to replace part of cement at 50, 60 and 70 wt.%. Compressive strength, density, volume of permeable pore space (voids) and water absorption of SCC were investigated. The total charge passed in coulombs was assessed in order to determine chloride resistance of SCC. The results show that binary blended cement with high level fly ash generally reduced the compressive strength of SCC at all test ages (3, 7, 28 and 90 days). However, ternary blended cement with fly ash and silica fume gained higher compressive strength after 7 days when compared to binary blended fly ash cement at the same replacement level. The compressive strength more than 60 MPa (high strength concrete) can be obtained when using high-calcium fly ash and silica fume as ternary blended cement. Fly ash decreased the charge passed of SCC and tends to decrease with increasing fly ash content, although the volume of permeable pore space (voids) and water absorption of SCC were increased. In addition when compared to binary blended cement at the same replacement level, the charge passed of SCC that containing ternary blended cement was lower than binary blended cement with fly ash only. This indicated that fly ash and silica fume can improve chloride resistance of SCC at high volume content of Portland cement replacement.     

Investigation into the synergistic effects in ternary cementitious systems containing portland cement, fly ash and silica fume

This research was primarily conducted to verify the presence of synergistic effects in ternary cementitious systems containing portland cement (OPC), class C fly ash (FA) and silica fume (SF). A subsequent objective of the study was to quantify the magnitude of the synergy and to determine its source. For a ternary mixture containing 20% FA and 5% SF by mass, the synergistic effect was observed mostly at later ages (7 days onward) and it resulted in an increased compressive strength and resistance to chloride ion penetration as well as a reduced rate of water absorption (sorptivity) compared to predictions based on individual effects of FA and SF in respective binary systems. The observed synergy was attributed to both chemical and physical effects. The chemical effect manifested itself in the form of an increased amount of hydration products. The physical effect associated with packing density was, somewhat contrary to general belief, not due to an optimized particle size distribution of the binder components of the ternary cementitious system. Instead, it was the result of smaller initial inter-particle spacing caused by lower specific gravities of both FA and SF which, in turn, led to a lower volumetric w/cm. If the mixture design was adjusted to account for these differences, the physical effect would be diminished.     

Decoupling the effects of chemical composition and fineness of fly ash in mitigating alkali-silica reaction

A comprehensive investigation was conducted to determine the individual effects of the chemical composition and particle size of fly ash on alkali-silica reaction (ASR). Test results indicated that the combined oxides content of fly ash showed a better correlation with the ASR expansions than its individual oxides. Mixtures containing finer fly ash fractions registered lower expansions than those containing the corresponding virgin fly ashes or its coarser fractions.Within the usual range of average particle size of 10 to 30 microns, of fly ash, the chemical composition had a more dominant influence on ASR mitigation than the particle size. However, when the average particle size of fly ash decreases below 10 microns, the fineness of fly ash becomes significant in mitigating ASR. In addition, the fineness of fly ash had a more significant influence in mitigating ASR in mixtures containing high-lime fly ashes than those containing low-lime fly ashes. Hence, reducing the particle size of fly ash to finer fractions is an effective strategy to mitigate ASR. The decoupling of the chemical composition of fly ash from its particle size indicated that ASR mitigation can be achieved with any fly ash having a D₅₀ below 5 μm. However, low-lime fly ashes were effective in mitigating ASR even without reducing their particle size.     

Hydration of quaternary Portland cement blends containing blast-furnace slag,siliceous fly ash and limestone powder

In this study the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days using X-ray diffraction and thermogravimetric analysis. In addition thermodynamic modeling was used to calculate the total volume of hydrates. Two blast-furnace slag contents of 20 and 30 wt.% were studied in blends containing fly ash and/or limestone at a cement replacement of 50 wt.%. In all cases the experiments showed the presence of C–S–H, portlandite and ettringite. In samples without limestone, monosulfate was formed; in the presence of limestone monocarbonate was present instead. The addition of 5 wt.% of limestone resulted in a higher compressive strength after 28 days than observed for cements with lower or higher limestone content. Overall the presence of fly ash exerts little influence on the hydrate assemblage. The strength development reveals that amounts of up to 30 wt.% fly ash can be used in quaternary cements without significant loss in compressive strength.     

