Hydration characteristics of tricalcium aluminate phase in mixes containing β-hemihydate and phosphogypsum
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 C2AHX and C4AHX. 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.
Section snippets
Preparation of tricalcium aluminate (C3A)
C3A had been prepared at the laboratory by firing at appropriate temperatures the stoichiometric composition of this phase using highly pure limestone (99.0% CaCO3) and technically pure A12O3 (99.0%) with molar ratio 3:1, CaO/Al2O3, respectively; then, the mixture was pressed under 100 kg/cm2 pressure. The prepared specimens were calcined at 1000 °C for 2 h. After cooling, they were crushed and ground in absolute ethanol to complete homogeneity, then remoulded and fired at 1350 °C for 3 h.
Chemically combined water contents
Chemically combined water contents of the prepared pastes from different mixes of tricalcium aluminate, β-hemihydrate and phosphogypsum are depicted in Fig. 3 as a function of curing time up to 168 h. Investigation of the results indicates that, the values of chemically combined water contents generally increase with the curing time from 6 up to 168 h. At early hydration period (6 h), chemically combined water contents decrease with the phosphogypsum content; this is due to the impurities
Conclusions
From the above findings, it can be concluded that:
- (1)
At early hydration period (6 h), the chemically combined water contents decrease with the proportion of phosphogypsum. With the increase of the amount of phosphogypsum (15 and 20 mass% phosphogypsum), the values of chemically combined water contents decrease at 6 h.
- (2)
The combined lime slightly increases with the increase of amounts of phosphogypsum in the mix.
- (3)
DTA shows that the endothermic peak at about 130–140 °C, which characterizes the
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