Anomalous tobermorite in autoclaved aerated concrete

Three samples of commercial products were examined by X-ray diffraction, electron diffraction, chemical analysis and d.t.a. - t.g.a. The cementing materials were highly crystalline Al-substituted 11 Å tobermorite with Ca/(Al+Si) = 0.81 and Al/(Al+Si) = 0.10 for specimen A, 0.85 and 0.10 for specimen B, and 0.84 and 0.07 for specimen C. All of these tobermorites showed anomalous thermal behaviour, i.e., the basal spacing did not shrink after heating at 300°C. The content of alkalis in all the tobermorites was insufficient to balance replacement of Si by Al. All the crystals gave SED patterns characteristic of crystals with (001) cleavage, similar to those given by normal specimens.     

Fracture toughness of autoclaved aerated concrete

So far, few experiments to determine fracture toughness of autoclaved aerated concrete have been reported in the literature. Tests on three different types of autoclaved aerated concrete using six different specimen geometries have been carried out. Four of the six specimen geometries chosen lead to nearly identical results for K_IC. The two other types of specimen were used to determine fracture energy G_f. K_IC increases with compressive strength of the material. Fracture energy is about one tenth of the corresponding value of normal concrete. The influence of rate of loading on K_IC can be expressed by means of a power law. These results suggest that linear elastic fracture mechanics is a suitable approximation for the calculation of crack propagation in autoclaved aerated concrete.     

Relations between structure and mechanical properties of autoclaved aerated concrete

Laboratory specimens of autoclaved aerated concrete were produced under varying conditions, mainly with cement and lime as binders. The type and amount of reaction products, the porosity and the pore size distribution were studied. Shrinkage and compressive strength were measured. The reaction products belonged to the tobermorite group of calcium silicate hydrates and the term crystallinity was defined as the percentage of 11.3 Å tobermorite out of the total amount of calcium silicate hydrates. The shrinkage decreased with increasing crystallinity while the compressive strength increased up to an optimum value. The strength also increased with increasing amounts of hydrates and with decreasing porosity. Other features of the reaction products were indicated by thermal behaviour and micropore size distributions and may have been of importance for the mechanical properties of the material.     

Influence of air-cooled slag on physicochemical properties of autoclaved aerated concrete

Lime and sand in autoclaved aerated concrete (AAC) were replaced by air-cooled slag (AS). The compressive strength and the type and nature of the hydration products were studied for samples autoclaved at 8 bar for different periods of times: 2, 6, 12 and 24 h. The hydration reactions were monitored by determining free-lime contents and combined water. The types of the hydration products were investigated using XRD and SEM/EDX. Slag substitutions for sand and lime up to 50% enhance the compressive strength, especially at short curing times (2 and 6 h). The optimum strength is obtained by 50% AS substitution for low-lime mixes (10% CaO) and 30% AS substitution for high-lime mixes (25% CaO). In high-lime mixes containing up to 30% AS, the initially formed fibrous calcium-rich CSH was changed to needle-like and lath-like 1.1 nm tobermorite. In low-lime mixes with AS-substitution, tobermorite appears at 2 h processing time with grass-like silica-rich CSH around quartz particles.     

Use of gasification residues in aerated autoclaved concrete

Two fine-grained residues produced by gasification of wastes were tested for their suitability in aerated autoclaved concrete (AAC). AAC typically has additional aluminium powder added to the raw ingredients to facilitate the formation of small gas bubbles which provide a low-density product. Both gasification residues had high amounts of metallic aluminium and thus were well suited for this application. Studies were conducted to determine whether the residue addition would eliminate the need for the extra aluminium powder for creating air voids. Simultaneously, the residues replaced some of the aggregate ground in the process, thus enabling savings in energy consumption. Initial laboratory tests determined how much residue could be added and what type of concrete mixture proportioning was needed. Multiple properties were tested on AAC with 10% residue addition, with and without the additional aluminium powder. After laboratory investigations, full-scale tests were done at the Siporex AAC factory in Finland. These tests revealed that the residues offer a promising resource for economic production of AAC without detrimental effects to the final products.     

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.     

Cracking in autoclaved aerated concrete: Experimental investigation and XFEM modeling

The paper aims to investigate and model cracking development in beams and deep-beams made of autoclaved aerated concrete (AAC). Fracture mechanics of AAC has been first studied by performing three-point bending tests on beams, similar to those commonly used for ordinary concrete elements. In some of these tests, crack growth has been also monitored by using ESPI laser technique. In this way, it has been possible to calibrate the main parameters of a proper cohesive law by means of extended finite element inverse analysis. Subsequently, cracking tests have been also performed on deep-beams, whose behavior is more representative of full scale walls. To validate the proposed cohesive law, deep-beam experimental behavior has been finally simulated through XFEM.     

