Calcium silicate structure and carbonation shrinkage of a tobermorite-based material

Carbonated autoclaved aerated concretes (AACs) show no shrinkage at a degree of carbonation approximately less than 20%. The ²⁹Si MAS NMR spectrum showed that at a degree of carbonation less than 25%, the typical double-chain silicate anion structure of tobermorite-11Å was well maintained and interlayer Ca ions were exchanged with protons. This corresponded to the absence of carbonation shrinkage at a degree of carbonation less than 20%. When the degree of carbonation increased from 25% to 50% up to 60%, the double-chain silicate anion structure of tobermorite-11Å was decomposed and Ca ions in the Ca-O layers were dissolved, showing a possible mechanism of carbonation shrinkage.     

Eight-year exploration of shrinkage in high-performance concrete

In this paper, an experimental and analytical exploration on the effect of water-binder ratio (w/b), silica fume and age on autogenous, carbonation, drying and total shrinkage of high-performance concrete (HPC) is outlined. Eight types of HPC were studied. Carbonation, internal relative humidity (RH) and strength were studied on specimens from the same batch of HPC that was used in the studies of shrinkage. The results indicate fairly good correlation between carbonation, shrinkage, w/b and RH. The type and amount of silica fume affected shrinkage.     

Structural Features of C–S–H(I) and Its Carbonation in Air—A Raman Spectroscopic Study. Part II: Carbonated Phases

The effects of carbonation of mechanochemically prepared C–S–H samples under ambient conditions for upto 6 months have been investigated by Raman spectroscopy and X-ray diffraction. The type and extent of carbonation are strongly dependent on the initial CaO/SiO₂ (C/S) ratio of the samples. Amorphous calcium carbonate hydrate is formed within minutes upon exposure to air. It crystallizes, over time, to give primarily vaterite at C/S≥0.67 and aragonite at C/S≤0.50. Calcite was not observed as a primary carbonation product within the time frame investigated. Decalcification upon storage also initiates silicate polymerization. The dimeric silicate units seen in the calcium-rich phases polymerize rapidly to yield Q² silicate moieties. After 6 months, broad bands are seen in most spectra, ascribed to poorly ordered silica. C–S–H phases with C/S ratios of 0.75 and 0.67 are the most resistant to carbonation, and even after 6 months of storage, Q² silicate units still dominate their structures. The ability of Raman spectroscopy to probe the short-range order of poorly crystalline materials is ideal for investigations of C–S–H structure. Additionally, the technique's sensitivity toward the various calcium carbonate polymorphs illuminates the sequence of carbonation and decalcification processes during aging of C–S–H. Of particular importance is the identification of amorphous calcium carbonate as the first carbonation product. Additionally, the formation of aragonite as a carbonation product is related to the presence of SiO₂ gel in the aged samples.     

Carbonation rates of concretes containing high volume of pozzolanic materials

The project studies the influence of fly ash and slag replacement on the carbonation rate of the concrete. The experimental work includes samples of pure Portland cement concrete (CEM I 42,5 R), blast-furnace slag concrete (CEM III-B), and fly ash blended concrete. To reveal the effect of curing on carbonation rate, the concretes were exposed to various submerged curing periods during their early ages. After that, the samples were subsequently exposed in the climate room controlling 20 °C and 50% RH until the testing date when the samples had an age of 5 months. Then, the accelerated carbonation test controlling the carbon dioxide concentration of 3% by volume, with 65% relative humidity were started to perform. The depth of carbonation can be observed by spraying a phenolphthalein solution on the fresh broken concrete surface. Finally, according to Fick's law of diffusion theoretical equations are proposed as a guild for estimating the carbonation rate of fly ash and blast-furnace slag concretes exposed under natural conditions from the results from accelerated carbonation tests.     

