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Studies on carbonation- carbonation mechanisms
The carbonation also occurs for anhydrous calcium silicates phases in presence of an adequate relative humidity as observed by [40]. Ill-crystallize calcium carbonate and the three crystalline forms of calcium carbonate are detected. Table 5 gathers results and observations from carbonation studies.
Black et al. [29] observed the formation of aragonite which they explained by the presence of silica gel or low C/S ratio C-S-H during the carbonation. Considering Sauman et al. [41] and Anstices et al. [42] results, Black et al. [29] explanation could justify the presence of aragonite. Nonetheless, in the case of C3S paste carbonation at low and high 𝑃𝐶𝑂2 [43] no aragonite was observed. Table 5: Reported results from literature on calcium carbonate polymorphism and carbonation kinetics. Moreover calcite was not observed in the case of natural carbonation even in presence of portlandite. These results show a discrepancy among the reported polymorphic distribution. Similar polymorphic distribution was observed for C3S at both high and low 𝑃𝐶𝑂2 but the properties of the silica gel obtained in each case are different. A lower silicate chain length and higher crosslinking was observed at high CO2 pressure. Consequently, the analysis of the reaction products cannot be limited to the polymorphic distribution alone. The nature of both the calcium carbonates formed and the silica gel should be considered. Besides the polymorphic abundance, the relative kinetics of the carbonation of the calcium bearing hydrates holds uncertainties. It was first described a mechanism involving solely portlandite carbonation. Then further studies reported the simultaneous carbonation of portlandite and C-S-H. Finally, Parrot and Killoh studies [46] outlined, in a 39 years old carbonated concrete, fully carbonated C-S-H in presence of portlandite.
Influence of the conditions
To take place, carbonation requires both the carbon dioxide and the calcium to be in aqueous phase but diffusion is much faster in gaseous than in aqueous phase. The higher carbonation kinetics is thus obtained between 40-60% RH [24,47–49]. Drouet’s conclusion on high alkalinity and LAC [16] study is that a RH corresponding to the triggering point of the capillary condensation is the optimum for the carbonation of each material considered. The hydric condition of the environment surrounding the materials controls the carbonation rate due to the dependency between the materials’ saturation and the external RH. The RH of the environment along with the microstructure impose the degree of saturation of the material. A fully saturated material carbonates slower than a partially saturated one. Indeed, carbonation will then be limited by the diffusion of CO2, that is 104 times lower in water compared to air [50]. Oppositely, the carbonation rate is also decreased when the saturation of the cement pore structure is too low (typically RH domain lower than 40%) to allow optimum CO2 dissolution in water. The polymorphism is also influenced by RH. Calcite is the polymorph obtained during carbonation in fully saturated medium and at high RH ≈ 80%. Drouet’s work [16] has shown a predominant proportion of metastable calcium carbonates phases during carbonation at low relative humidity [16]. This was confirmed by the study of the carbonation at increasing RH from other materials than cementitious materials’, such as calcium hydroxide nanoparticles [51]. The transformation from metastable phases to a more stable phase is thought to be limited at low RH, which would explain the absence of significant conversion to stable phases [52].
Impact of temperature and pH:
Temperature and pH strongly influence the polymorphs obtained. Tai and Chens [53] investigation on calcium carbonates yielded at 24 and 58°C shows a dependency with respect to both the temperature and the pH. They highlight several tendencies. At 58°C, above pH 11, the main polymorph found is calcite, while below a pH value of 10.5, aragonite is the predominant calcium carbonate. At a lower temperature, 24°C, three domains are observed, calcite is the main polymorph above a pH value of 12, between 12 and 11 aragonite is predominant, and below 10 vaterite is the main polymorph. The tendency is clear at high pH, irrespective of the temperature calcite seems to be the predominant polymorph and at high temperature and low pH, aragonite is the predominant species. However, at lower temperature a more contrasted behaviour is observed since aragonite is found within a medium pH range (10-12) and vaterite is the main species at lower pH values.
Impact of w/b and curing period:
The curing period allows the hydration to extend in time, which is beneficial for LAC that have longer hydration period than high alkalinity materials [4]. That hydration is closely related to the mechanical resistance of the material, given the higher proportion of C-S-H yielded. A higher curing period is observed to induce both a more refined porosity due to the formation of C-S-H but also a porosity which is less connected [17], the latter impacts the microstructure therefore the progress of the carbonation. The same tendency is evidenced during natural carbonation of OPC at different w/b = 0.30 and 0.50, namely the decrease in total porosity and the clogging of porosity between 100 and 10 nm.
