S. Martínez-Ramírez

Alkali-activated fly ash/slag cements: Strength behaviour and hydration products

Abstract The activation of fly ash/slag pastes with NaOH solutions have been studied. The parameters of the process studied are: activator concentration (NaOH 2 and 10 M), curing temperature (25°C and 65°C), and fly ash/slag ratios (100/0, 70/30, 50/50, 30/70, and 0/100). The equations of the models describing the mechanical behaviour of these pastes have been established as a function of the factors and levels considered. The ratio of fly ash/slag and the activator concentration always result to be significative factors. The influence of curing temperature in the development of the strength of the pastes is lower than the contribution due to other factors. At 28 days of reaction, the mixture 50% fly ash/50% slag activated with 10 M NaOH and cured at 25°C, develop compressive mechanical strengths of about 50 MPa. The nature of the reaction products in these pastes has been studied by insoluble residue in HCl acid, XRD, FTIR and MAS NMR. It has been verified that slag reacts almost completely. It has also been determined that the fly ash is partially dissolved and participates in the reactive process, even in pastes activated at ambient temperature. The main reaction product in these pastes is a hydrated calcium silicate, like CSH gel, with high amounts of tetracoordinated Al in its structure, as well as Na ions in the interlayer spaces. No hydrated alkaline alumino-silicates with three-dimensional structure characteristics of the alkaline activation of fly ashes were formed.

MICRO-RAMAN SPECTROSCOPY APPLIED TO DEPTH PROFILES OF CARBONATES FORMED IN LIME MORTAR

Abstract Carbonation takes place in building materials when atmospheric CO 2 reacts with Ca 2+ present in the pore solution. Of the three crystallized forms of calcium carbonate, calcite is the most thermodynamically stable. Raman spectroscopy is a very useful technique for distinguishing between calcite, aragonite and vaterite. In the present study, micro-Raman techniques are used for the first time to establish the existence of various forms of calcium carbonate at different depths in fully carbonated lime mortar, in trials conducted at a temperature of 20 °C and 75% relative humidity in a chamber with atmospheric CO 2 . A new model is introduced for samples containing two or three polymorphs. The size of the calcite crystals formed, also determined with this technique, was estimated to be between 25 and 30 μm.

Infrared Spectroscopy in the Analysis of Building and Construction Materials

In the characterization of building and construction materials, the most frequently analytical tool performed have been X-ray diffraction but also, thermal analysis and microscopic techniques. Nowadays, infrared and other spectroscopic techniques have become as a useful, non-destructive and easy technique to study the phase composition of initial but also the evolved materials due to their exposure to the climatic conditions. Moreover, by using this tool is possible the detection of crystalline but also the amorphous phases very frequently developed on certain cementitious materials, mainly at early ages. The infrared spectroscopy is used both to gather information about the structure of compounds and as analytical tool to assess in qualitative and quantitative analysis of mixtures.

Studies on degradation of lime mortars in atmospheric simulation chambers

Abstract It is well known that the presence of pollution increases the degradation of some building materials. In order to understand the influence of individual pollutants as well as the role of oxidants and water in lime mortar degradation, those materials have been exposed in atmospheric simulation chambers. According to the pollutants used in the chambers (NO, NO 2 and SO 2 ), NO 2 − and NO 3 − or SO 3 2− and SO 4 2− have been analysed and related with the reaction between the lime mortar and the pollutant. The reactivity order of the different pollutant (NO, NO 2 and SO 2 ), in presence and absence of water and/or oxidant has been determined.

Carbonation process and properties of a new lime mortar with added sepiolite

Abstract Sepiolite is a clay mineral (Si12Mg8O30(OH)4(OH2)4) which due to its fibrous structure has considerable absorbent and adsorbent properties making of it an ideal material for supporting different products, such as toxines, oil, biocides, etc. In this work, the influence of the sepiolite on the carbonation process of lime mortars with sand lime 3 1 ratio, and also on the physical, chemical, mechanical and rheological properties as well as the microstructural characteristics of the same mortars has been studied. It has been proved that sepiolite slows down the rate of carbonation process in these lime mortars due to its capacity for water adsorption without affecting the mechanical behaviour of the mortars. However, it has been observed that sepiolite affects the microstructural development of the composite. The influence of the sepiolite on the rheological behaviour of these lime mortars has also been established by a mathematical equation which correlates the viscosity and the content of sepiolite in the mortar. This equation can also determine, for different water ( lime + sand ) ratios, the optimum amount of sepiolite which could be added to the mortar for achieving the best plasticity.