Sodium carbonate activated slag as cement replacement in autoclaved aerated concrete

This paper aims to study the suitability of fully replacing cement by sodium carbonate activated slag in producing autoclaved aerated concrete (AAC). The material properties of the product are characterized in terms of green strength development, mechanical properties, pore related properties such as porosity and thermal conductivity, shrinkage and reaction products. The produced alkali-activated slag-based AAC (ASAAC) shows comparable material properties to the designed cement-based reference AAC samples by giving a compressive strength of −25% with raw density of +18%, thermal conductivity of +13% with a porosity of −5% and drying shrinkage of +5.5%. Besides, a relatively higher crystallinity of calcium silicate hydrates and Al incorporation in the chain of C-S-H is observed for ASAAC products. Furthermore, significant reductions in cost, energy consumption and CO₂ emission are foreseen.     

Capillary absorption of water and n-decane by autoclaved aerated concrete

An autoclaved aerated concrete was one of three construction materials used in the round-robin study of hygral properties carried out in the European Commission funded Heat, Air and Moisture Standards Development [HAMSTAD] project (Roels S et al., Journal of Thermal Envelope and Building Science 2004 27 307–325). The material has fine micron-scale matrix porosity generated by the packing of thin tobermorite 11Å plates; and coarse mm-scale aeration pores arising from the foaming of the wet mix. We treat the material as having a strongly bimodal pore size distribution. Capillary absorption does not obey simple t^1/2 kinetics. We report here the results of liquid uptake tests using both water and n-decane to investigate the cause of the imbibition behaviour. The transport properties are modelled in a Sharp Front analysis as a parallel combination of absorption into the coarse aeration pores and into the fine matrix pores. The aeration pores have weak capillary suction and absorption into these pores reaches capillary rise equilibrium during the test. The Sharp Front model is applied here for the first time separately to subsets of the total porosity. The matrix sorptivity of the autoclaved aerated concrete studied (density 450 kg m^− 3, porosity 0.82) is about 0.23 mm min^− 1/2.     

Preparation and properties of sulphur-infiltrated autoclaved concrete

Autoclaved concrete specimens were infiltrated with liquid sulphur at 150°C. The rates of sulphur infiltration, the mechanical properties and the durability of the infiltrated specimens were all found to be greatly influenced by the nature of the porosity of the uninfiltrated parent concretes. The size distribtuions of pores in the specimens were found to be controlled by the CaO/SiO₂ (mole) ratio of the binder.     

Statistical modeling of the geometrical structure of the system of artificial air pores in autoclaved aerated concrete

The geometrical structure of the system of artificial air pores in autoclaved concrete is studied, based on samples from laboratory and industry. The single pores are approximated by spheres, the corresponding radius distribution is determined by means of stereological methods. The resulting distribution is not a classical standard distribution, but instead requires a mixture of three components, one of them Gaussian. The pore system as a whole is approximated by the penetrable-concentric-shell or “cherry-pit” model, which consists of spheres that are partially penetrable. The corresponding hard cores form a random packing of hard spheres. The goodness-of-fit of the model is tested by means of information available from planar sections: the section profile radius distribution and the pair correlation function of profile centers.     

Low temperature fracture behaviour and AE characteristics of autoclaved aerated concrete(AAC)

For a quantitative understanding of freezing damage on Autoclaved Aerated Concrete(AAC) and fiber reinforced AAC(i.e., RAAC), the influence of water and temperature (R.T. ∼ −20°C) on those materials have been studied by the investigation of AE characteristics, the fracture mechanics J-integral test and SEM observation. Furthermore, using the AE frequency analysis based on the frequency energy density distribution ratio (EDDR), the micro-fracture process for various test conditions has been interpreted.The AE activities and fracture toughness showed a large difference depending on the water content and temperature. All the AE events emitted during the fracture toughness tests could be classified into 6 groups. Also, the AE sources were considered paying particular attention to the micro-crack formation, the friction of inter-matrix and the fiber breaking behaviors at fracture. Noting that the AE is emitted during the drying process, the drying shrinkage damage was discussed.     

Temperature-dependent physical properties in physical gas absorption

A solution for interface temperature rise in physical gas absorption due to heat of solution was obtained by Danckwerts [1,2] with the limitation that the pertinent physical properties—diffusion coefficient, solubility of the gas and thermal conductivity of the liquid remained constant. This paper complements Danckwerts' contribution by considering the pertinent physical properties as temperature-dependent, the dependence being of the form:$\text{Q}\text{Qi}$ = exp [γ_Q(T ? T_i)]. Wagner [3] has obtained solutions for unsteady-state diffusion when the diffusion coefficient is similarly dependent on concentration. Wagner's solutions are adapted to provide a solution for the present problem. Two approximate solutions, one valid at high Lewis Number and the other when Lewis Number approaches unity are also presented. For the normal values of activation energies, Danckwerts' model is shown to be accurate when interfacial temperature rise is less than one percent of the absolute temperature of bulk liquid. The extent of departure from the Danckwerts' model is shown to depend on combinations of activation energies of the pertinent physical properties.     