Representativeness of the carbonation pressure:
The effect of the concentration of CO2 raises the question of the representativeness between accelerated carbonation and natural carbonation. The aim of an accelerate carbonation is to access advanced carbonation states that are indicative of a natural progress of the chemical, the mineralogical, and the microstructural properties. Studies at several CO2 concentration from 1% to 100% [5,6,54–59] are available. They reveal that several modifications are closely related to the exposure at high CO2 concentration, namely the change in microstructure and the carbonation kinetics reflected through the carbonation depth. The carbonation depth tends to increase when the CO2 concentration is increased, from 0 to 50% of CO2 in volume [60]. The alteration of pores of diameter less than 10 nm was shown to be greater during high concentration carbonation. Those pores were reduced from 11.7 to 1.85% at 20% CO2 and only to an extent of 5% when the CO2 concentration was set to 3%. Literature seems to agree on the use of low concentration during carbonation without a consensus on the most appropriate concentration to use. Several studies has proposed 3% to be an adequate concentration [5,59].
Thermogravimetric behaviour before and after carbonation
The Differential of the thermogravimetric profiles (Figure 1) mainly exhibited the typical C(-A)-S-H’s weight loss, which persisted until 850°C. Nonetheless, two domains could be distinguished, the first from 0 to 725°C and the second which spread from 725 to 850°C. The loss exhibited in the first domain was attributed to C(-A)-S-H’s dehydroxylation and water loss (peak with minima at 180°C). C-A-S-Hs demonstrated a more pronounced weight loss in the 300 to 525°C domain compared to C-S-Hs (Figure b,c). This weight loss could be imputed to the presence of minor phases such as hydrated calcium aluminate not detected by XRD due to their minor content and their lack of crystallinity. Those species could not be quantified since their weight loss is not resolved from the one of the C-A-S-H. The loss observed in the second domain (from 725 to 850°C) was associated to the formation of wollastonite [48].
Table of contents :
FIRST PART
I. INTRODUCTION
II. LITERATURE REVIEW
1.1 The large-scale frame of the Low-pH materials
1.2 The hydration of cementitious materials
1.3 Properties of low pH materials:
1.4 The reaction of carbonation
1.4.1 Mechanism of calcium carbonates formation
1.5 The carbonation reaction in cementitious materials
1.5.1 The carbonation of the CaO-H2O-SiO2 system.
1.5.2 Studies on carbonation- carbonation mechanisms
1.5.3 Consequences of carbonation
1.5.3.1 Influence of the conditions
III. MATERIALS AND METHODS
1.1 Program and overall approach
1.2 Materials and methods
1.2.1 Materials
1.2.1.1 Powders
1.2.1.2 The pastes
1.2.2 Methods
1.3 Discussion on the methods used
CHAPTER 1: MODEL SYSTEM FOR CEMENTITIOUS MATERIALS
1.1 Abstract
1.2 Introduction
1.3 Materials and methods.
1.4 Results
1.5 Discussion
1.6 Conclusion
1.7 Supplementary materials
CHAPTER 2: ROLE OF LOW Al CONTENT IN C-S-H STRUCTURE AND EVOLUTION IN PRESENCE AND IN ABSENCE OF CO2
1.1 Abstract
1.2 Introduction
1.3 Materials and methods
1.3.1 Materials
1.3.2 Methods
1.4 Results
1.4.1 Thermogravimetric behaviour before and after carbonation
1.4.2 MAS NMR studies of pristine samples
1.4.2.1 29Si MAS NMR of pristine
1.4.2.2 27Al MAS NMR
1.4.2.2.1 Al MQMAS NMR
1.4.2.2.1.1 Low C/S ratio: C/S = 0.80 & Al/Si = 0.05-0.1
1.4.2.2.1.2 Samples at high C/S: the case of 0.95 and C/S > 0.95
1.5 Evolution of the C-A-S-H
1.5.1 Aging of the pristine product in absence of CO2
1.5.2 Effect of the carbonation on C-A-S-H
1.5.2.1 Mineralogical properties of the pristine and the carbonated materials
1.5.2.2 DTG of carbonated product
1.5.2.3 29Si in the carbonated product
1.5.2.4 Al in the carbonated product (1D+2D)
1.6 Discussion
1.7 Conclusion
1.8 Supplementary materials
CHAPTER 3: CARBONATION IN DIFFUSIVE SYSTEM: MODEL PASTES AND INDUSTRIAL Low alcalinity Cement
1.1 Introduction
1.1.1 Materials
1.1.2 Methods
1.1.3 Results
1.1.4 Discussion
1.1.5 Conclusion
1.2 Appendices
1.3 Carbonation of an industrial LAC
2. CONCLUSIONS AND PROSPECTS
SUMMARY IN FRENCH
