Gérard Platret

Bio: Gérard Platret is an academic researcher from University of Paris. The author has contributed to research in topics: Carbonation & Portlandite. The author has an hindex of 9, co-authored 16 publications receiving 1482 citations.

The use of thermal analysis in assessing the effect of temperature on a cement paste

Abstract: Upon heating, the cement paste undergoes a continuous sequence of more or less irreversible decomposition reactions. This paper reports studies on a cement paste fired to various temperature regimes up to 800 °C in steps of 100 °C for a constant period of 24 h. This work has been carried out using thermal analysis technique to study the effect of temperature in the mineralogical composition of cement hydration products. The thermal decomposition of the cement paste is analysed with the thermogravimetric analysis (TGA) and the derivative thermogravimetric analysis (DTG) curves. Such techniques can be used to determine fire conditions and the consequent deterioration expected in the cement paste. Therefore, the aim of this work is to have a better knowledge of the reactions that take place in a cement paste during a fire and thus to be able to determine the temperature history of concrete after a fire exposure. The results show that even if the dehydroxylation reaction is reversible, the portlandite formed during the cooling has an onset temperature of decomposition lower than the original portlandite and can thus be considered as a tracer for determining the temperature history of concrete after a fire exposure.

Measurement methods of carbonation profiles in concrete: Thermogravimetry, chemical analysis and gammadensimetry

Abstract: This paper deals with two experimental methods to determine carbonation profiles in concrete. Gammadensimetry is a non-destructive test method able to measure the total penetrated CO2 and to monitor the carbonation process during laboratory accelerated tests. The second method is thermogravimetric analysis (TGA) supplemented with chemical analysis (CA): as TGA is performed on a small mortar sample not representative of the whole tested concrete, CA is needed to proportion the sample cement content, the sand content and to correct the TGA results becoming thus representative of the concrete mix. Consequently, TGA-CA gives accurate quantitative profiles in carbonated cementitious materials. Results are reported for an ordinary Portland cement paste, and three concrete mixes, containing siliceous or calcareous aggregates. The CO2 mass loss due to carbonation occurs from 530 to 950 °C, which overlaps the temperature range of the calcareous aggregate dissociation. To solve the problem, the origin of CaCO3 is carefully analyzed. Calcium carbonate ensuing from C–S–H carbonation dissociates in a lower temperature range than the more stable one ensuing from portlandite carbonation and from limestone, which enables C–S–H carbonation to be distinguished from calcareous aggregates. Therefore, TGA-CA allows the CaCO3 ensuing from C–S–H carbonation to be measured and to calculate the portlandite degraded by carbonation. Thus, the total calcium carbonates profiles can be deduced even when calcareous aggregates is present in the concrete mix.

Investigation of the Carbonation Front Shape on Cementitious Materials: Effects of the Chemical Kinetics

Abstract: Carbonation depth-profiles have been determined by thermogravimetric analysis and by gammadensitometry after accelerated carbonation tests on ordinary Portland cement (OPC) pastes and concretes. These methods support the idea that carbonation does not exhibit a sharp reaction front. From analytical modelling, this feature is explained by the fact that the kinetics of the chemical reactions become the rate-controlling processes, rather than the diffusion of CO2. Furthermore, conclusions are drawn as to the mechanism by which carbonation of Ca(OH)2 and C–S–H takes place. Carbonation gives rise to almost complete disappearance of C–S–H gel, while Ca(OH)2 remains in appreciable amount. This may be associated with the CaCO3 precipitation, forming a dense coating around partially reacted Ca(OH)2 crystals. The way in which CO2 is fixed in carbonated samples is studied. The results indicate that CO2 is chemically bound as CaCO3, which precipitates in various forms, namely: stable, metastable, and amorphous. It seems that the thermal stability of the produced CaCO3 is lower when the carbonation level is high. It is also shown that the poorly crystallized and thermally unstable forms of CaCO3 are preferentially associated with C–S–H carbonation.

Mechanisms of k-struvite formation in magnesium phosphate cements

Abstract: The formation of magnesium phosphate cements is based on an acid-base reaction between potassium dihydrogen phosphate and magnesium oxide. The reaction leads to the formation of k-struvite as main product. These cements, due to their specific properties, represent an interesting alternative solution for niche applications, such as cold weather repair materials or waste treatment materials. However, scarce information is found in the literature when it comes to certain aspects linked to the consolidation of the material. The main purpose of this study is thus to better understand the influence of both the pH and the species that are present in solution on the precipitation of the k-struvite. The present study highlights the fact that the consolidation of the material involves the formation of an initial transition phase. Indeed throughout the reaction, newberyite is first formed. With the pH increase, newberyite is dissolved, and these conditions enhance the crystallization of k-struvite.

