ICE - Institution of Civil Engineers - Virtual Library

In this study, a series of new cement minerals, Ba-bearing calcium sulpho-ferritealuminates, were synthesized by the substitution of Fe³⁺ for Al³⁺ ions of C_2·75B_1·25A₃S(1·75CaO·1·25BaO·1Al₂O₃·CaSO₄). Differential thermal analysis—thermogravimetric analysis was used to determine the optimum sintering temperature and 1300°C was found to be a suitable clinkering temperature for the formation of minerals. The composition and structure of the clinker minerals were studied by X-ray diffraction and scanning electron microscopy energy dispersive X-ray spectroscopy. The results indicated that the size of the crystal was about 2 μm in diameter and the composition of Al₂O₃ can be substituted for Fe₂O₃, with a value of 0 ∼ 1·25. The development of strength was investigated and the results showed that the strength of Ba-bearing calcium sulpho-ferritealuminates was higher than C_2·75B_1·25A₃S, and the optimal composition is 0·25.

Self-compacting concrete (SCC) can be placed without any external compaction avoiding some health risks as well as environmental problems. In order to obtain its key properties, a large amount of fine particles and a new generation of superplasticisers can be used. An experimental programme was set up to study the pore structure of SCC, including the influence of limestone filler and polycarboxylate ether superplasticiser. Mercury intrusion porosimetry (MIP) was used as a testing method. In this study tests were performed on samples of hardened cement paste of several ages in order to evaluate the relative influence of various parameters like water—cementitious (W/C), cement—powder (C/P) and water—powder ratio and to investigate the difference between traditional and self-compacting concrete. It was concluded that adding limestone powder as a filler and superplasticiser has an important influence on the pore structure of the cement paste. Decreasing the C/P and keeping the amount of water and powder constant induces a more porous cement paste. Increasing the W/C and keeping the amount of cement and powder constant leads to a coarser cement paste.

Currently the prediction of the strength of blended cement constitutes a major preoccupation leading the promoters of such constructions to be more and more interested in predicting the timing of removal of the framework or the prestressing at suitable ages. The prediction of the strength of the slag concrete, using simple models of prediction such as Feret's model, with the introduction of the concept of the equivalent binder, was studied and this led to the definition of an efficiency coefficient of the slag which defines the latter according to the cement. This efficiency coefficient, once deduced, characterised the slag from the El-Hadjar steel factory (Algeria) well and made it possible to predict the strength of a slag concrete of a given age and replacement level from the data of a reference concrete made without slag. This coefficient is defined on the basis of the age, the replacement level, the specific surface area of the slag and the curing temperature.

The various waterproofing admixtures may prevent water from penetrating concrete in different ways, but primarily by rendering it hydrophobic. Spectroscopic techniques have proved to be the appropriate tools for studying cement hydration in the presence of different admixtures. A combination of Fourier Transform-Raman and ²⁹ Si nuclear magnetic resonance (NMR) spectroscopy were used here to monitor OPC hydration and C-S-H gel formation with two different waterproofing admixtures. The Q¹/Q⁰ ratio was determined with the former technique to evaluate the influence of the waterproofing substances on C-S-H gel formation rate and intensity, whereas ²⁹Si NMR was employed to establish the relative amount of C-S-H gel formed at different hydration times as well as to calculate the main chain length.

A multi-method approach was used for the investigation and comparison of alkali-activated slag binders (AAS), pure slag and ordinary Portland cement (OPC). X-ray fluorescence, X-ray powder diffraction, granulometry, calorimetry, thermo-gravimetric analysis and environmental scanning electron microscope investigations of the microstructure with energy dispersive X-ray analyses were used to characterise the cements and their hydrate phases. In addition, the chemical composition of the pore solution, including the different sulphur-containing ions, was analysed. The precipitation mechanisms during binder hydration in the AAS and OPC systems exhibit significant differences: in AAS the formation of the ‘outer product’ C-S-H is much faster than in OPC. The high Si concentrations in the pore solution during the early hydration of AAS are related to the fast dissolution of Na-metasilicate. The fast reaction of Na is an important factor for the voluminous precipitation of C-S-H within the interstitial space already during the first 24 h. In addition to the Na-metasilicate component, the high fineness of the slag represents a further important factor for the fast hydration of AAS. The small slag particles (< 2 μm) are completely dissolved or hydrated within the first 24 h, whereas hydration of the larger particles is much slower. The fast formation of a gel-like matrix in AAS is the product of a fast ‘through solution’ precipitation, which contrasts with the slower dissolution-precipitation mechanism of a ‘topotactic’ growth of C-S-H in OPC. The chemical and mineralogical characterisation of solid and liquid phases and their changes with time are the basis for thermodynamic modelling of the corresponding hydration process, which is presented in a second paper.