Evaluation of a lime-mediated sewage sludge stabilisation process. Product characterisation and technological validation for its use in the cement industry

This paper describes an industrial process for stabilising sewage sludge (SS) with lime and evaluates the viability of the stabilised product, denominated Neutral, as a raw material for the cement industry. Lime not only stabilised the sludge, raised the temperature of the mix to 80-100°C, furthering water evaporation, portlandite formation and the partial oxidation of the organic matter present in the sludge. Process mass and energy balances were determined. Neutral, a white powder consisting of portlandite (49.8%), calcite (16.6%), inorganic oxides (13.4%) and organic matter and moisture (20.2%), proved to be technologically apt for inclusion as a component in cement raw mixes. In this study, it was used instead of limestone in raw mixes clinkerised at 1400, 1450 and 1500°C. These raw meals exhibited greater reactivity at high temperatures than the limestone product and their calcination at 1500°C yielded clinker containing over 75% calcium silicates, the key phases in Portland clinker. Finally, the two types of raw meal (Neutral and limestone) were observed to exhibit similar mineralogy and crystal size and distribution.

Microstructure studies on Portland cement pastes obtained in highly alkaline environments

For the last decade, the mechanism of the alkali activation process has been broadly studied. However, there still are some uncertainties in the understanding of the mentioned process. In order to improve the knowledge of the alkali activation process, some properties of a cement paste obtained through the hydration of an ordinary Portland cement with some high-alkaline solutions have been investigated. The flexural strengths of the pastes decrease as the alkalinity of the hydrating solution increases. The opposite behaviour is observed in the evolution of porosity. Microstructural analysis by scanning electron microscopy confirms the results obtained in a previous paper.

Formation of thaumasite in carbonated mortars

Abstract The main objective of this study was to investigate the formation of thaumasite in carbonated Portland cement mortars. Another important purpose was to study the role that ettringite (AFt) plays in the process of formation of Th and its influence in the deterioration of the mortars. Work was carried out with mortar prisms, elaborated using two different cements: one of them with low Al 2 O 3 content (exempt of C 3 A) and the other one with high Al 2 O 3 content. Additions of different amounts of gypsum and/or calcium carbonate were also done. After three months of curing at 21 °C and 100% RH mortars were submitted to an accelerated carbonation process until total transformation of Ca(OH) 2 into CaCO 3 . Then mortar prisms were partially immersed in distilled water and kept at temperature ranging from 0 to 5 °C for one year. A mineralogical, micro-structural and physical characterisation was carried out on samples at different ages. The thaumasite formation rate was much lower in those mortars elaborated with the cement exempt of C 3 A than in those mortars made with the cement having high proportion of C 3 A when they are conserved in water. The study of transversal sections of prisms reveals the presence of a white expansive product (thaumasite) not always homogeneously distributed.

Quantitative analysis of mineralized white Portland clinkers: The structure of Fluorellestadite

Fluorellestadite, Ca 10 (SiO 4 ) 3 (SO 4 ) 3 F 2 , has been synthesized as single phase. This compound crystallizes in the apatite type structure, s.g. P6 3 /m, with parameters a =9.4417(1) A, c =6.9396(1) A and V =535.76(1) A 3 . The refinement of its crystal structure converged to R WP =12.33% and R F =4.58%. The atomic parameters have been used to analyze the phase content of mineralized white Portland clinkers. These clinkers contain Ca 3 SiO 5 , Ca 2 SiO 4 , Ca 12 Al 14 O 32 F 2 and Ca 10 (SiO 4 ) 3 (SO 4 ) 3 F 2 . The agreement between the elemental composition inferred from the Rietveld phase analysis and that measured by XRF is noteworthy. This comparison does not take into account the presence of amorphous phases and unmodeled elemental substitutions in crystalline phases. Similar Rietveld studies on commercial white Portland clinkers are also shown to be feasible.

Sorption of indium (III) onto carbon nanotubes.

Indium has numerous applications in different industrial sectors and is not an abundant element. Therefore appropriate technology to recover this element from various process wastes is needed. This research reports high adsorption capacity of multiwalled carbon nanotubes (MWCNT) for In(III). The effects of pH, kinetics, isotherms and adsorption mechanism of MWCNT on In(III) adsorption were investigated and discussed in detail. The pH increases improves the adsorption capacity for In(III). The Langmuir adsorption model is the best fit with the experimental data. For the kinetic study, the adsorption onto MWCNT could be fitted to pseudo second-order. The adsorption of indium(III) can be described to a mechanism which consists of a film diffusion controlled process. Metal desorption can be achieved with acidic solutions.