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Browsing by Author "Castro, Javier"

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    Evaluating the hydration of high volume fly ash mixtures using chemically inert fillers
    (2018) De la Varga, Igor; Castro, Javier; Bentz, Dale P.; Zunino, Franco; Weiss, Jason
    Fly ash is frequently used as a replacement for cement in concrete. However, questions remain regarding the influence that fly ash has on the hydration of cement. This paper examines physical aspects (e.g., surface nucleation, cement particle spacing) and chemical aspects (e.g., pozzolanic and hydraulic reactions) of the fly ash and cement in mixtures containing high volumes of fly ash. In addition to using fly ash, a chemically inert filler was used consisting of a blend of fine silica sands with approximately the same particle size distribution as that of the fly ash. The paper compares reactivity results from 1) cement, 2) cement-fly ash and 3) cement-inert filler systems. Isothermal calorimetry measurements are used to quantitatively evaluate the role played by the fly ash in hydration of high volume fly ash mixtures. The results provide a decoupling of the physical and chemical effects of high volume fly ash on cement hydration. (C) 2017 Elsevier Ltd. All rights reserved.
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    Evaluation of Portland and Pozzolanic cement on the self-healing of mortars with calcium lactate and bacteria
    (2020-10) González, Álvaro; Parraguez, Araceli; Corvalán, Liliana; Correa, Néstor; Castro, Javier; Stuckrath, Claudia; González, Marcelo
    This article studies the effect of Portland and Pozzolanic cement on the crack-healing of mortars prepared with Bacillus pseudofirmus bacteria and calcium lactate as a nutrient. Mixtures were prepared with the same content of cement, water, sand, and Light Weight Aggregate. Results showed an inverse relationship between crack width and self-healed area, with most of the healing occurring between 0 and 21 days of sample conditioning. The healing on samples with calcium lactate only and calcium lactate + bacteria was significantly higher than in control samples. Results indicate that Portland cement better promotes the self-healing of cracks compared to Pozzolanic cement.
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    Fluid transport in high volume fly ash mixtures with and without internal curing
    (2014) De la Varga, Igor; Spragg, Robert P.; Di Bella, Carmelo; Castro, Javier; Bentz, Dale P.; Weiss, Jason
    The transport of fluid and ions in concrete mixtures is central to many aspects of concrete deterioration. As a result, transport properties are frequently measured as an indication of the durability that a concrete mixture may be expected to have. This paper is the second in a series investigating the performance of high volume fly ash (HVFA) mixtures with low water-to-cementitious ratios (w/cm) that are internally cured. While the first paper focused on strength and shrinkage, this paper presents the evaluation of the transport properties of these mixtures. Specifically, the paper presents results from: rapid chloride migration (RCM), rapid chloride penetration test (RCPT), apparent chloride diffusion coefficient, surface electrical resistivity, and water absorption. The test matrix consisted of mortar samples with two levels of class C fly ash replacement (40% and 60% by volume) with and without internal curing provided with pre-wetted lightweight fine aggregates (LWA). These mixtures are compared to plain ordinary portland cement (OPC) mortars. The results indicate that HVFA mixtures with and without internal curing provide benefits in terms of reduced transport coefficients compared to the OPC mixtures
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    Massive volume fly-ash concrete: A more sustainable material with fly ash replacing cement and aggregates
    (2015) Rivera, Felipe; Martínez, Patricia; Castro, Javier; López, Mauricio
    Fly ash (FA) has been widely used to improve concrete sustainability for many years; however, the amount of FA has been limited by the relatively low cement contents in concrete. In this study, massive volume fly-ash concrete was developed that maximizes the use of FA in concrete through its use as both a cement and aggregate replacement. Concrete containing as much as 728 kg of FA per cubic meter was produced with a compressive strength greater than 30 MPa, a low permeability measured in terms of chloride ion permeability (2300C at 56 d) and electrical resistivity (60 Omega-m at 56 d), and a decreased environmental impact. (C) 2015 Elsevier Ltd. All rights reserved.
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    Reducing setting time of blended cement paste containing high-SO3 fly ash (HSFA) using chemical/physical accelerators and by fly ash pre-washing
    (2018) Zunino, Franco; Bentz, Dale P.; Castro, Javier
    Reducing the carbon footprint of the cement industry has become one of the main concerns of researchers in the field. This study explores different strategies to reduce the setting retardation effect of high-SO3 fly ash (HSFA) on cement paste. The SO3 phase was found to correspond to hannebachite (CaSO3·0.5H2O). Chemical (calcium chloride), physical (fine limestone powder), and pre-washing strategies were investigated as means to reduce or eliminate the retardation. Each of these strategies showed some potential to decrease the retardation effect. A combination of fine limestone powder and HSFA pre-washing showed almost the same accelerating power as the calcium chloride, offering a good alternative when chloride incorporation is restricted. The retardation effect can be associated with a combined extension of the induction period and a depression of the initial silicate reactions of the clinker phases. A methodology to assess the hannebachite content based on a thermogravimetric analysis (TGA) technique is proposed, allowing a good alternative control approach for field conditions or for where X-ray (XRD or XRF) equipment is not readily available.
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    Thermo-mechanical assessment of concrete microcracking damage due to early-age temperature rise
    (2015) Zunino, Franco; Castro, Javier; López, Mauricio
    The pursuit of high early-age strength concrete has led to mixtures with higher heat of hydration rates at early ages which produces higher temperatures and an overall increased risk of cracking. This study uses a two-phase micromechanical model to compute thermal stresses based on both coefficient of thermal expansion (CTE) and elastic Young's modulus (E) mismatches between aggregates and the cementitious matrix. Concrete specimens were prepared using four types of coarse aggregates (different CTE and E), and subjected to temperature cycles to generate thermal cracking. Fluorescence microscopy, compressive strength, dynamic elastic Young's modulus, and electrical resistivity were used to characterize the effect of this induced thermal cracking. Experimental results were in agreement with the two-phase model and it was concluded that the interaction pressure (P) between phases could be used to estimate the impact on the mechanical and transportation properties of a temperature gradient at early age. (c) 2015 Elsevier Ltd. All rights reserved.

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