DSpace Repository :: Browsing by Author "Zunino, Franco"

Browsing by Author "Zunino, Franco"

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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.
Low-temperature curing strength enhancement in cement-based materials containing limestone powder
(01/06/2017) Bentz, Dale P.; Stutzman, Paul; Zunino, Franco
With the ongoing sustainability movement, the incorporation of limestone powder in cementitious binders for concrete in the U.S. has become a subject of renewed interest. In addition to accelerating the early age hydration reactions of cementitious systems by providing additional surfaces for nucleation and growth of products, limestone powder is also intriguing based on its influence on low-temperature curing. For example, previous results have indicated that the utilization of limestone powder to replace one quarter of the fly ash in a high volume fly ash mixture (40-60% cement replacement) produces a reduction in the apparent activation energy for setting for temperatures below 25 degrees C. In the present study, the relationship between heat release and compressive strength of mortars at batching/curing temperatures of 10 and 23 degrees C is investigated. For Portland-limestone cements (PLC) with limestone additions on the order of 10%, a higher strength per unit heat release is obtained after only 7 d of curing in lime water. Surprisingly, in some cases, the absolute strength of these mortar cubes measured at 7 d is higher when cured at 10 degrees C than at 23 degrees C. Solubilities vs. temperature, reaction stoichiometries and enthalpies, and projected phase distributions based on thermodynamic modeling for the cementitious phases are examined to provide some theoretical insight into this strength enhancement. For a subset of the investigated cements, thermogravimetric analysis, quantitative X-ray diffraction, and scanning electron microscopy are conducted on 7-d paste specimens produced at the two temperatures to examine differences in their reaction rates and the phases produced. The strength enhancement observed in the PLC cements is related to the cement hydration products formed in the presence of carbonates as a function of temperature.
Recycling of hydrated cement pastes by synthesis of alpha'(H)-C2S
(2017) Serpell, Ricardo; Zunino, Franco
Hardened cement paste in concrete wastes can be a valuable precursor material for the production of recycled cements. In the reported study, X-ray diffraction data of cementitious materials obtained by thermal processing of hardened pastes were quantitatively analyzed using Rietveld refinement to explain the effect of process parameters on their hydration reactivity and on the strength gain of pastes made with them. The parameters studied were annealing temperature, residence time, and cooling rate. Across the annealing temperature range explored C2S polymorphs were found to comprise the larger fraction of the resulting materials. However, their relative concentrations varied. Results indicate alpha'(H)-C2S formed at low temperature is highly reactive and remains stable on cooling due to its smaller crystallite size, whereas at higher temperatures most of it converts to the less reactive beta-C2S on cooling. Accordingly, materials obtained at lower temperatures exhibited higher heats of hydration and much higher strength gain rates.
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.
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.

Browsing by Author "Zunino, Franco"

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