Hydration characteristics of tricalcium aluminate phase in mixes containing β-hemihydate and phosphogypsum

Abstract

The tricalcium aluminte phase was prepared from pure chemicals on a laboratory scale. Five mixes were formulated from the prepared C₃A phase, β-hemihydate, phosphogypsum, calcium hydroxide and quartz. Different mixes were hydrated at various time intervals, namely, 6, 24, 72 and 168 h. The kinetics of hydration was measured from chemically combined water and combined lime contents. The phase compositions and microstructures of the hydrated products were studied by X-ray diffraction (XRD), differential thermal analysis (DTA)/TG, scanning electron microscopy (SEM) techniques and FT-IR spectroscopy. This work aimed to study the effect of partial to full substitution of phosphogypsum by β-hemihydate on the hydration characteristics and microstructures of tricalcium aluminte phase. The results showed that the combined lime slightly increases with the increase of amounts of phosphogypsum. The XRD patterns showed the increase in the intensities of monosulphate and different forms of calcium aluminate (C₄AH₁₃ and C₄AH₁₉) with phosphogypsum content. Ettringite is less stable than monosulphoaluminate, so it transformed into monosulpho-aluminate after 24 h, which persisted up to 168 h. The mechanism of the hydration process of C₃A phase in the presence of phosphogypsum proceeds in a similar path as with β-hemihydate. Phosphogypsum reacts with C₃A in the presence of Ca(OH)₂ forming sulphoaluminate hydrates, which are responsible for setting regulation in cementitious system.

Introduction

Large quantities of industrial by-products are produced every year by chemical and agricultural industrials. These materials have dual problems of disposal and health hazards. In Abo-Zaabal, Cairo, Egypt, a few factories produces phosphatic fertilizer and chemical industries, which generate large quantities of phosphogypsum that are currently being disposed by damping into pond or wastelands. In Egypt, phosphogypsum is produced from phosphoric acid manufacture by dihydrate process. The annual production of phosphogypsum is about 300,000 tons, which causes serious storage and environmental pollution. Due to increasing concerns about environmental pollution, it is important to utilize these wastes as building materials to save the environmental from degradation. The use of phosphogypsum is of interest for many researchers [1], [2], [3], [4], [5], [6].

Attempts have been made by several authors [7], [8], [9] to use phosphogypsum and fluorogypsum in the manufacture of cement. The impurities of P₂O₅, F⁻, organic matter and alkalis present in the gypsum adversely affect the setting and hardening of cement [10]. The effect of waste phosphogypsum and fluorogypsum on the properties of Portland cement and Portland slag cement was studied. The natural gypsum was blended with by-product gypsum to reduce the adverse effect of impurities of P₂O₅ and F⁻ present in this type of gypsum.

This work aimed to study the possibility of using phosphogypsum as a set controller to the hydration process of C₃A in the presence and in the absence of β-hemihydate. Although the proportion of C₃A in OPC is relativity low, it exerts a very significant role on the setting property and heat of hydration during hydration. In a previous study, it has been shown that the presence of phosphogypsum in cement pastes did not adversely affect the setting times and improves the workability of cement pastes. Phosphogypsum enhances the kinetics of hydration and mechanical properties as well as the rate of hydration as indicated from differential thermal analysis (DTA) up to 90 days [11].

In the system CaO·Al₂O₃·H₂O, different hydration products of calcium aluminate hydrates have been identified. CAH₁₀, C₂AH₈ and C₄AH₁₉ were formed at temperatures below 50 °C. It was found that the metastable hydration products of C₃A were C₂AH_X and C₄AH_X. In saturated lime solution, the formation of C₄AH₁₃ or C₃AH₆ formation depends on the condition of hydration reactions [12], [13].

It was stated that the role of gypsum in the retardation of the reaction of C₃A is mainly due to the reduction of the solubility of C₃A as a result of the sulphate reaction. The formation of thin layer of ettringite on the surface of anhydrous C₃A crystal will control the diffusion of SO₄²⁻ ions through this layer. The formation of ettringite directly on the C₃A grains produces a sort of crystallization pressure, which leads to bursting of the ettringite layer caused by the pressure of crystallization. This burst section is sealed by newly formed ettringite. When the sulphate content is depleted (insufficient sulphate ions to allow formation of ettringite), on further hydration of the C₃A, ettringite converts to monosulphate [14], [15].

A comparison study of the hydration characteristics of C₃A in the presence of raw gypsum and phosphogypsum was done.