The influence of NaCl on the reactivity of high alumina cement in water: Pore-solution and solid phase characterization
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The influence of NaCl (3% of Cl" by weight of cement) on the reactivity of High Alumina Cement (HAC) in water has been studied over a period of one month. The changes in microstructure were monitored by x-ray diffraction and scanning electron microscopy. The pore-solution, extracted by the application of high mechanical pressure (500 MPa), was studied for the chemical composition and changes caused by chloride and sodium ions.
I. INTRODUCTION The knowledge of the pore-solution chemistry of hydrating high alumina cement (HAC) pastes and the changes resulting from reactions with internal or external agents is of great interest for their implications on the durability of the hardened material. The ionic species dissolved in the pore-solution establish different equilibria depending on the external agents. The higher the pH levels of pore-solution are, the higher the damage to the HAC. This is due to the amphoteric character of alumina phases that are hydrolyzed or dissolved at certain pH levels. In a recent paper,1 Gaztanaga, Gofii, and Sagrera studied the chemical composition of pore-solution and related it to the hydrated solid phases of HAC pastes made with a water/cement ratio of 0.5, without chloride ion. The aim of the present paper is to study the influence of sodium chloride (NaCl) on the microstructure of the hydrated HAC, bonding capacity of chloride and alumina phases, and their conversions. pH modifications caused by Na+ and the implications on the possible hydrolysis of hydrated calcium aluminate phases have also been studied. II. EXPERIMENTAL A Spanish commercial Fondu high alumina cement (HAC) of the chemical composition given in Table I has been used in the studies. Pastes of water/cement ratio 0.5, containing 3% of Cl" by weight of cement, were made by dissolving the required quantities of NaCl in
the mix water (de-ionized) and stored in sealed plastic cylinders at 100% RH and 20 °C for different periods up to 30 days. After 15 h of mixing, the samples were cooled in ice for 9 h to avoid heating from the strongly exothermic hydration reaction. The pore-solution was extracted by application of high mechanical pressure (500 MPa).2'3 The values of the initial and the final setting times, as determined by the Vicat Needle apparatus (ASTM C191), are shown in Table II. The concentrations of sodium and potassium in the pore-solution were determined by atomic absorption spectroscopy with Perkin-Elmer 305 equipment, and those of calcium and aluminum by inductively coupled plasma (ICP) emission spectroscopy carried out with a Yobin Ybon 38 VHR. Chloride concentration was measured by titration against AgNO 3 with potentiometric determination of the end point using an automatic MemoTitrator Mettler DL40RC. The pH was measured with a combined electrode for pH range 0-14. It was not possible to measure the OH~ concentration due to the interference of the AH 3 phase, which is precipitated during the titration against HC1. XRD patterns were recorded on a Philips PW 1730 diffractometer using a graphite monochromator
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