A. Guerrero

ACCELERATED CARBONATION OF FRIEDEL'S SALT IN CALCIUM ALUMINATE CEMENT PASTE

Knowledge of the stability of Friedels salt is important due to its capacity for binding chloride. Free chloride is one of the more aggressive ions for steel embedded in concrete, contributing greatly to corrosion. This article reports on a study in which the stability of Friedels salt with respect to carbonation has been investigated in calcium aluminate cement (CAC) pastes containing NaCl (3% of Cl- by weight of cement). Carbonation was carried out on a powdered sample in flowing 5% CO2 gas at 65% relative humidity to accelerate the process. At an intermediate carbonation step, a part of the sample was washed and dried up to 10 cycles to simulate a dynamic leaching attack. The authors then analyzed the simulated pore solution, with X- ray diffraction (XRD), pH and Cl- analyses. The authors conclude that the carbonation of Friedels salt does not produce a significant increase of soluble chloride, thus the chloride is trapped in another solid phase. Chloride is effectively eliminated from the carbonated sample after 10 cycles of simulated semidynamic leaching.

Activation of the fly ash pozzolanic reaction by hydrothermal conditions

The effect of hydrothermal treatment on the pozzolanic reaction of two kinds of Spanish fly ashes from coal combustion (ASTM class F) is discussed. Characterization of the compounds formed as a result of hydrothermal treatment and the changes provoked in the starting fly ashes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared (IR) spectroscopy. α-C2SH, CSH gel, different solid solutions of katoites (the cubic crystallographic variety of hydrogarnets series (C3ASH4)) and a mixed oxide (CaFe2O4) were formed depending on the kind of fly ash. The hydrated compounds are precursors of a new kind of low-energy cement called fly ash belite cement (FABC); besides, they have potential properties to intercalate toxic ions and therefore can be used as immobilization systems of these ions.

HYDRAULIC ACTIVITY AND MICROSTRUCTURAL CHARACTERIZATION OF NEW FLY ASH-BELITE CEMENTS SYNTHESIZED AT DIFFERENT TEMPERATURES

The influence of the synthesis temperature of a new fly ash–belite cement (FABC) on its hydraulic activity and microstructural characteristics is discussed in this work. Three types of FABC were synthesized at 700, 800, and 900 °C after a previous hydrothermal treatment at 200 °C of the fly ash, CaO, and water mixture. The hydraulic activity of the pastes was evaluated during a period of 200 days from mixing through the combined water content, previously determined from thermogravimetric analyses, and the hydration advance degree by x-ray diffraction (XRD). The microstructure characterization of the pastes was studied by scanning electron microscopy (SEM).

Mechanical properties, pore size distribution, and pore solution of fly ash-belite cement mortars

The mechanical properties, pore size distribution, and extracted pore solution of fly ash-belite cement (FABC) mortars were studied for a period of 200 days. The influence of the calcination temperature, which ranged from 700 to 900°C, of the fly ash-belite cement was discussed. The evolution with hydration time of the pore size distribution was followed by mercury intrusion porosimetry, and the results correlated with those of flexural and compressive strength. The pore solution was expressed and analyzed at different times of hydration.

The role of the fly ash pozzolanic activity in simulated sulphate radioactive liquid waste

Effects of the very concentrated sulphated salts that accompanied the low (LLW) and medium level wastes (MLW), on the cementitious characteristics of materials employed in the Spanish disposal are discussed. The study was carried out according to the Koch-Steinegger test, at the temperature of 40°C during a period of 365 days. Samples of a specific backfilling fly ash-pozzolanic-cement mortar, were immersed in a simulated radioactive liquid waste very rich in sulphate (2.05 M) and sodium (4.1 M) ions. In addition to the mechanical behavior, the changes of the microstructure characteristics of the mortar were evaluated by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and extracted pore solution analyses. The results showed the important role played by the fly ash pozzolanic reactivity which is activated markedly by the ingress of sulphate and sodium ions towards the porous microstructure. Accordingly, the porosity is strongly refined and the mechanical properties enhanced.