A multiscale micromechanical approach to model the deteriorating impact of alkali-silica reaction on concrete

The alkali-silica reaction (ASR) in concrete is one of the most harmful deterioration processes, which leads to expansion and cracking of the material. To understand the evolution of ASR in concrete and its deteriorating impact on the material, a multiscale material model, from aggregate to concrete level, is proposed. The concrete, which at macro scale is considered a homogeneous material, is micromechanically modelled by a matrix-cracks system, in which each phase is uniform and behaves elastically. The damage criterion, associated to the cracks, is formulated on the basis of linear fracture mechanics theory. The model, which is analytically solved, is based on a limited numbers of input parameters, to be determined via micro and macro experimental investigations. The model is able to predict the non-linear behaviour of concrete subject to uniaxial loading in good agreement with code formulations, which are usually input for numerical analyses of structures. For the case of ASR-affected material, the model overestimates the degradation rate of mechanical properties as a function of the expansion. On the contrary, the relationship between stiffness and strength deterioration is correctly approximated. Various model modifications are explored suggesting that the assumption of elastic behaviour of each phase should be reconsidered.     

Microanalysis and optimization-based estimation of C-S-H contents of cementitious systems containing fly ash and silica fume

In this study, a new approach to characterize hardened pastes of pure portland cement as well as those containing cement with supplementary cementitious materials (SCM) was adopted using scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDS) microanalyses. The volume stoichiometry of the hydration reactions was used to estimate the quantities of the primary and secondary calcium silicate hydrate (C-S-H) and the calcium hydroxide produced by these reactions. The 3D plots of Si/Ca, Al/Ca and S/Ca atom ratios given by the microanalyses were compared with the estimated quantities of C-S-H to successfully determine the Ca/Si ratio of eleven different cementitious systems at four different ages using a constrained nonlinear least squares optimization formulation by General Algebraic Modeling System (GAMS). The estimated mass fraction of calcium hydroxide from the above method agreed well with the calcium hydroxide content determined from the thermogravimetric analyses (TGA).     

Assessment of in-situ alkali-silica reaction (ASR) development of glass aggregate concrete prepared with dry-mix and conventional wet-mix methods by X-ray computed micro-tomography

A recent study showed that concrete products prepared with the dry-mix method have better alkali-silica reaction (ASR) resistance than that prepared by the wet-mix method. But the mechanism of ASR in dry-mix glass concrete remains unclear. Meanwhile, the techniques such as Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) cannot reflect the in-situ evolution of the microstructure with the progress of the ASR. In this study, two common casting methods, the wet-mix and dry-mix methods, were adopted to prepare the glass concrete. The non-destructive X-ray computed micro-tomography (X-ray μCT) technique was applied to observe the pore geometries of both the wet-mix and dry-mix glass concrete, to determine their porosities using 3D volumes and to investigate the generation of cracks during ASR development. This study firstly observed the irregular pore geometry of the glass concretes by quantitatively comparing the pore geometries using the sphericity developed by Wadell for both dry-mix and wet-mix glass concrete. The test results of the porosity measured by 3D volume showed that the porosity of the dry-mix glass concrete decreased after the ASR test. However, no obvious change was observed in the porosity of the wet-mix glass concrete. This change may be attributed to the large pores in the dry-mix glass concrete which can accommodate the ASR gel. Through the in-situ observation using 3D X-ray μCT, no new cracks were generated in the dry-mix glass concrete during the progressive development of ASR. On the contrary, new cracks which were filled with ASR gel were densely distributed in the wet-mix glass concrete, which led to failure of the concrete matrix. This is because the expansive ASR gel formed could not be accommodated by the limited pore space in the wet-mix glass concrete, and the swelling pressure of ASR gel induced new cracks in the wet-mix glass concrete.     

Swellings due to alkali-silica reaction and delayed ettringite formation: Characterisation of expansion isotropy and effect of moisture conditions

This paper investigates the effect of different conditions on the development of concrete expansions due to alkali-silica reaction (ASR), delayed ettringite formation (DEF) and their combination. The presence of products of the two reactions has been observed during structure diagnosis. The aim of this research is to study the two reactions in concretes with close mix designs but with various types of aggregate and moisture conditions. Measurements performed in the three directions of stress-free specimens showed that DEF expansions could be considered as isotropic for stress-free material. DEF expansions were largely influenced by the storage conditions (immersed in water or in sealed conditions). The volume of storage water modified the kinetics. Under sealed conditions, no expansions were measured for mortar containing non-reactive aggregate, while small positive strains were obtained for mortar containing reactive aggregate. In all cases, new water supply caused fast, large expansions. The different effects of alkali leaching and moisture conditions on DEF and ASR expansions are discussed.     

