Crystallization of calcium hydroxide in the presence of sulphonated melamine formaldehyde superplasticizer

Abstract

Calcium hydroxide (portlandite) is one of the main solid constituents of hydrated Portland cement. It is not present in dry Portland cement clinker, but develops during the course of setting and strength gain, mainly as the tricalcium silicate and dicalcium silicate phases of clinker react with water. The mechanism of its formation is clear. Within a few seconds of mixing cement with water, the aqueous phase becomes supersaturated with respect to Ca(OH)2. Nucleation soon occurs, and thereafter free Ca(OH)2 is present; continuing hydration of solid phases maintains the aqueous phase at saturation. Thus, Ca(OH)2 forms by a solution-precipitation mechanism [1]. A completely hydrated cement paste contains 20-25% Ca(OH)2. Although this is intimately mixed with other phases [2], notably a gel-like calcium silicate hydrate designated in shorthand nomenclature C-S-H, the crystalline Ca(OH)2 may be readily observed by scanning electron microscopy (SEM) and quantitatively measured by selective dissolution, by X-ray diffraction or by thermogravimetric analysis (TGA). The high Ca content of modern Portland cements leads to high Ca(OH)2 contents which, in turn, has led to speculation concerning the contribution that it makes to the strength of pastes, mortars and concretes. The crystal structure of Ca(OH)2 is well known; it is a layer lattice structure in which individual symmetrical layers are electrically neutral, each having the net composition Ca(OH)2. Thus, layers are bonded together only by van der Waals or hydrogen bonds, with the result that the pseudo-hexagonal crystals have perfect basal cleavage. Of course, the gel matrix is itself hydrogen bonded, but in an irregular manner, so it does not readily propagate fracture. Thus, it would appear that Ca(OH)2 crystals may be a potential strength-limiting factor of cement paste. A wide range of organic and inorganic admixtures are added to cements for a variety of purposes: to increase fluidity, to accelerate or retard setting, as waterproofers, etc. [3]. The colloidal nature of the gel phase is very sensitive to the presence and amount of these additives. However, it is less well known that the presence of these agents also influences the crystal size and morphology, and hence other properties, of the more crystalline components. This was suspected on the basis of previous studies, suggesting that the presence of sulphonated melamine formaldehyde (SMF; widely used as a superplasticizer) altered the morphology 712 and size of Ca(OH)~ crystals [4]. This letter reports the effect of SMF superplasticizer on the morphology and crystallization of Ca(OH)2 from simulant cement pore fluids. 5 ml 0.024 ~ CaCI2 was mixed with 1 ml 0.24 NaOH in a plastic centrifuge tube and sealed to avoid carbonation. To the solutions was added 0, 0.5, 1.0 and 2.0 wt % SMF. This produced about 6 ml of a solution, which was found on analysis to be about 20 mM Ca 2÷ and 40 mM OH-, roughly equivalent to the saturation solubility of Ca(OH)2 in water at 18-20 °C. After 3 days the solutions were centrifuged to separate the solid, and excess water was removed. The Ca(OH)2 crystals were washed with isopropanol twice and kept under isopropanol until they were dried before examination by SEM. In another experiment, after centrifuging and removing excess water, Ca(OH)2 crystals from the same compositions were washed twice with a very small amount of deionized water and kept in isopropanol to be used for infrared analysis. Fig. 1 shows the SEM micrographs of the Ca(OH)z crystals obtained from the control solution: the SEM micrographs of the Ca(OH)2 crystals from the solution with 1 and 2% SMF are shown in Figs 2 and 3, respectively. The micrographs clearly indicate how SMF modifies the crystal morphology of the Ca(OH)> The crystals from the control solution develop a pseudo-hexagonal, prismatic morphology. However, in the presence of 1% SMF, well-shaped flattened pseudo-hexagonal plates form; occasional growth spirals are visible on sur