Maria Alba Cincotto

29Si and 27Al high-resolution NMR characterization of calcium silicate hydrate phases in activated blast-furnace slag pastes

Abstract Within the framework of improving mechanical properties of activated blast-furnace slag cements, a set of hardened pastes of 28 days age were analyzed by 29Si and 27Al high-resolution nuclear magnetic resonance (NMR) at 9.4 T. Structural and compositional differences among C-S-H phases obtained with different activation and curing conditions were characterized by NMR. Activation of the slag was done with compounds of different alkalinity (sodium silicate, sodium hydroxide, calcium hydroxide and gypsum), and under steam and wet curing. Parameters characterizing the extent of the hydration reaction, the polymerization degree of the aluminum-silicate chains and the Al/Si ratio in C-S-H were obtained from NMR spectra. 29Si-NMR spectra indicated that connectivites of silicate tetrahedra in all pastes are compatible with the “dreierkette” structural model of C-S-H. A substantial degree of polymerization of the aluminum-silicate groups in C-S-H was observed in pastes resulting from activation with sodium silicate and gypsum/calcium hydroxide blend. Steam curing and a higher alkali concentration enhanced the incorporation of Al in C-S-H. Even with low concentration of alkali, through steam curing it is possible to obtain a degree of incorporation of Al in C-S-H as high as in the case of higher alkali addition.

Chemical–mineralogical characterization of C&D waste recycled aggregates from São Paulo, Brazil

This study presents a methodology for the characterization of construction and demolition (C&D) waste recycled aggregates based on a combination of analytical techniques (X-ray fluorescence (XRF), soluble ions, semi-quantitative X-ray diffraction (XRD), thermogravimetric analysis (TGA-DTG) and hydrochloric acid (HCl) selective dissolution). These combined analytical techniques allow for the estimation of the amount of cement paste, its most important hydrated and carbonated phases, as well as the amount of clay and micas. Details of the methodology are presented here and the results of three representative C&D samples taken from the São Paulo region in Brazil are discussed. Chemical compositions of mixed C&D aggregate samples have mostly been influenced by particle size rather than the visual classification of C&D into red or grey and geographical origin. The amount of measured soluble salts in C&D aggregates (0.15-25.4mm) is lower than the usual limits for mortar and concrete production. The content of porous cement paste in the C&D aggregates is around 19.3% (w/w). However, this content is significantly lower than the 43% detected for the C&D powders (<0.15 mm). The clay content of the powders was also high, potentially resulting from soil intermixed with the C&D waste, as well as poorly burnt red ceramic. Since only about 50% of the measured CaO is combined with CO(2), the powders have potential use as raw materials for the cement industry.

Indirect and direct Chapelle's methods for the determination of lime consumption in pozzolanic materials

In this work, comments are made about indirect methods and direct Chapelles method applied to the determination of the reactivity of pozzolanic materials. The Chapelles method is based on the lime-pozzolan reaction and quantifies the pozzolanic reactivity of any material intended to be applied by the cement industry. This lime consumption determination by the pozzolan through this reaction can be made with mass proportions - lime:pozzolan 1:1, as originally proposed by Chapelle or 2:1, as specified in Brazilian and French Standards. Comparative results with both proportions are presented for sugar cane bagasse ash, rice rusk ash, silica fume, fly ash, and metakaolin, commonly studied in our country. Statistical calculations showed that for some materials of similar characteristics to the researched RHA, FA e MK at issue, essays carried out with only 1g of CaO may be underestimating the amounts of CaO consumed per gram of pozzolanic material that can be obtained. Comments were made about the Brazilian and French Standards based on this method and emphasized the expression of lime consumption per mass of the amorphous phase of these materials, as determined by X ray diffraction analysis with the Rietveld refinement method.