The anticorrosive effect of fly ash, slag and a Greek pozzolan in reinforced concrete

The influence of the addition of 15% and 30% fly ash, 15% and 30% of a Greek natural pozzolan and 50% granulated blastfurnace slag to ordinary Portland cement on the corrosion resistance of the reinforcing bars was studied in a program of long-term exposure to seawater. The use of blended cements resulted in a decrease in the corrosion rate, especially after long exposure times. The most effective protection was rendered by the 30% fly ash mix. This performance was related to the chloride content and the chloride binding capacity of the blended cements.     

Arsenic and lead release from fly ash stabilized/solidified soils under modified semi-dynamic leaching conditions

A fly ash-based stabilization/solidification (S/S) technique was investigated using field soil samples contaminated with arsenic (As) and lead (Pb). A semi-dynamic leaching test was used to evaluate the effectiveness of the S/S treatment. By assessing the cumulative fractions of leached As and Pb, the effective diffusion coefficient (D(e)) and a leachability index (LX) were measured and used for evaluating the effectiveness of the S/S treatment. Overall, As release was reduced by 98.3% and Pb release was reduced by 98.5% upon addition of 25% Class C fly ash. The mean D(e) decreased significantly and the mean LX was always above 9 for all treated samples, indicating that the treated soils were acceptable for "controlled utilization". The mechanism controlling As leaching from all treated samples appeared to be a mixture of wash-off and diffusion. Diffusive As release was proportional to fly ash content. The mechanism controlling Pb leaching when samples were treated with 25% fly ash appeared to be wash-off.     

SEM/EDS and XRD characterization of raw and washed MSWI fly ash sintered at different temperatures

The disposal of fly ash generated during municipal solid waste incineration (MSWI) may pose a significant risk to the environment due to the possible leaching of hazardous pollutants, such as toxic metals. Sintering technology attracted more attention than the vitrification process because of its low energy needed. Generally, a preliminary washing treatment of raw fly ash with water was necessary for this sintering technology. This study investigated the composition and morphology of raw fly ash (RFA) and washed fly ash (WFA) at different sintering temperatures, and examined the newly formed minerals during sintering. Toxicity characteristic leaching procedure (TCLP) tests were carried out to investigate the effect of the washing treatment and sintering process on the leaching performance of heavy metals in fly ash. Results showed that, with an increase of sintering temperature more complex aluminosilicates were formed; the incorporation of Mg, Fe and Pb into the aluminosilicates occurred during the sintering process at higher temperatures (800 and 900 degrees C). The washing treatment reduced the leachable concentration of Cd, Pb and Ni, but increased that of Cr. A CaCrO(4) compound was considered as a potential soluble species.     

Synthesis of Na-X zeolite from low aluminum coal fly ash: Characterization and high efficient As(V) removal

Na-X zeolite was successfully prepared from low aluminum coal fly ash (LACFA) via fusion-hydrothermal treatment. The influence of various synthesis parameters was investigated, including aluminum additives (AlCl₃·6H₂O, Al(NO₃)₃·9H₂O, AlF₃·3H₂O and NaAlO₂), dosages of NaAlO₂, weight ratio of LACFA/NaOH, crystallization temperature and time. The results indicated that the addition of Al species played a key role in the synthesis process of purity Na-X zeolite, and the corresponding relative crystallinity of the obtained samples was in the order of Na-X zeolite_(Al)???Na-X zeolite_(Cl)?>?Na-X zeolite_(N)?>?Na-X zeolite_(F) (Here, “Al”, “Cl”, “N” and “F” represent NaAlO₂, AlCl₃·6H₂O, Al(NO₃)₃·9H₂O and AlF₃·3H₂O, respectively). And the optimal operating conditions were: the weight ratio of LACFA/NaOH?=?0.83, short crystallization time 360?min, low crystallization temperature 90?°C and 0.038?mol NaAlO₂. Additionally, X-ray fluorescence (XRF), X-ray powder diffraction (XRD), Fourier Transforms Infrared (FT-IR) and Scanning Electron Microscopy (SEM) were employed to determine the resultant samples. Based on the results of As(V) adsorption experiment, the experimental data was suitable fitted by Freundlich adsorption isotherm model, and the theoretical maximum adsorption capacity was 27.79?mg/g at pH?=?2.14. The kinetics studies suggested that “surface reaction” was the rate-determining step of adsorption process, and the thermodynamics studies indicated that the adsorption process was spontaneous and endothermic. These results deemed that the LACFA was suitable for preparing Na-X zeolite_(Al), and the obtained Na-X zeolite_(Al) was served as a promising adsorbent to remove As(V) from acid wastewater.     

