Crystallisation of calcium hydroxide in early age model and ordinary cementitious systems

https://doi.org/10.1016/j.cemconres.2007.01.001Get rights and content

Abstract

The distribution and morphology of CH particles in various cementitious systems was investigated to determine which parameters influence their nucleation and growth. Morphological image analysis of SEM-BSE images of polished cross-sections was used to measure CH distribution, particle size and shape within the microstructure. It was observed that CH crystallises according to two main patterns, the first consisting of a few, large, convoluted and unevenly distributed clusters, while the second consists of numerous smaller particles, evenly and closely distributed in the matrix. The growth of CH in Portland cement type systems seems to be the result of a specific interaction between the availability of gypsum and alumina, with CH crystals nucleating in the vicinity of gypsum particles.

Introduction

Calcium hydroxide (CH) or portlandite is well known as one of the main products of cement hydration, along with C–S–H. Although extensive work has been done on the hydration of Portland cement (PC) as well as on blended systems, the formation of CH has received relatively little attention compared to other hydrate phases. It is well recognised that CH plays a critical role in buffering the pH of the pore solution, so protecting reinforcement from corrosion. On the other hand, it is implicated in the formation of efflorescence and is considered to be more susceptible to leaching. A good review on the properties of CH can be found in the special volume of Materials Science of Concrete especially dedicated to calcium hydroxide [1].

From a microstructural point of view, several issues such as the crystallinity of CH, its growth and the size, morphology and distribution of clusters in the microstructure have to be considered. Glasser [2] identifies two types of CH in PC: the first corresponds to physically discrete crystallites and the second to labile particles initially present in C–S–H gel with high Ca/Si ratio and responsible for its capability to donate calcium [3], [4]. Several authors have proposed that the structure of C–S–H could consist of the intermixing of tobermorite-like or jennite-like structural units with continuous protolayers of CH [5], [6], [7], [8], [9].

Although the existence of amorphous or microcrystalline CH in C–S–H seems established, the vast majority of this phase in cementitious systems occurs in the form of micrometer-sized crystals. Depending on the hydration conditions, CH in PC is known to occur as massive clusters and/or as isolated hexagonal crystals. However, there have been very few systematic studies of the nucleation and growth of these crystals, either in terms of their size and spatial distribution or of the way this is influenced by the growth environment. Only two previous publications could be found concerning these aspects: Diamond [10] investigated the morphology of CH clusters in 3- and 100-day-old pastes by means of SEM image analysis. He concluded that CH exists as discrete particles with a mean size distribution of 7 μm, that none of these particles were euhedral (hexagonal platelet-like) and that the growth of CH is constrained and limited by the tortuous free space available.

Kjellsen and Justnes [11] reported on the microstructure of hydrated pure C3S pastes. Although PC contains about 70% of C3S, the microstructure of C3S pastes differs significantly from that of cement pastes. They suggested that the morphological differences of CH in C3S and PC (respectively, large concentrated masses vs. finely dispersed particles with complex and irregular shapes) could be attributed to the difference in pH of the systems although they did not look at this effect in their study. Kjellsen and Justnes [11] suggested that the ions present as impurities in alite (impure form of C3S) induce more abundant CH nuclei in PC than in C3S. However, although they used Mg- and Al-stabilised alite instead of pure C3S, the number of CH nuclei remained limited. To date, none of the hypotheses advanced to explain the variations of CH morphology have been validated experimentally.

The present study looks at the distribution and morphology of CH in various cementitious systems in order to assess which parameters influence its nucleation and growth. Polished sections of pastes aged from a few hours to several days were prepared for SEM backscattered electron (BSE) imaging. Morphological image analysis was used to measure CH distribution, particle size and shape within the microstructure. The differences observed are discussed and correlated with the chemical environment of ions in solution in the fresh cement pastes.

Section snippets

Materials and methods

Several hydrated systems ranging from pure tricalcium silicate (C3S—the main phase of PC) to PC were prepared and examined (Table 1, Table 2). The pure phases C3S and C3A were synthesised by several heating/grinding cycles of stoichiometric mixes of pure reagent grade reactants: CaCO3 and SiO2 at 1650 °C for C3S and CaCO3 and Al2O3 at 1050 °C for C3A. In the case of polyphase systems of C3S and C3A, the two phases were produced and sintered together at 1500 °C. The purity of the synthesised

Nucleation and growth of CH in the early stages of hydration of C3S and PC

After 1 day of hydration, the difference between the C3S and PC systems is clearly established as illustrated in Fig. 1. As reported by Kjellsen and Justnes [11], CH in C3S grows only from a few nuclei and rapidly extends into very large clusters which totally envelop C3S grains, preventing their further hydration. At later stages of hydration, CH seems to grow preferentially on the existing clusters rather than elsewhere in the matrix. The development of CH in PC is radically different:

Conclusion

Several model systems of hydrated cement phases have been studied by means of Scanning Electron Microscopy imaging and morphological image analysis in order to understand the occurrence and development of CH in cementitious systems. It has been shown that CH crystallises according to two main patterns:

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    the first, C3S type, generates few, large, convoluted and randomly distributed clusters,

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    the second, PC type, only observed in the presence of C3A and gypsum, produces numerous small particles,

Acknowledgements

The authors thank Christian Vernet and Hélène Di Murro from Lafarge LCR for their helpful advice in synthesising the model phases.

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