Erich D. Rodríguez

Performance of alkali-activated slag mortars exposed to acids

The acid resistance of 60-day cured alkali-activated slag (AAS) mortars, in comparison with Portland cement, is assessed. Specimens are exposed to hydrochloric, nitric and sulphuric acids at pH 3.0, and acetic acid (CH3COOH) at pH 4.5, for 150 days, with specimens also immersed in water as a control. Negligible changes in compressive strength are identified in Portland cement and AAS binders during exposure to mineral acids, while compressive strength increases during immersion in water. However, exposure to CH3COOH reduces strength, and increases pore volume, in both Portland cement and AAS mortars. AAS performs better than Portland cement in CH3COOH, associated with the lower initial permeability of the specimens, along with the low CaO/SiO2 ratio typical of AAS. Decalcification of the AAS binder through formation of calcium acetate leaves an aluminosilicate type gel that can hinder the further ingress of acids, contributing to the high acid resistance of this binder.

Increase of the reactivity of densified silica fume by sonication treatment

Five silica fumes from different manufacturers were subjected to ultrasonic treatment in order to decrease particle agglomeration and improve particle dispersion. The effectiveness of the sonication was observed as a reduction in particle size distribution of sonicated silica fume (SSF) compared to non-sonicated silica fume. SSF was added to Portland cement, and then the hydrated paste was analysed by thermogravimetric analyses (TGA/DTG) and scanning electron microscopy (SEM/EDX). The results were compared with those of control pastes made with untreated densified silica fume (DSF), as well as a reference cement paste of ordinary Portland cement (OPC). A maximum grade of de-agglomeration by the sonication was obtained, with a high volume of particles of diameter less than 1 μm. Images obtained by transmission electron microscopy (TEM) of the SSF showed sintered particles that could not be fragmented by the treatment. Micro-structural characterisation results showed an increase in the reactivity of the silica fume after the treatment.

Structure of Portland cement pastes blended with sonicated silica fume

Application of power ultrasound to enhance dispersion of commercial densified silica fume leads to increased compressive strengths and refinement of the pore structure in mortars, compared with those that are untreated. This was attributed to the enhanced pozzolanic reactivity achieved by particle dispersion through sonication, leading to higher consumption of portlandite during curing, and formation of a calcium silicate hydrate gel with a higher degree of cross-linking than is identified in specimens with densified silica fume. This suggests that with the use of sonicated silica fume, it is possible to reduce the required quantity of admixture in blended cements to achieve specified performance, with the additional advantage of the formation of a highly densified structure and refined pore network, contributing to potential improvements in durability.

Role of Adsorption Phenomena in Cubic Tricalcium Aluminate Dissolution

The workability of fresh Portland cement (PC) concrete critically depends on the reaction of the cubic tricalcium aluminate (C3A) phase in Ca- and S-rich pH >12 aqueous solution, yet its rate-controlling mechanism is poorly understood. In this article, the role of adsorption phenomena in C3A dissolution in aqueous Ca-, S-, and polynaphthalene sulfonate (PNS)-containing solutions is analyzed. The zeta potential and pH results are consistent with the isoelectric point of C3A occurring at pH ∼12 and do not show an inversion of its electric double layer potential as a function of S or Ca concentration, and PNS adsorbs onto C3A, reducing its zeta potential to negative values at pH >12. The S and Ca K-edge X-ray absorption spectroscopy (XAS) data obtained do not indicate the structural incorporation or specific adsorption of SO42- on the partially dissolved C3A solids analyzed. Together with supporting X-ray ptychography and scanning electron microscopy results, a model for C3A dissolution inhibition in hydrated PC systems is proposed whereby the formation of an Al-rich leached layer and the complexation of Ca-S ion pairs onto this leached layer provide the key inhibiting effect(s). This model reconciles the results obtained here with the existing literature, including the inhibiting action of macromolecules such as PNS and polyphosphonic acids upon C3A dissolution. Therefore, this article advances the understanding of the rate-controlling mechanism in hydrated C3A and thus PC systems, which is important to better controlling the workability of fresh PC concrete.

