Cecilia Paredes

Hydration and strength evolution of air-cured zeolite-rich tuffs and siltstone blended cement pastes at low water-to-binder ratio

Abstract This contribution is the second part of an in-depth study on the hydration and strength evolution of blended cement pastes at a water to binder (W/B) ratio of 0.3, cured by two different methods. The blended cement pastes showed significant hydration up to 7 days, when almost all of the hydration products had already formed; thereafter, carbonation played an important role up to, and possibly beyond, 91 days. Likewise, the hydration of alite (tricalcium silicate, Ca3SiO5, C3S) proceeded up to 14 days and then started to slow down. However, the hydration of belite (dicalcium silicate, Ca2SiO4, C2S) was affected most strongly, as it nearly ceased, under the air-curing conditions. During hydration, some of the blended cement pastes had a larger calcium hydroxide (CH) content than the unblended (plain) ones. The accelerating effects of the addition of supplementary cementitious materials (SCMs), the air-curing conditions and the low W/B ratio may explain these unusual results. Under these experimental conditions, the water incorporated into hydrates was about 50% of the total amount of water used during full hydration of the cement pastes. The pozzolanic reaction predominated during the early ages, but disappeared as time passed. In contrast, the carbonation reaction increased by consuming ~45% of the total amount of CH produced after aging for 91 days. Only one blended cement paste reached the compressive strength of the plain cements. The blended cement pastes containing 5% of the zeolitic tuffs, Zeo1 or Zeo2, or 10% of the calcareous siltstone, Limo, developed the greatest compressive strength under the experimental conditions used in this study.

Hydration process of zeolite-rich tuffs and siltstone-blended cement pastes at low W/B ratio, under wet curing condition

An extensive study in blended cement pastes that comprised two different experimental settings was carried out so as to analyse the hydration process and compressive strength evolution up to 91 days. The aim of this study was to understand the hydration process using zeolite-rich tuffs and siltstone as supplementary cementitious materials at low water-to-binder ratio (W/B = .3) under wet curing condition. It was observed that there were two competing reactions, i.e. pozzolanic reaction and carbonation, during the hydration process, thus leading to a decrease in the content of calcium hydroxide (CH); however, carbonation played a more important role than the pozzolanic reaction in consuming it at the given W/B ratio. The total amount of CH consumed by pozzolanic reaction was likely around 8%, while carbonation transformed around 19% of the total amount of CH at 91 days. Although fully hydrated cement paste incorporates .23 g of water per g of cement, only 60% of this value was used at such a W/B ratio. In addition, the amount of normalised water in hydrates increased as the dosage increased by factor of .158% per 1% of dosage. Finally, the optimal dosages at which Zeo1, Zeo2 and Limo showed the highest compressive strength were in the ranges of 12.5–15%, 17.5–20% and 10–12.5%, respectively .

Comparison between curing conditions of zeolite and siltstone cement pastes at low W/B ratio after 91 days

This study deals with a comparison between water and air cured blended cement pastes at a Water-to-binder (W/B) ratio of 0.3 after ageing for 91 days. Supplementary Cementitious Materials (SCMs) like zeolitic tuffs are widely used to improve compressive strength by the pozzolanic reaction. In this study, mordenite rich tuff, a mixture of clinoptilolite-heulandite-mordenite-calcite, and calcareous siltstone were blended with Ordinary Portland cement (OPC) at levels ranged from 5 to 27.5%. Using quantitative X-ray diffraction (QXRD), Thermal gravimetric analysis – Differential scanning calorimetry (TGA-DSC), as well as density measurements and compressive strength, some hydration parameters such as content of calcium hydroxide (CH), anhydrous cement phases, calcite, and water in hydrates, density and the mechanical performance were determined. The results showed that hydration process of blended cement pastes was uncomplete at ages as late as 91 days regardless of curing conditions. The air cured cement pastes showed a less content of CH, water in hydrates, and compressive strength, but instead a higher density and content of carbonate-like minerals with respect to their water-cured counterparts. The pozzolanic reaction scarcely proceeded at a W/B ratio of 0.3 regardless of curing condition. Among the SCMs, mordenite rich tuff blended cement pastes presented the best compressive strength values, when was cured under water; the others showed better values of compressive strength in air curing condition. Keywords—Zeolitic tuffs, Hydration process, blended cement pastes, pozzolanic reaction. Digital Object Identifier (DOI):http://dx.doi.org/10.18687/LACCEI2016.1.1.165 ISBN: 978-0-9822896-9-3 ISSN: 2414-6390

Consejerías Académicas en la ESPOL: evolución e impacto en la comunidad politécnica.

Resumen– El trabajo de investigacion presenta una descripcion general del proceso de consejerias academicas en la ESPOL desde su inicio y evolucion y el impacto que genera de manera integral junto con las iniciativas institucionales establecidas en el desempeno academico de los estudiantes evidenciado en su promedio general de calificaciones, ademas de presentar datos sobre la percepcion de los estudiantes sobre este proceso y recomendaciones sobre acciones futuras.