Microstructure and Shrinkage Behavior of Autoclaved Aerated Con-crete(AAC) -Comparison of Vietnamese and Japanese AACs-

Abstract

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.

Utilization of Solid Waste in the Production of Autoclaved Aerated Concrete and Their Effects on its Physio-mechanical and Microstructural Properties: Alternative Sources, Characterization, and Performance Insights

“…So limiting the incorporation of lime in the mix prevents the increment in drying shrinkage. Fly ash also increases the drying shrinkage due to the presence of a higher surface area (Lam et al, 2018). The quantity of air-entraining agent, superplasticizers, and silica fumes has no influence on drying shrinkage ; whereas the incorporation of alumina-rich substances, such as bauxite, alumina-rich cement and blast furnace slag can decrease the percentage of drying shrinkage by 30-100% because of the development of a greater number of tobermorite (Alexanderson, 1979).…”

“…The shrinkage mechanism of AAC is still unclear. It has not been thoroughly explored for the wide range of saturation conditions to dry conditions at various range of relative humidity (RH) (Lam et al, 2018). Drying shrinkage of AAC has been studied at lower and higher value of RH.…”

“…Result shows that at higher RH (> 65%), capillary tension theory plays important role for the drying shrinkage mechanism; whereas at lower RH (< 65%) the shrinkage of AAC has a linear connection with the surface energy change. It has been determined that the primary factor for AAC to shrink under low RH circumstances (RH 65%) is a shift in the surface free energy (Lam et al, 2018).…”

“…where 'F' is absorbed water per surface area with time (t); 'C' is the capillary coefficient and depends on number of participated pores during the suction process. Moisture condition plays a decisive role in water absorption (Lam et al, 2018). Water vapor diffusion increases in the dry state because all pores are vacant in the dry state; whereas, in the regions where humidity is high, the capillary condensed water can fill some pores due to which these pores will not contribute to water vapor diffusion, and consequently water moves in it through the capillary forces only (Prim & Wittmann, 1983).…”

“…There are at least two types of pores in AAC, which are called "macropores" and "micro-pores". Macro-pores are pores with diameters more than 50 nm and micro-pores have pore sizes lower than 2 nm (Lam et al, 2018). The other one is called "meso-pores" and they exist between the gaps of these two ranges.…”