Effect of silicate modulus and metakaolin incorporation on the carbonation of alkali silicate-activated slags

Accelerated carbonation is induced in pastes and mortars produced from alkali silicate-activated granulated blast furnace slag (GBFS)-metakaolin (MK) blends, by exposure to CO₂-rich gas atmospheres. Uncarbonated specimens show compressive strengths of up to 63 MPa after 28 days of curing when GBFS is used as the sole binder, and this decreases by 40-50% upon complete carbonation. The final strength of carbonated samples is largely independent of the extent of metakaolin incorporation up to 20%. Increasing the metakaolin content of the binder leads to a reduction in mechanical strength, more rapid carbonation, and an increase in capillary sorptivity. A higher susceptibility to carbonation is identified when activation is carried out with a lower solution modulus (SiO₂/Na₂O ratio) in metakaolin-free samples, but this trend is reversed when metakaolin is added due to the formation of secondary aluminosilicate phases. High-energy synchrotron X-ray diffractometry of uncarbonated paste samples shows that the main reaction products in alkali-activated GBFS/MK blends are C-S-H gels, and aluminosilicates with a zeolitic (gismondine) structure. The main crystalline carbonation products are calcite in all samples and trona only in samples containing no metakaolin, with carbonation taking place in the C-S-H gels of all samples, and involving the free Na⁺ present in the pore solution of the metakaolin-free samples. Samples containing metakaolin do not appear to have the same availability of Na⁺ for carbonation, indicating that this is more effectively bound in the presence of a secondary aluminosilicate gel phase. It is clear that claims of exceptional carbonation resistance in alkali-activated binders are not universally true, but by developing a fuller mechanistic understanding of this process, it will certainly be possible to improve performance in this area.     

Analysis of an accelerated carbonation test with severe preconditioning

In this paper, an analysis of the accelerated carbonation test according to the French Standard XP P18-458 is presented. It aims at a better understanding of the carbonation kinetics in the early days of this test. The influence of preconditioning by oven-drying was studied by means of carbonation depths monitoring (phenolphthalein spraying and thermogravimetric analysis) and measurements of relative humidity (RH) inside concrete samples. The main results question the relevance of the preconditioning because it does not establish a hydric balance between the tested samples and the carbonation chamber. Moreover, an absence of carbonation close to the sample surface was revealed during the early days of testing. This is explained by the very low water content resulted from oven-drying. This gives finally an explanation of the whole carbonation kinetics recorded during this test.     

超细碳酸钙对硫铝酸盐水泥基双液注浆材料性能的影响

为了进一步改善硫铝酸盐水泥基双液注浆材料(SCGM)的力学性能和抗碳化性能,采用气-液碳化法合成了超细碳酸钙(SC),研究了SC掺量对SCGM凝结时间、水化硬化性能、抗碳化性能及微观结构的影响规律.结果表明:SC可以促进SCGM的水化,提高力学性能及抗碳化性能;当SC掺量为3％(质量分数)时,养护6 h和28 d的抗压...     

Effect of carbonation on the hydro-mechanical properties of Portland cements

We evaluate experimentally the effect of carbonation on the hydro-mechanical properties of Portland cement. Samples were carbonated at 90 °C and 28 MPa under wet supercritical CO₂. Two types of carbonation features were achieved, either the samples were homogeneously carbonated or they displayed sharp carbonation fronts. Using a tri-axial apparatus, the static elastic moduli and the mechanical strength were measured at in-situ pressure conditions (28 MPa) and showed a degradation of the mechanical properties of the samples where a carbonation front prevailed. Water and gas permeabilities were measured and showed that the samples with a carbonation front exhibit a stress sensitive permeability. P and S elastic wave velocities were measured to evaluate dynamic (ultrasonic range, 1 MHz) elastic moduli. The use of an effective medium theory approach enabled us to characterize the density and distribution of cracks within the samples. This approach outlines that the samples which developed a carbonation front were damaged.     

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C₃S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C-S-H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C-S-H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C-S-H at Ca/Si ∼ 1.2 and eventually to the decomposition of C-S-H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C-S-H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.     

