•Backscattered electron microscopy used to study ITZ of mortar with RAP aggregates.•Larger, more porous ITZ forms from RAP aggregates compared to dolomite.•Less CH and C-S-H are present in ITZ near ...the RAP interface.•Silica fume does not significantly improve ITZ properties in mortar with RAP.
Reclaimed asphalt pavement (RAP), when used as a coarse aggregate, has been shown to reduce bulk concrete strength and modulus. Part I of this study quantifies and compares the interfacial transition zone (ITZ) for mortar with RAP aggregates relative to dolomite aggregates through image analysis of backscattered electron micrographs. The ITZ with RAP aggregates was larger and more porous with less calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) at the asphalt interface compared with dolomite aggregates. The CH morphology was not significantly affected, although the presence of the asphalt layer may be affecting the CH growth. The addition of silica fume reduced the porosity and size of CH particles in the ITZ with RAP, but not sufficiently to be similar to the ITZ of the dolomite mortar. The microstructural changes caused by RAP aggregates, primarily the larger, more porous ITZ, provide strong evidence for the observed reduction in concrete strength and modulus.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The type of steel furnace slag (SFS), including electric arc furnace (EAF) slag, basic oxygen furnace (BOF) slag, ladle metallurgy furnace (LMF) slag, and argon oxygen decarburization (AOD) slag, can ...significantly affect the composite properties when used as an aggregate or as a supplementary cementitious material in bound applications, such as concretes, mortars, alkali-activated materials, and stabilized soils. This review seeks to collate the findings from the literature to express the variability in material properties and to attempt to explain the source(s) of the variability. It was found that SFS composition and properties can be highly variable, including different compositions on the exterior and interior of a given SFS particle, which can affect bonding conditions and be one source of variability on composite properties. A suite of tests is proposed to better assess a given SFS stock for potential use in bound applications; at a minimum, the SFS should be evaluated for free CaO content, expansion potential, mineralogical composition, cementitious composite mechanical properties, and chemical composition with secondary tests, including cementitious composite durability properties, microstructural characterization, and free MgO content.
•Asphalt cohesion is the dominant failure mode between asphalt and cement.•Chemical treatments to RAP can increase interfacial bond energy between asphalt and cement.•Higher ITZ porosity is the ...reason for reduced modulus in concrete with RAP aggregates.•Asphalt cohesion failure is the primary reason for reduced mechanical properties in concrete with RAP.
Reclaimed asphalt pavement (RAP), when used as an aggregate in concrete, will reduce bulk concrete strength and modulus. While Part I of this study investigated the properties of the interfacial transition zone (ITZ), Part II focuses on the nature of the cement-asphalt bond. Several chemical oxidative treatments of the asphalt were found to improve the interfacial cement-asphalt bond energy without affecting the ITZ porosity and size. Based on surface free energy measurements, the failure mode was estimated to occur preferentially as asphalt cohesion rather than cement-asphalt adhesion or ITZ cohesion. Based on the findings from Parts I and II, RAP aggregates reduce concrete strength and modulus because of: (1) the higher porosity in the ITZ, which produces a lower bulk modulus and allows for easier crack initiation, and (2) the preferential asphalt cohesion failure, which occurs rather than adhesive failure of the cement-asphalt interface or cohesive failure of the ITZ.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Digital holographic microscopy in reflection mode is used to track in situ, real-time nanoscale topography evolution of cleaved (104) calcite surfaces exposed to flowing or static deionized water. ...The method captures full-field holograms of the surface at frame rates of up to 12.5s−1. Numerical reconstruction provides 3D surface topography with vertical resolution of a few nanometers and enables measurement of time-dependent local dissolution fluxes. A statistical distribution, or spectrum, of dissolution rates is generated by sampling multiple area domains on multiple crystals. The data show, as has been demonstrated by Fischer et al. (2012), that dissolution is most fully described by a rate spectrum, although the modal dissolution rate agrees well with published mean dissolution rates (e.g., 0.1μmolm−2s−1 to 0.3μmolm−2s−1). Rhombohedral etch pits and other morphological features resulting from rapid local dissolution appear at different times and are heterogeneously distributed across the surface and through the depth. This makes the distribution in rates measured on a single crystal dependent both on the sample observation field size and on time, even at nominally constant undersaturation. Statistical analysis of the inherent noise in the DHM measurements indicates that the technique is robust and that it likely can be applied to quantify and interpret rate spectra for the dissolution or growth of other minerals.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•RCA performs best in concrete when in a partially-saturated moisture condition.•Two-stage mixing could improve the concrete strength properties.•Shrinkage of concrete with RCA was not affected by ...the mixing method.
