Experimental Study on the Effect of Different Pozzolans on Improving Mechanical Properties and Water Absorption of Autoclaved Aerated Concrete

Increasing builders waste of autoclaved aerated concrete (AAC) require new concepts for future recovery processes. There are two main aspects, underlining this basic necessity for developing AAC recycling models. Environment protection ranks first because of the risk of groundwater pollution by compounds leached out from AAC waste during its deposition. The second aspect is raw materials preserving because of the high content of the valuable potential recyclable calcium- silicate-hydrate phase 11 Å tobermorite (5CaO 6SiO2 5H2O) besides pure quartz and minor aluminate. In general the main constituents of AAC amount around 40–50 wt-% for tobermorite and approx. 30–40 wt-% for quartz. The tobermorite phase is causing the AAC strength and forms hydrothermally at 180–200 °C and 10–12 bar pressure during autoclaving from the raw materials lime, quartz, and water. Minor parts of aluminum powder for pore-forming and small amounts of cement and anhydrite for better handling of the AAC-green bodies are the further additives of the AAC raw materials mixture. The high silicate content, as well as the valuable calcium parts, display AAC waste as an interesting raw material for zeolite formation as known from the treatment of fly ashes and slags. According to their outstanding properties zeolites are used in sorption techniques, catalysis, molecular sieving, and ion exchange processes, and in previous studies, we already could show zeolite hydro sodalite formation beside hydrogarnet and other valuable calcium- and sodium aluminosilicates applying NaAlO2 as an aluminum source in the reaction mixture. Those previous syntheses were performed in water and under low alkaline (1 m NaOH) and low acid (1 m citric acid) conditions. This mild reaction milieu was found to be responsible for relatively low AAC conversion rates and the formation of multiphase products. In reference, the aluminum source NaAlO2 was added to the AAC always before the leaching reactions were performed. In contrast, the presented study investigates leaching of pure AAC in stronger alkaline media of 4–8 m NaOH and the combination with acid treatment, before the aluminate is added for the final crystallization process. This procedure is expected to be much more effective to synthesize uniform zeolite products at 100% AAC conversion rates, as shown in the following experimental study. Autoclaved Aerated Concrete (AAC) used in low-rise buildings and infilled frames as a structural member. One of the weaknesses of AAC is low mechanical strength. In addition, AAC blocks absorb water of mortar which can lead to executive problems. In this paper, the effect of silica fume, zeolite and granulated blast furnace slag (7%, 14% and 21% by weight of cement) was investigated on improving mechanical properties and water absorption of AAC. The compressive and tensile strength tests and water absorption test was conducted on 10 x 20 cm cylindrical and 10 x 10 x10 cm cubical specimens. The results showed that pozzolanic materials can improve mechanical properties and water absorption of AAC. The compressive strength for AAC mixes containing silica fume, zeolite, and granulated blast furnace slag, increased up to 184%, 200%, and 172% compared with AAC control mix. In addition, the use of pozzolanic materials with the ratio of 21% by weight of cement improved tensile strength of AAC up to 25%. Generally, silica fume, zeolite and granulated blast furnace slag in different replacement levels decreased water absorption up to 50%, 45%, and 35%, respectively.