Effects of Steam Addition during Calcination on Carbonation Behavior in a Calcination/Carbonation Loop

The effects of steam addition during calcination on the carbonation behavior of calcium‐based sorbents in cyclic calcination/carbonation experiments were investigated. Variations in the CaO conversion rate during carbonation were measured to evaluate the influence of operating conditions and particle size on the carbonation reaction in kinetic‐ and diffusion‐controlled reaction regimes. Surface sintering and particle aggregation during cyclic calcination/carbonation affected the sorbent surface area, pore volume, and possibly the pore size, resulting in less sorbent recyclability and a trigger time retard in the fast kinetic‐controlled carbonation. Steam addition during calcination positively affected the recyclability of the sorbents and altered the carbonation behavior.     

A discussion of the papers “Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO cements” and “Enhancing the carbonation of MgO cement porous blocks through improved curing conditions”, by C. Unluer & A. Al-Tabbaa

This paper is a discussion of two recent papers by Unluer & Al-Tabbaa which analysed accelerated carbonation of reactive MgO blocks. We suggest that the authors have incorrectly analysed key data, leading to overstated claims of MgO carbonation. Based on the reassignment of their X-ray diffraction data, it is proposed that little MgO carbonation occurred in the samples discussed in those papers, with CaCO₃ instead forming during accelerated carbonation. We also draw attention to the thermodynamic instability of nesquehonite under ambient conditions, which calls into question the long-term stability of these binders.     

Accelerated protocol for measurement of carbonation through a crack surface

This paper introduces a method for accelerating experiments to quantify gaseous carbonation of cementitious materials through a sheltered crack surface. To date the majority of measurements of carbonation have focused upon the determination of the carbonation reaction through an open material face with no restriction to gaseous exposure. Experiments to determine the extent of carbonation through a crack surface can verify the extent to which restrictions of gaseous exposure can alter rates of carbonation into the crack surface as well as the depth into the crack to which the reaction occurs.The paper demonstrates that with experimental data the accelerated protocol can produce differences in outcomes in time intervals that are short relative to those in which the reaction occurs naturally. The experiment conducted to demonstrate the viability of the accelerated protocol involved measuring differences in the penetration of carbonation into the crack surface that resulted from differences in crack width. A byproduct of this experiment was a measurement of the depth into the crack (from the material face) to which carbonation occurs. It is not the intent of the paper to develop a theory of rates of carbonation, but rather to demonstrate that statistical differences are obtainable with the accelerated protocol.     

Carbonation of FBC ash by sonochemical treatment

This work explores the sonochemical-enhanced carbonation of FBC ash for direct disposal in landfills. Tests have been conducted using four ashes originating from three commercial CFBC boilers. Experiments with additives such as NaCl and seawater have also been carried out. Tests were performed at low (20°, 40 °C) and high (60°, 80 °C) temperatures. Sonicated samples were analyzed using TGA, TGA-FTIR and XRD techniques to determine the influence of other calcium compounds (OCC). The particle size reduction brought about by sonication was quantified using wet sieving. The ash reactivity displays a strong temperature dependency with almost complete carbonation achieved in minutes at higher temperatures. Additives were found to increase the level of hydration of the ashes, in line with previous work; however, carbonation levels were unaffected. TGA, TGA-FTIR and XRD analysis of the samples indicated participation of OCC, which were also formed during hydration.     

碳化过程中大水灰比水泥浆体孔结构的变化(英文)

使用特殊的增黏剂与聚羧酸减水剂,制备了掺加石灰石粉、高炉矿渣、硅灰等混合材的普通波特兰水泥浆体和和低热硅酸盐水泥浆体(水粉比为1.0)。这些水泥浆体在20℃的水中养护4年后基本完全水化。这些硬化水泥浆体在5%(质量分数)CO2、相对湿度66%和温度20℃条件下进行碳化,对比研究碳化前后水泥浆体孔结构的变化。结果显示:碳化浆体内孔直径大于10nm的孔体积明显减少;碳化浆体的孔径分布向大孔径范围偏移;掺加混合材的硬化水泥浆体结构明显趋于松散;与不掺加任何混合材的水泥浆体相比,掺加混合材的水泥浆体的孔径更大。     

Microstructural investigations of naturally and artificially weathered autoclaved aerated concrete

