María Teresa Blanco-Varela

Chemical stability of cementitious materials based on metakaolin

The alkali activation of metakaolin is a way of producing high strength cementitious materials. The processing of these materials has been the subject of numerous investigations. The present paper describes the results of a research project initiated to study the stability of these materials when exposed to aggressive solutions. Prisms of mortar made of sand and alkali-activated metakaolin were immersed in deionized water, ASTM sea water, sodium sulfate solution (4.4% wt), and sulfuric acid solution (0.001 M). The prisms were removed from the solutions at 7, 28, 56, 90, 180, and 270 days. Their microstructure was characterized and their physical, mechanical, and microstructural properties were measured. It was observed that the nature of the aggressive solution had little negative effect on the evolution of microstructure and the strength of these materials. It was also found that the 90-day and older samples experienced a slight increase in their flexural strengths with time. This tendency was most pronounced in those samples cured in sodium sulfate solutions. This behavior may be related to the change in microstructure of the cementitious matrix of the mortars cured longer than 90 days. Some of the amorphous material present had crystallized to a zeolite-like material belonging to the faujasite family of zeolites.

Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate

Abstract In this work, the relationship between the composition of pore solution in alkali-activated slag cement (AAS) pastes activated with different alkaline activator, and the composition and structure of the main reaction products, has been studied. Pore solution was extracted from hardened AAS pastes. The analysis of the liquids was performed through different techniques: Na, Mg and Al by atomic absorption (AA), Ca ions by ionic chromatography (IC) and Si by colorimetry; pH was also determined. The solid phases were analysed by XRD, FTIR, solid-state 29 Si and 27 Al NMR and BSE/EDX. The most significant changes in the ionic composition of the pore solution of the AAS pastes activated with waterglass take place between 3 and 24 h of reaction. These changes are due to the decrease of the Na content and mainly to the Si content. Results of 29 Si MAS NMR and FTIR confirm that the activation process takes place with more intensity after 3 h (although at this age, Q 2 units already exist). The pore solution of the AAS pastes activated with NaOH shows a different evolution to this of pastes activated with waterglass. The decrease of Na and Si contents progresses with time. The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q 3 units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass.

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.

Influence of the starting kaolin on alkali-activated materials based on metakaolin. Study of the reaction parameters by isothermal conduction calorimetry

The properties of the product obtained through alkaline activation of metakaolin are directly influenced by the characteristics of the starting kaolin. For the study of this influence, a complete characterization of two spanish kaolins used to synthesised the material has been carried out. A JAF conduction calorimeter was used to follow metakaolin reaction with NaOH solutions. Calorimetric data were obtained isothermally at 45°C with variable solution/solid ratio and NaOH solution concentrations varying from 12 to 18 M.

Atmospheric deterioration of ancient and modern hydraulic mortars

Different types of ancient and recent hydraulic mortars were collected from well-documented archaeological, historic and modern buildings in various geographical locations (urban, suburban, rural and maritime) of Italy, Spain and Belgium, representative of different environmental impacts, types and degrees of deterioration. A synthesis of the characteristics of the collected samples is presented, along with the identification of the formation products that occurred on the sample surfaces as a result of the reaction of the mortars with atmospheric pollutants. The analyses were performed by means of optical microscopy (OM), X-ray diffractometry (XRD), scanning electron microscopy (SEM-EDX) and ion chromatography (IC). The results obtained prove that sulphation processes takes place on hydraulic mortars, leading to gypsum formation on the external surface of the samples. Through the reaction of gypsum with the aluminate hydrate of the binder, ettringite formation was found to occur on a cement-based restoration mortar sampled in Antwerp.

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.

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.

Behaviour of cement mortars containing an industrial waste from aluminium refining: Stability in Ca(OH)2 solutions

Abstract The physical and chemical interaction between a solid industrial waste from aluminium refining and saturated Ca(OH)2 solution, as well as the effects of substituting siliceous sand for the waste on the physical and mechanical properties of mortars were studied. The waste is a solid that contains reactive alumina capable of combining with the calcium hydroxide. These reactions result in stable and insoluble compounds. This alumina, together with the halite (also present in the waste composition), chemically react with a saturated solution of Ca(OH)2, giving as a main reaction product the so-called Friedels salt (Ca4Al2Cl2O6 · 10H2O). Stratlingite and Si-hydrogarnets were among other products detected. The waste has a high specific surface area. The cement/waste mixtures therefore require a higher quantity of mixing water than cement/sand mixtures. The result is a decrease of the mechanical strengths and an increase of the total porosity. However, a decrease of the average size of the pores occurs, which can have a positive effect on the durability of the final material.

Effect of Dry Deposition of Pollutants on the Degradation of Lime Mortars with Sepiolite

Abstract The behaviour of lime mortars containing sepiolite or sepiolite plus pentaclorophenol in atmospheric simulation chambers has been studied. The pollutant gases used in this study have been NO, NO2, and SO2. The studies has been done in wet and dry conditions as well as with and without ozone. In the case of NO and NO2, the aggressive agent would be HNO3, which reacts with lime mortar binder CaCO3, producing Ca(NO3)2 formation. On this process, the influence of the presence of an oxidant as well as water has been studied. The catalyst effect of the oxidation is known to be accelerated by water presence, so mortars exposed to NO + O3 + H2O and NO2 + O3 + H2O environments undergo a greater salts formation than those exposed to the rest of the aggressive media. The three mortars’ behaviours are similar and independent of their composition, producing small amount of salts in every case. When SO2 is the gas used, the aggressive agent of mortar will be H2SO4 that reacts with mortar binder, CaCO3 to produce calcium sulphate in the form of gypsum. Oxidant and/or water influence has been studied, and the (SO2 + O3 + H2O) chamber was found to be the one with greater aggressivity to mortars. However, in these conditions, sepiolite presence within the mortars delayed gypsum formation.

Behaviour of Repair Lime Mortars by Wet Deposition Process

Abstract New repair mortars with biocide properties were subjected to acid rain wet/dry cycles in aggressive environment simulation chambers. Those mortars based on lime had a sepiolite addition as a biocide support. The aim of this work was to determine the influence of both the clay and the biocide on mortar behaviour under aggressive solutions. The aggressive solutions used were the following: 1) H2SO4 solution, with pH = 4.5; and 2) “acid rain” solution with pH = 3.5. In the first case (H2SO4 solution), neither the sepiolite nor the biocide had a significant influence on mortar behaviour, while in the second aggressive medium (“acid rain”), sepiolite plus biocide, a decrease in the reaction between the mortar and the “acid rain” were noted.