高延性纤维增强偏高岭土-粉煤灰基地聚合物在不同环境下的自愈合性能

在1%、2%及3%不同程度预加单轴直接拉伸应变破坏下,研究了3天、7天及28天龄期的高延性聚乙烯醇（PVA）纤维增强偏高岭土-粉煤灰基地聚合物（PVA/MK-FA EGC）在空气中和干湿循环条件下的裂缝分布及自愈合性能。结果表明：PVA/MK-FA EGC结合了传统高延性纤维增强水泥基复合材料（ECC）及地聚合物二者的优点,表现出了明显的多缝开裂特性和应变硬化行为。2~5 mm的裂缝间距、小于25 μm的最大残余裂缝宽度给裂缝的自愈合提供了更加有利的条件。带缝试件在不同环境中自愈合后,裂缝数量大大下降,极限拉伸应变可达3.8%以上,大部分试件的极限拉应变及最终应力均能超过对比试件,空气中的养护环境更加有利于PVA/MK-FA EGC材料的自愈合。裂缝内颗粒表面覆盖有凝胶状的地聚合产物,可能增强了体系中的纤维/基体界面,使力学性能恢复。     

Strength and durability properties of polyester concrete using pet and fly ash wastes

Recycled poly(ethylene terephthalate), PET, mainly recovered from plastic beverage bottles, can be used to produce unsaturated polyester resins. In turn, these resins can be mixed with inorganic aggregates (sand and gravel and fly ash waste), to produce polyester concrete (PC). The strength and durability properties of plain and steel-reinforced polyester concrete (PC) using unsaturated polyester resins based on recycled PET and fly ash fillers are discussed in this paper. The recycling of PET and fly ash in PC helps in reducing the cost of the material and alleviating an environmental problem posed by waste materials. The material may effectively be used in many construction applications such as utility, transportation and building components, and the repair and overlay of pavements, bridges and dams.     

Influence of recycled aggregates and high contents of fly ash on concrete fresh properties

This study presents the fresh properties of concrete with supplementary cementitious materials (SCM) and recycled concrete aggregates (RCA), with emphasis on the feasibility of using high volumes of fly ash (FA) in RCA concrete. For this purpose, two mix families (0% coarse RCA and 100% coarse RCA) were produced, both with and without superplasticizers (SP). The coarse natural aggregates (NA) were replaced with coarse RCA at 0% and 100%, respectively. For each of the mentioned families, three incorporation levels (0%, 50% and 100%) of fine RCA were used with 0%, 30% and 60% of FA, resulting in 28 compositions. Each mix was tested in the fresh state by means of slump, density and air content. The results of this study show that RCA decreased the slump of concrete mixes, but the required water content can be minimized by incorporation FA. Regardless of the water absorption of the aggregates, for a given fine RCA incorporation ratio and the same ratio of FA, no increase in water content is required to obtain the same target slump as in the reference concrete. On the other hand, for a given coarse RCA incorporation ratio, a five times lower FA ratio is enough to obtain the same target slump as in the reference concrete. Air voids in concrete mixes were more affected by the shape of the aggregates than by their water absorption. The air content of concrete mixes increased as the incorporation levels of FA and RCA increased. However, in comparison with the individual effects, the air content decreased by combining the incorporation of both FA and RCA. Moreover, the rate of reduction in fresh density by increasing the incorporation of RCA and FA was similar in concrete mixes with and without SP.