Higher quality recycled concrete aggregates (RCA) were used as coarse aggregates in a concrete mixture. The initial aggregate moisture condition (oven dry, partially saturated, fully saturated) and mixing method (normal, two-stage) were assessed in order to limit the differences between concrete containing RCA and virgin aggregates. Concrete with partially-saturated RCA using two-stage mixing was beneficial for improving the compressive and split tensile strengths. For this material, the mixing method did not affect the concrete free drying shrinkage at any age. Overall, the two-stage mixing method increased the concrete workability and could potentially improve the concrete strength properties, particularly when the RCA is partially saturated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The research study described in this paper investigated the potential to use steel furnace slag (SFS) as a stabilizing additive for clayey soils. Even though SFS has limited applications in civil ...engineering infrastructure due to the formation of deleterious expansion in the presence of water, the free CaO and free MgO contents allow for the SFS to be a potentially suitable candidate for clayey soil stabilization and improvement. In this investigation, a kaolinite clay was stabilized with 10% and 15% ladle metallurgy furnace (LMF) slag fines by weight. This experimental study also included testing of the SFS mixtures with the activator calcium chloride (CaCl2), which was hypothesized to accelerate the hydration of the dicalcium silicate phase in the SFS, but the results show that the addition of CaCl2 was not found to be effective. Relative to the unmodified clay, the unconfined compressive strength increased by 67% and 91% when 10% and 15% LMF slag were utilized, respectively. Likewise, the dynamic modulus increased by 212% and 221% by adding 10% and 15% LMF slag, respectively. Specifically, the LMF slag fines are posited to primarily contribute to a mechanical rather than chemical stabilization mechanism. Overall, these findings suggest the effective utilization of SFS as a soil stabilization admixture to overcome problems associated with dispersive soils, but further research is required.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
In situ digital holographic microscopy is used to characterize the dissolution flux of polycrystalline cubic tricalcium aluminate (C3A-c). The surface dissolves at rates that vary considerably with ...time and spatial location. This implies a statistical distribution of fluxes, but an approximately steady-state median rate was obtained by using flowing solutions and by reducing the water activity in the solution. The dissolution flux from highly crystalline C3A-c depends on the water activity raised to an empirically derived exponent of 5.2 and extrapolates to a median flux of – 2.1 μmol m–2 s–1 in pure water with an interquartile range of 3.2 μmol m–2 s–1. The flux from a less crystalline source of C3A-c has an empirical water activity exponent of 4.6 and an extrapolated median flux of only −1.4 μmol m–2 s–1 in pure water with an interquartile range of 1.9 μmol m–2 s–1. These data suggest that the bulk dissolution rate of C3A-c can vary by at least 30% from one source to another and that variability in the local rate within a single material is even greater because of the heterogeneous spatial distribution of structural characteristics (i.e., degree of crystallinity, chemical impurities, and defects).