On the correlation between permeability properties and strength of concrete

In this paper an attempt is made to study the correlation between the permeability and the strength of concrete by means of new methods of measuring the permeability of air and water in concrete. The scatter of the results is rather large, probably due to the heterogeneous character of concrete containing aggregate particles which are large in comparison with the test cavity. Further research is needed for a more definite evaluation of the methods, which can be used in the laboratory as an additional research technique and in the field as an additional research technique and in the field as an in-situ test method.     

In situ time-resolved X-ray diffraction of tobermorite formation in autoclaved aerated concrete: Influence of silica source reactivity and Al addition

The hydrothermal formation of tobermorite during the processing of autoclaved aerated concrete was investigated by in situ X-ray diffraction (XRD) analysis. High-energy X-rays from a synchrotron radiation source in combination with a newly developed autoclave cell and a photon-counting pixel array detector were used.To investigate the effects of the silica source, reactive quartz from chert and less-reactive quartz from quartz sand were used as starting materials. The effect of Al addition on tobermorite formation was also studied. In all cases, C-S-H, hydroxylellestadite and katoite were clearly observed as intermediates.Acceleration of tobermorite formation by Al addition was clearly observed. However, Al addition did not affect the dissolution rate of quartz. Two pathways, via C-S-H and katoite, were also observed in the Al-containing system. These results suggest that the structure of initially formed C-S-H is important for the subsequent tobermorite formation reactions.     

Mechanical and physical properties of flyash concrete after hydrothermal storage

In order to study the effect of fly-ash addition on the hydrothermal reaction in concrete an experimental program was conducted. Specimens of normal weight concrete made with ordinary Portland cement and fly-ash were exposed to 95 °C and relative humidities (RH) in between 95 % and 100 %. After storing the samples for 3, 6 and 12 months the following residual properties were studied: compressive strength, splitting tensile strength, content of calcium hydroxide, pH-value of concrete suspensions and carbonation depth. Alterations of the concrete microstructure were investigated by mercury porosimetry, DTA and X-ray technique. The results indicated that hydrothermal reactions proceed incomplete if environmental humidities are below 100 % RH. Because most of the pores dry out the liquid phase reactions cannot progress any more. However, during the drying process the hydrothermal reactions can take place to a considerable extent, if fly-ash is added to the concrete. At 100 % RH also concretes made without fly-ash indicate a nearly complete consumption of the free Ca(OH)₂. However, in the course of the tests none of the investigated specimens showed pH-values below the critical value of 9.5.     

Measurement of principal permeability with the channel flow experiment

A framework for channel flow permeability measurement in resin transfer molding (RTM) is developed in this paper. For the channel flow experiment, five possible experimental configurations are identified, of which two have not yet been mentioned in the literature. Starting from effective permeability, the entity measured in the experiment, explicit formulas for principal permeability and its orientation in two and three dimensions are developed. These formulas are applied to (published) experimental results that demonstrate their validity. The practical problems that prevent three-dimensional permeability measurement are discussed. Next, a framework for two-dimensional channel flow permeability measurement is proposed. All known channel flow permeability measurement methods are classified according to this new framework, and where appropriate, differences are discussed. An important finding emerging from this comparison is that two different definitions of Darcy's law are used today in permeability measurement.     

Studies on forecasting of carbonation depth of slag high performance concrete considering gas permeability

The carbonization resistance ability of concrete is a very important index of durability. Based on the standard specimen test, the relationship between the gas permeability of slag high performance concrete and the carbonation depth is studied, and the law of influence on carbonation depth of slag high performance concrete by the gas permeability is concluded. The carbonation speed coefficient decided by gas permeability and the influence coefficient of the actual environment condition are introduced on the basis of carbonization theoretical model, and the prediction model of carbonation depth of slag high performance concrete is proposed. It proves that the model has a good accuracy through the experiment.     

Dynamical analysis of gas fluctuations in an aerated vessel

The dynamics of gas fluctuations in a gas-liquid mixing vessel agitated by an axial pumping impeller near the full dispersion condition has been experimentally investigated. The transition region from full dispersion to partial dispersion has been identified in the plots of the specific power drawn with respects to the gas flow rate. The wavelet analysis performed on torque signal measurements revealed the existence of a competition between two dynamical systems—one representing vortex-type cavities and the other representing large gas-filled cavities. The fluctuation rate increased until it reached a maximum value at specific values of the flow number, and then decreased with further increase in the flow number.A space phase study revealed a transition from low amplitude oscillation attractor types to aperiodic attractor types as the flow number was increased.     

Effect of UV crosslinking and physical aging on the gas permeability of thin glassy polyarylate films

The effect of crosslinking by UV irradiation on the gas permeation properties of thin films (thickness ≤1 μm) made from two benzophenone-based polyarylates were examined. In addition to the permeation response to UV crosslinking in these two polymers, the effects of crosslinking on the rate of physical aging was also explored. The sequence of physical aging and crosslinking, as well as reversal of the aging process was studied in order to separate the similar effects of aging and crosslinking. The results show that crosslinking very thin films can greatly improve the long-term performance of membranes when compared to noncrosslinked films of similar thickness.