We investigated the microstructure and shrinkage behavior of autoclaved aerated concrete (AAC) from several manufacturers in Vietnam comparing with Japanese AAC. Three types of Vietnamese AAC and one type of Japanese AAC were used for powder X-ray diffraction, scanning electron microscopy, mercury intrusion porosimetry, and shrinkage tests. The experimental results show that the main hydration products of AAC that used fly ash as silica materials is semi-crystalline calcium silicate hydrate, while the ones of others are tobermorite; but the tobermorite crystals of AACs from some manufacturers in Vietnam are disordered structures and lack of interlocking among tobermorite crystals. The pore size distribution of all Vietnamese AAC are single peak, whereas Japanese AAC is bimodal. The pore distribution characteristics of AACs significantly influenced their shrinkage behavior and the shrinkage of Vietnamese AAC is higher than that of Japanese AAC at intermediate relative humidity (RH). The capillary tension is the principle shrinkage mechanism for AAC materials at high RH (above about 65%) to cause local minimum shrinkage of Japanese AAC at high RHs, while the change in surface free energy is dominant at low RH conditions.
In Vietnam, the current amount of thermal ash (fly ash and bottom) remains nearly 100 million tons, causing great environmental and social impacts. The recycling and reuse of this source of waste is an effective solution towards sustainable development. The paper introduces the results of study on some properties and particle size distribution of aggregate from thermal ash, Duyen Hai factory for refractory concrete. Particle composition is calculated and selected according to the density of particle size arrangement with the maximum number of points of contact. The continuous particle size distributions of thermal ash is calculated by Andersen’s formula with Dmax = 5 mm. The bulk density and porosity of the particle mixture corresponding to the vibrating modes is determined. By the experimental planning method (Design of experiments - DoE), the optimal aggregate particle composition was determined with the calculated value of n = 0.387 and the vibration time of 90 s gives the maximum bulk density of 1313.17 kg/m3 and the smallest actual porosity is 34.69%.
In recent years, the utilizations of industrial by-product as substitute for non-renewable material in cement and concrete production has received significant attention for sustainable development goals. In this study, fly ash blended cement was produced by replacing 5%, 10%, 15%, 20%, 25%, 30% and 35% by weight of Portland Cement with fly ash. In addition, the loss on ignition (LOI) content of fly ash, which used for the 20% replacement ratio, varies from 5 to 20% to assess the effect of unburnt carbon content in fly ash on the strength activity index of fly ash blended cement. The experimental results show that the addition of fly ash reduces the compressive strength of the mortar and strength activity index of fly ash blended cement at early ages. However, the presence of fly ash increases the strength of the mortar and the strength activity index of fly ash blended cement at longer ages when the fly ash replacement ratio is lower than 25%. The unburnt carbon in fly ash does not reduce the strength of the mortar and strength activity index of fly ash blended cement when its content does not exceed 20%.
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