The microstructural changes in autoclaved aerated concrete (AAC), particularly due to chemical degradation, have been investigated. The carbonation process has been studied on naturally and artificially weathered AAC by spectrographic and microscopic analysis. Visual inspections of unexposed and aged AAC were made by means of light optical microscopy and scanning electron microscopy (SEM), while chemical and structural analysis were based on X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The results obtained from two different experimental exposure set-ups, i.e., natural and artificial weathering, are presented. Thin-section images clearly indicate leaching out of the surface layer resulting in open larger air voids. Both naturally and artificially weathered samples displayed similar ageing characteristics in terms of mineralogical changes. The XRD patterns confirm that tobermorite were gradually transforming into calcium carbonate with exposure time. Calcite and gypsum were the two main crystal structures growing during weathering as detected in the SEM+EDS examinations.     

Influence of carbonation and carbonation methods on leaching of metals from mortars

Mortars with varying water-to-cement (w/c) ratio were spiked with heavy metals. After hardening, some samples were carbonated in a CO₂ chamber, others were carbonated using supercritical CO₂ (SCC). Porosity and carbonation depth of treated and untreated samples were compared. Leaching was tested using the diffusion (NEN 7345) and extraction test (based on DIN 38414-S4). The results show that carbonation decreases the porosity of the samples. The decrease is more important with increasing w/c ratio. Carbonation runs deeper into the monolith with increasing w/c ratio. In the supercritical method, not all samples were carbonated to the same extent. The diffusion test shows that carbonation decreases leaching of Na, K, Ca, Ba, Cu, and Pb and increases leaching of Mg. Ni leaching depends on the pH induced by carbonation. Influence of pH and formation of metal carbonates is evaluated with the extraction test and enhances understanding of metal leaching in the diffusion test.     

碳化作用下轻骨料混凝土干缩变形及影响规律

采用碳化试验和干燥收缩试验相结合,研究了轻骨料混凝土在碳化作用下的干缩变形发展规律及其影响因素。结果表明,加速碳化作用下,轻骨料混凝土的干缩变形显著增长,而随着矿物掺合料的掺入,干缩率得到一定程度的抑制,其中掺超细粉煤灰轻骨料混凝土碳化干缩率最小;骨料预湿完全条件下轻骨料混凝土水胶比越大,碳化干缩变形越大;轻骨料混凝土在高浓度CO2加速碳化作用下干缩变形显著高于自然碳化混凝土;而在相对湿度50%条件下碳化时,其干缩率要大于其它湿度条件下的试样;另外,轻骨料预湿后混凝土碳化干缩率要低于未预湿轻骨料混凝土。因此,在实际工程中,选择合适的矿物掺合料、水胶比、养护条件以及骨料的预湿工艺对轻骨料混凝土的减缩防裂具有较好的效果。     

Forced and natural carbonation of lime-based mortars with and without additives: Mineralogical and textural changes

We have studied the carbonation process in different types of mortars, with and without pozzolana or air-entraining additives, subject to a CO₂-rich atmosphere and compared the results with those of similar naturally carbonated mortars. We used X-ray diffraction technique to demonstrate that high CO₂ concentrations favour a faster, more complete carbonation process with 8 days being sufficient to convert portlandite into 90 wt.% calcite. Full carbonation, however, is not reached during the life-span of the tests, not even in forced carbonation experiments. This could be due to at least one of the following phenomena: a premature drying of samples during carbonation reaction, the temperature at which the carbonation process was carried out or the reduction of pore volume occupied by newly formed calcite crystals. This last option seems to be the least probable. We observed a more prolific development of calcite crystals in the pores and fissures through which the carbonic anhydride flows. Under natural conditions, carbonation is much slower and similar levels are not reached for 6 months. These differences suggest that the carbonation process is influenced by the amount of CO₂ used.

Both the mineralogy and texture of mortars vary depending on the type of additive used but the speed of the portlandite–calcite transformation does not change significantly. Pozzolana produces hydraulic mortars although the quantity of calcium aluminosilicate crystals is low. The air-entraining agent significantly alters the texture of the mortars creating rounded pores and eliminating or reducing the drying cracks.