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The surface normal dissolution flux of beta dicalcium silicate is quantified using in situ digital holographic microscopy. The dissolution flux in highly undersaturated flowing solutions depends on ...the water activity to an empirically determined power of 1.7. A heterogeneous distribution of reactive surface sites (e.g., crystalline defects) at the surface leads to a distribution of local fluxes. Accounting for the nominally unreactive and slowly reactive surface sites, the median of the macroscopic forward reaction dissolution flux distribution is −1.0 μmol m−2 s−1 with an interquartile range of 0.3 μmol m−2 s−1. Ex situ X-ray photoelectron spectroscopy indicates that the surface calcium-to-silicon ratio decreases by 10% after some dissolution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Recent topography measurements of gypsum dissolution have not reported the absolute dissolution rates, but instead focus on the rates of formation and growth of etch pits. In this study, the in situ ...absolute retreat rates of gypsum (010) cleavage surfaces at etch pits, at cleavage steps, and at apparently defect-free portions of the surface are measured in flowing water by reflection digital holographic microscopy. Observations made on randomly sampled fields of view on seven different cleavage surfaces reveal a range of local dissolution rates, the local rate being determined by the topographical features at which material is removed. Four characteristic types of topographical activity are observed: 1) smooth regions, free of etch pits or other noticeable defects, where dissolution rates are relatively low; 2) shallow, wide etch pits bounded by faceted walls which grow gradually at rates somewhat greater than in smooth regions; 3) narrow, deep etch pits which form and grow throughout the observation period at rates that exceed those at the shallow etch pits; and 4) relatively few, submicrometer cleavage steps which move in a wave-like manner and yield local dissolution fluxes that are about five times greater than at etch pits. Molar dissolution rates at all topographical features except submicrometer steps can be aggregated into a continuous, mildly bimodal distribution with a mean of 3.0μmolm−2s−1 and a standard deviation of 0.7μmolm−2s−1.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Digital holographic microscopy (DHM) is a surface topography measurement technique with reported sub-nanometer vertical resolution. Although it has been made commercially available recently, few ...studies have evaluated the uncertainty or noise in the phase measurement by the DHM. As current research is using the DHM to monitor surface topography changes of dissolving materials under flowing water conditions, it is necessary to evaluate the effect of water and flow rate on the uncertainty in the measurement. Uncertainty in this study was concerned with the temporal standard deviation per pixel of the reconstructed phase. Considering the effects of solution flow rate, magnification, objective lens type (air or immersion), and experimental configuration, measurements under static conditions in air and in water with an immersion lens yielded the smallest amount of uncertainty (mean of ≤ 0.5 nm up to 40× magnification). Increasing the water flow rate resulted in an increase in mean uncertainty to ≤ 0.6 nm up to 40× with an immersion lens. Observations of a sample through a glass window at 20× magnification in flowing water also yielded increasing uncertainty, with mean values of ≤ 0.5 nm, ≤ 0.8 nm, and ≤ 1.1 nm for flow rates of 0 mL min−1, 15 mL min−1, and 33 mL min−1. Different hologram acquisition rates (12.5 s−1 and 25 s−1) did not significantly impact the uncertainty in the phase. Collecting holograms in single-wavelength versus dual-wavelength modes did impact the uncertainty, with the mean uncertainty at 10× magnification for the same wavelength being ≤ 0.5 nm from the single-wavelength mode compared to ≤ 1.5 nm from the dual-wavelength mode. When the quantified uncertainty was applied to simulated dissolution data, lower limits of measured dissolution rates were found below which the measured data may not be distinguishable from the uncertainty in the measurement. The limiting surface-normal dissolution velocity is −10−11.7 m s−1 for experiments with an immersion lens in flowing water conditions and −10−11.7 m s−1, −10−11.4 m s−1, and −10−11.0 m s−1 for static (0 mL min−1), slow (≤ 15 mL min−1), and fast (≤ 109 mL min−1) flowing water conditions in experiments with a glass window, respectively. The data presented by this study will allow for better experimental design and methodology for future dissolution or precipitation studies using DHM and will provide confidence in the data produced in postprocessing.
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