Role of cement type on carbonation attack

The carbonation of two hydrated ordinary portland cements of alkali content 1.03% or 0.43% Na 2 O equivalent and hydrated calcium aluminate cement (0.1% Na 2 O equivalent) was studied in a semi-dynamic atmosphere of 100% CO 2 , and 65% relative humidity at 20 ′ I °C, for a period of 100 days. The changes of the microstructure before and during the carbonation were characterized by x-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy. The kinetics of the process was evaluated from the total CaCO 3 content by means of thermogravimetric analysis. The changes of the mechanical flexural strength were also studied. The pore solution was collected and analyzed before and after different periods of time. The results were compared with those obtained under natural carbonation conditions. The results showed that the alkali content of cement does not influence the kinetics of the process when the carbonation is accelerated. In the case of natural carbonation, an induction period is produced in the ordinary portland cement of low alkali content and calcium aluminate cement. The carbonation rate of calcium aluminate cement is the slowest for accelerated and natural carbonation.

Durability of new fly ash-belite cement mortars in sulfated and chloride medium

Abstract The durability of mortars fabricated with two kinds of low-energy cements containing hydrothermally activated Spanish fly ashes is presented. The study has been carried out by means of the Koch–Steinegger test at 21±2°C for 180 days. The durability was evaluated from the changes of flexural strength of the mortars after immersion in the potential aggressive solution (0.5 M NaCl+0.5 M Na2SO4) compared with those obtained in similar samples immersed in demineralized water. The changes in microstructure were followed by mercury intrusion porosimetry (MIP) and mineralogy by X-ray diffraction (XRD).

Efficiency of a blast furnace slag cement for immobilizing simulated borate radioactive liquid waste.

The efficiency of a blast furnace slag cement (Spanish CEM III/B) for immobilizing simulated radioactive borate liquid waste [containing H3BO3, NaCl, Na2SO4 and Na(OH)] has been evaluated by means of a leaching attack in de-mineralized water at the temperature of 40 degrees C over 180 days. The leaching was carried out according to the ANSI/ANS-16.1-1986 test. Moreover, changes of the matrix microstructure were characterized through porosity and pore-size distribution analysis carried out by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) and thermal analysis (TG). The results were compared with those obtained from a calcium aluminate cement matrix, previously published.

Effect of the starting fly ash on the microstructure and mechanical properties of fly ash–belite cement mortars

Abstract The effect of two kinds of Spanish fly ashes ASTM (class F) on the mechanical and microstructure parameters of fly ash–belite cement (FABC) mortars was studied for up to 200 days from mixing. Pore-size distribution analyses were carried out by mercury intrusion porosimetry, and microstructure was investigated by scanning electron microscopy (SEM). The pore solution was expressed and analyzed at different periods of hydration. The results showed that the alkali and iron content of the starting fly ash strongly influenced the kinetics of hydration of the FABC cements. If the time factor is deleted, general correlation is obtained between the mechanical and microstructure parameters studied, irrespective of both the starting fly ash characteristics and the calcination temperature.

Durability of Class C fly ash belite cement in simulated sodium chloride radioactive liquid waste: Influence of temperature

This work is a continuation of a previous durability study of class C fly ash belite cement (FABC-2-W) in simulated radioactive liquid waste (SRLW) that is very rich in sulphate salts. The same experimental methodology was applied in the present case, but with a SRLW rich in sodium chloride. The study was carried out by testing the flexural strength of mortars immersed in simulated radioactive liquid waste that was rich in chloride (0.5M), and demineralised water as a reference, at 20 and 40 degrees C over a period of 180 days. The reaction mechanism of chloride ions with the mortar was evaluated by scanning electron microscopy (SEM), porosity and pore-size distribution, and X-ray diffraction (XRD). The results showed that the FABC mortar was stable against simulated chloride radioactive liquid waste (SCRLW) attack at the two chosen temperatures. The enhancement of mechanical properties was a result of the formation of non-expansive Friedels salt inside the pores; accordingly, the microstructure was refined.