A Scanning Transmission X-ray Microscopy Study of Cubic and Orthorhombic C3A and Their Hydration Products in the Presence of Gypsum

This paper shows the microstructural differences and phase characterization of pure phases and hydrated products of the cubic and orthorhombic (Na-doped) polymorphs of tricalcium aluminate (C3A), which are commonly found in traditional Portland cements. Pure, anhydrous samples were characterized using scanning transmission X-ray microscopy (STXM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and demonstrated differences in the chemical and mineralogical composition as well as the morphology on a micro/nano-scale. C3A/gypsum blends with mass ratios of 0.2 and 1.9 were hydrated using a water/C3A ratio of 1.2, and the products obtained after three days were assessed using STXM. The hydration process and subsequent formation of calcium sulfate in the C3A/gypsum systems were identified through the changes in the LIII edge fine structure for Calcium. The results also show greater Ca LII binding energies between hydrated samples with different gypsum contents. Conversely, the hydrated samples from the cubic and orthorhombic C3A at the same amount of gypsum exhibited strong morphological differences but similar chemical environments.

Evaluación microestructural de geopolímeros basados en metacaolín y fuentes alternativas de sílice expuestos a temperaturas altas

Los geopolimeros se obtienen a partir de la activacion alcalina de minerales naturales, desechos o subproductos industriales para generar un producto con caracteristicas ceramicas que presentan ventajas a nivel ambiental y con propiedades similares y en algunos casos superiores frente a materiales tradicionales. El presente estudio evalua el efecto de la exposicion a 600° y 1200°C de sistemas geopolimericos basados en metacaolin (MK) con la utilizacion de activadores basados en KOH. Se evaluo la utilizacion de una ceniza de cascarilla de arroz (RHA) y un humo de silice comercial (SF) como fuentes alternativas de silice para la preparacion del activador alcalino. La exposicion a las temperaturas elevadas conduce a la deshidratacion de los productos formados durante la activacion del MK y cambios estructurales asociados a la transformacion del gel geopolimerico a estructuras cristalinas tipo leucita (KAlSi2O6) y kalsilita (KAlSiO4), lo cual promueve la densificacion y reduccion de la porosidad del material. La presencia de particulas de MK sin reaccionar en los materiales geopolimericos condujo a la formacion de mullita (2Al2O3 · SiO2), despues del tratamiento a 1200°C

Effect of Gypsum on the Early Hydration of Cubic and Na-Doped Orthorhombic Tricalcium Aluminate

The tricalcium aluminate (C3A) and sulfate content in cement influence the hydration chemistry, setting time and rheology of cement paste, mortar and concrete. Here, in situ experiments are performed to better understand the effect of gypsum on the early hydration of cubic (cub-)C3A and Na-doped orthorhombic (orth-)C3A. The isothermal calorimetry data show that the solid-phase assemblage produced by the hydration of C3A is greatly modified as a function of its crystal structure type and gypsum content, the latter of which induces non-linear changes in the heat release rate. These data are consistent with the in situ X-ray diffraction results, which show that a higher gypsum content accelerates the consumption of orth-C3A and the subsequent precipitation of ettringite, which is contrary to the cub-C3A system where gypsum retarded the hydration rate. These in situ results provide new insight into the relationship between the chemistry and early-age properties of cub- and orth-C3A hydration and corroborate the reported ex situ findings of these systems.

Evaluación térmica de sistemas geopoliméricos basados en metacaolín con incorporación de humo de sílice y ceniza de cascarilla de arroz

The effect of exposing metakaolin-based geopolymers to a temperature of 1200 °C was assessed using as alkaline activator a mixture of potassium hydroxide with different sources of silica: a commercial potassium silicate (PS), rice husk ash (RHA), and silica fume (SF). The amounts of metakaolin and activator in the mixture were adjusted to obtain molar ratios of 2.5 for SiO 2 /Al 2 O 3 and 0,28 for K 2 O/SiO 2 . The substitution of 50% PS with RHA and SF was also studied, and the resistance to compression after exposing the mixture to different temperatures between 300 and 1200 °C was assessed. The respective physical analyses, such as determination of volumetric changes, were performed, and the study was complemented with a microstructural analysis conducted by X-ray diffraction and electron microscopy. At room temperature, the geopolymers presented mechanical strengths ranging between ~30 and ~36 MPa. At 1200 °C, materials originally amorphous were transformed into crystalline-type structures, specifically leucite and kalsilite. The best thermo- mechanical performance of the geopolymers was achieved by replacing 50% PS with CCA, which when exposed to 1200 °C retains 44% of its mechanical strength as compared with 26% retained by the reference material.