Study of the influential factors on the rheological behavior of adhesive mortar available in the market

Rheology studies the deformation and flow of matter and seeks to describe the deformations of material depending on the time they are exposed to mechanical actions. This paper proposes to understand the behavior of fresh mortar adhesives in Squeeze Flow tests based on rheological characterization of different commercial compositions. In the tests, flow of material results from the application of a compressive load on the sample in the fresh state which causes displacement due to stresses generated during radial shear flow. We performed further physical characterizations of mortars and viscosity on material passing the through a sieve with a 0.075 mm sieve opening with the objective of basing the analysis of the results obtained from Squeeze Flow tests. It was observed that the mortars studied showed differences in rheological behavior, probably resulting from synergistic action of some of the composition parameters, especially the particle size distribution and particle morphology.

Hydration of Portland cement with red mud as mineral addition

The use of red mud from Bayer process in association with Portland cement has been shown to be promising because of its good performance on the hardened state. However, the physicochemical interactions during the cement reaction have not been explored in depth in the literature. As the red mud is rich in iron, aluminates, silicate and, mainly, sodium components, the interaction with Portland cement is rather complex. Additionally, the large amount of finer particles can introduce another variable in the development of hydrated compounds. This work was aimed at evaluating the impact from partial substitution of Portland cement for different contents of red mud collected in an alumina production plant in the northeast of Brazil. The hydration reaction was monitored by isothermal conduction calorimetry and the setting time of cement by the Vicat test. The results indicate that the residue of bauxite ore affects the chemical reaction of cement, due to nucleation effects, packing and dilution, and the high amount of sodium and soluble aluminates from the red mud causes the formation of sodium silicoaluminate hydrate (NASH) and a greater amount of hydrated calcium aluminate, which had no effect on the setting time.

Influência da estrutura molecular dos policarboxilatos na hidratação do cimento Portland

O objetivo deste trabalho e apresentar os dados de literatura que enfatizam a influencia da estrutura molecular de policarboxilatos na hidratacao do cimento, estudada por ensaios de calor de hidratacao, tempos de pega e analise termogravimetrica em pastas de cimento. Com essa mesma metodologia foram avaliados dois policarboxilatos, agente ativo de dois aditivos nacionais cujas estruturas moleculares diferem no tamanho das cadeias laterais. Tanto os dados da literatura quanto os resultados dos ensaios realizados mostram que o fator da estrutura molecular do policarboxilato que determina o retardo da pasta de cimento e a densidade de carga anionica presente no sistema, a qual e maior quanto menor a densidade de cadeias laterais. Adicionalmente, se comparada a influencia na hidratacao do cimento de policarboxilatos com igual densidade de cadeias laterais, porem, de tamanhos diferentes, utilizados em igual quantidade em massa, observa-se que aqueles com cadeias laterais menores geram maior retardo.

Monitoring of Portland cement chemical reaction and quantification of the hydrated products by XRD and TG in function of the stoppage hydration technique

Cement hydration occurs according to a sequence of reactions whose products have a direct influence on the working properties of concretes and mortars during their use. The understanding of the evolution of its formation is quite simple and focused on monitoring the identification of resulting hydrate formed and their corresponding quantification by content of bound water after hydration stoppage and drying at certain set periods of time. Among the more common techniques are the methods of direct drying, where free water is eliminated by evaporation or sublimation or by replacing the existing water with organic solvents and then evaporating them. The existing doubts about the effect of these treatments in the change of the hydrate composition make it difficult to obtain an accurate determination. This work aimed to assess the impact of the stoppage of the hydration reaction of CPV (cement Portland type V from Brazilian market) in the results obtained by freeze-drying or immersion in an organic solvent as isopropanol or acetone, followed by substitution for diethyl ether. This last step is aimed at dispensing with the need for elevated temperatures or other aggressive solvent elimination methods such as evaporation under low pressure. The hydration was monitored at the previously set periods, and the hydrated products were analyzed by thermogravimetry and X-ray diffractometry. The results indicate that the technique of immersion on isopropanol and drying with diethyl ether is the most suitable one, as it does not affect the hydrated product composition.