Two Experimental Methods to Determine Carbonation Profiles in Concrete

Abstract: Carbon dioxide in the air can penetrate into concrete, dissolve in the interstitial solution, and react with the reinforcing steel in the reinforced concrete. This carbonation is one of the major causes of deterioration in reinforced concrete because it leads to a pH reduction and mass increase partly due to the precipitation of calcite. This article reports on two experimental methods that provide carbonation profiles that are related to the amount of chemically-fixed carbon dioxide at various depths in a concrete sample. The first method, using gamma-ray measurement, is nondestructive and is suited for samples submitted to laboratory tests, because the knowledge of the initial state is needed as a reference to obtain the difference between the virgin reference state and the carbonated one. The second method, based on thermogravimetric analysis, can be used on laboratory samples, as well as on core samples taken from concrete structures. The authors describe, validate, and compare these innovative methods by investigating various cementitious materials. Gammadensimetry is nondestructive, which makes it possible to monitor the progression of the carbonation in the same sample subjected to either natural or accelerated carbonation. The authors conclude that gamma-ray measurement allows the determination of the kinetics of the carbonation phenomenon and the validation of the mathematical models.

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

Abstract: The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.

Performance enhancement of recycled concrete aggregate – A review

Abstract: Recycled concrete aggregate differ from natural aggregate as the former contains hardened cement mortar. The adhered cement mortar on recycled concrete aggregate has higher porosity and water absorption and lower strength than natural aggregate do. It has negative effects on the mechanical properties and durability of fresh and hardened concrete made with recycled concrete aggregate. Therefore, it will facilitate the applications of recycled concrete aggregate if the adhered cement mortar can be enhanced. Removing and strengthening the adhered mortar are the two main methods for enhancing the properties of recycled concrete aggregate. This paper reviews the published enhancement methods for recycled concrete aggregate, and points out their advantages and disadvantages so as to facilitate the selection and further development of suitable enhancement methods for recycled concrete aggregate. It suggests that carbonation treatment is an efficient and feasible method for improving the mechanical properties and durability of recycled concrete aggregate. Carbonation treatment of recycled concrete aggregate is not only an efficient way for enhancing the properties of recycled concrete aggregate, but also an environmental friendly approach.

Mix design and properties assessment of Ultra-High-Performance Fibre Reinforced Concrete (UHPFRC)

Abstract: This paper addresses the mix design and properties assessment of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC). The design of the concrete mixtures is based on the aim to achieve a densely compacted cementitious matrix, employing the modified Andreasen & Andersen particle packing model. One simple and efficient method for producing the UHPFRC is utilised in this study. The workability, air content, porosity, flexural and compressive strengths of the designed UHPFRC are measured and analyzed. The results show that by utilizing the improved packing model, it is possible to design UHPFRC with a relatively low binder amount. Additionally, the cement hydration degree of UHPFRC is calculated. The results show that, after 28 day of curing, there is still a large amount of unhydrated cement in the UHPFRC matrix, which could be further replaced by fillers to improve the workability and cost efficiency of UHPFRC.

Investigation of the carbonation mechanism of \{CH\} and C-S-H in terms of kinetics, microstructure changes and moisture properties

Abstract: The purpose of this article is to investigate the carbonation mechanism of CH and C-S-H within type-I cement-based materials in terms of kinetics, microstructure changes and water released from hydrates during carbonation. Carbonation tests were performed under accelerated conditions (10% CO2, 25 °C and 65 ± 5% RH). Carbonation profiles were assessed by destructive and non-destructive methods such as phenolphthalein spray test, thermogravimetric analysis, and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was carried out at different ages from 1 to 16 weeks of CO2 exposure on cement pastes of 0.45 and 0.6 w/c, as well as on mortar specimens (w/c = 0.50 and s/c = 2). Combining experimental results allowed us to improve the understanding of C-S-H and CH carbonation mechanism. The variation of molar volume of C-S-H during carbonation was identified and a quantification of the amount of water released during CH and C-S-H carbonation was performed.

Measurement methods of carbonation profiles in concrete: Thermogravimetry, chemical analysis and gammadensimetry

Abstract: This paper deals with two experimental methods to determine carbonation profiles in concrete. Gammadensimetry is a non-destructive test method able to measure the total penetrated CO2 and to monitor the carbonation process during laboratory accelerated tests. The second method is thermogravimetric analysis (TGA) supplemented with chemical analysis (CA): as TGA is performed on a small mortar sample not representative of the whole tested concrete, CA is needed to proportion the sample cement content, the sand content and to correct the TGA results becoming thus representative of the concrete mix. Consequently, TGA-CA gives accurate quantitative profiles in carbonated cementitious materials. Results are reported for an ordinary Portland cement paste, and three concrete mixes, containing siliceous or calcareous aggregates. The CO2 mass loss due to carbonation occurs from 530 to 950 °C, which overlaps the temperature range of the calcareous aggregate dissociation. To solve the problem, the origin of CaCO3 is carefully analyzed. Calcium carbonate ensuing from C–S–H carbonation dissociates in a lower temperature range than the more stable one ensuing from portlandite carbonation and from limestone, which enables C–S–H carbonation to be distinguished from calcareous aggregates. Therefore, TGA-CA allows the CaCO3 ensuing from C–S–H carbonation to be measured and to calculate the portlandite degraded by carbonation. Thus, the total calcium carbonates profiles can be deduced even when calcareous aggregates is present in the concrete mix.