A. Macías

Impact of carbon dioxide on the immobilization potential of cemented wastes: Chromium

Portland cement and blended cements containing blast furnace slag afford both physical and chemical immobilization of chromium. Chromium occurs in aqueous solutions in two oxidation states, Cr(III) and Cr(VI). Slag-containing cements are very effective at removing Cr(VI) from the internal pore fluid, probably by reducing Cr(VI) to less soluble Cr(III). Carbon dioxide attack, or carbonation, is probably the most common form of concrete environmental attack and it promotes changes to the cement chemical composition and physical properties that can affect the long-term retention of heavy metals. In the present paper the effect of carbonation on the immobilization of Cr(III) and Cr(VI) has been studied in both Portland cements and blended cements containing blast furnace slag. The results show that although Portland cements matrices are more resistant to carbonation than slag-containing cement matrices, the increase of chromium content in pore solution is more marked for Portland matrices. After 60 days the pore fluid of carbonated Portland cement spiked with 50000 ppm Cr(III) or Cr(VI) contains 20 or 40000 ppm respectively, whereas after carbonation of slag blends for the same time, the corresponding pore fluid Cr contents are 1 and 16000 ppm respectively.

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.

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.

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).

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.

Limitations of Köch-Steinegger test to evaluate the durability of cement pastes in acid medium

The durability of ordinary Portland cement and ground granulated blast furnace slag pastes in buffered acetic/acetate medium (pH 4.5) was studied by means of the Koch-Steinegger test. Results show that flexural strength measurement is not a good parameter to evaluate the degradation degree of cement paste in acid medium because two effects take place with opposite consequences on flexural strength as a result of acid attack: a densification of the cement paste in the specimen core and a degradation of the outer surface with loss of resistance.

Hydrothermal treatment of fly ash from municipal solid waste incineration

The effect produced by hydrothermal treatment of fly ashes from municipal solid waste incineration (MSWFA) is discussed in this work. The more interesting results showed that practically 100% of chloride and metal Al from the fly ash are dissolved. CSH gel and katoite (C 3 ASH 4 ) are the main cemented hydrated phases formed after 1 hour of hydrothermal treatment.

CHARACTERIZATION OF ADMIXTURE AS PLASTICIZER OR SUPERPLASTICIZER BY DEFLOCCULATION TEST

A deflocculation test based on the method proposed by Verhasselt and Pairon has been applied to different mixes of cement-admixtures to distinguish quickly between plasticizer and superplasticizer. The admixtures used were commercial products containing different proportions of lignosulfonates, sulfonated naphthalene formaldehyde condensate, and sulfonated melamine formaldehyde condensate. Three different cements were used: a low tricalcium aluminate (C3A = 2.5%), a high-alkali content (Na2O equivalent = 0.95%), and a high-fineness portland cement (S.A. Blaine = 500 sq m/kg). In addition, the minimum active dosage of superplasticizer was calculated. The influence of the type of cement on the minimum active dosage is clearly shown by the results obtained through this method.

Influence of the cement type on the stabilisation of fly ashes from municipal solid waste incineration

Municipal Solid Waste Incineration Residues (MSWIR) and specially fly ashes (MSWFA) from gas cleaning devices contain dangerous salts and toxic ions, which may be released to the environment. Therefore, fly ashes must be stabilised before their landfill. In this paper the influence of cement type on the stabilisation of MSWFA from gas-cleaning devices is presented. The cements used were, Foundu calcium aluminate cement (CAC), ordinary Portland cement (OPC) and OPC with 80% replacement by ground granulated blast furnace slag addition (OPC/BFS).