OPTIMIZATION OF PRESSURE AND CURING TIME IN PRODUCING AUTOCLAVED AERATED CONCRETE

ABSTRACT

Most autoclaved aerated concrete (AAC) is produced from materials such as quartz sand, lime, Portland cement and other constituent categorized as pozzolanic materials. The density of the concrete is less than 1000 kg/m3. Using less Portland cement and being easy to be formed are the advantages of applying AAC as bricks at the construction site. In this paper, the material to produce AAC were composed of Portland cement, calcined Sidoarjo volcano-mud, lime, sand and Aluminum powder as foaming agent. Specimens were cured in autoclaved at varied pressures at a different period of time to obtain an optimum curing system.

The results showed that the autoclaving system  using pressure of 14 bars for six hours were recommended for the curing system. The averages of compressive strength of specimens at 7days were 2.4 MPa with the density of 817 kg/m3. Longer curing system resulted in lower compressive strength.



INTRODUCTION

Concrete is widely used in construction industry because it's compressive strength. Concrete is also easily molded as desired. However, the weight itself is relatively great, so that when a building is attacked by an earthquake, the earthquake fprces will be amplified because the force is proportional to the weight of these reasons, the current developing of concrete material technology is creating buildings with a strong concrete based material but reducing the weight of non-structural elements by applied lightweight concrete.

By using less volume of cement, a lightweight concrete was made from silica fume, perlite, fly ash, quartz sand, lime, gypsum and other materials that are categorized as materials for lightweight concrete. It is not only having density less than 1 g/cm3, but conveniently formed and mobilized during construction. They are one of the advantages of lightweight concrete compared with normal concrete. In the manufacturing process, lightweight concrete is cured in a steam pressure system for 20 hours at a high temperature at around 190 centigrade and the steam pressures up to 20 bars or 2 MPa.

Sidoarjo Mud (Lusi) is a material having potential to reduce cement content in making lightweight concrete. The activated siliceous mud can be applied as cement substitution. It has also substantial fine particles but high shrinkage so that the mud is usually incorporated with additional materials such as fly ash or silica sand to increase the conpressive strength and its stability. To activate tohe oxide content, the mud is calcined at a certain temperature. The optimum calcination temperature usually is kept constant at 600 - 800 centigrade for at least two hours.

Aluminum powder is generally applied in making lightweight concrete to accelerate the foaming process to reduce the density of concrete. This process causes a chemical reaction which releases gas. After the mixture hardens, porous concrete is formed. Usually, after setting the lightweight concrete were cured in an autoclave at a temperature of 120 - 250 centigrade and a pressure of 5 - 20 bars for 8 - 20 hours. Autoclaving significantly increases the compressive strength due to its high temperature and pressure to produce a stable form of voids. Final strength obtained depends on the pressure and duration of autoclaving process. The process increases the strength and reduces the density of lightweight concrete.

This study investigated the effect of pressure and autoclaved curing time on the mechanical properties of autoclaved aerated concrete made with Sidoarjo mud. The effect of lime addition to the mixture was also studied.



MATERIALS

An Ordinary Portland Cement (OPC) having specific gravity of 3.1 g/cm3 was used to mix with Sidoarjo mud with specific gravity of 2.6 g/cm3. The mud was prepared with a particle size up to 75 mm. A lime containing 98% of hydrated lime was added to accelerate the hydration process. The specific gravity of the lime is 2.3 g/cm3. The aluminum powder was applied as a foaming agent. Chemical composition of material is listed in Table 1. The mineral content in Sidoarjo mud is shown in Fig.1.



METHODS

Three identical cubical specimens size of 5x5x5 cm3 from each variation were cast and settled for 24 hours before remolding. Specimens were leveled and settled into a vacuum machine for 10-15 minutes to reduce the moisture remaining in the pores of concrete. Specimen were cured in an autoclave with a certain pressure. Stem pressures from the autoclave were applied ranging from 5,9, and 14 bars, while the length of the treatment process ranging from 2,3,4,6,8 and 10 hours.C

ompression test and density of lightweight concrete were analyzed at the age of seven days. The results were compared with the non-autoclaved specimens curing at room temperature for 14,25 and 56 days. Variations in pressure and length of treatment will be recommended in this paper.

RESULTS AND DISCUSSIONS

The Effect of Pressures and Lime Content.

Fig.2 illustrates the relationship between the addition of lime to the compressive strength. Specimens were cured in the autoclave at a pressure of 5,9 and 14 bar for four hours. In the picture, it is shown that the addition of lime decreases the compressive strength. Specimen P10-1 which containing 10% of lime showed the highest strength compared with other specimens. When the specimens were cured with steam pressure at 14 bars, the strength increased. The highest strength compared with other specimens. When the specimens were cured with steam pressure at 14 bars, the strength increased. The highest strength was also provided by specimens P10-1.

As well known, an excessive lime added to the mixture of cement paste decrease the strength. However, by performing about 15 - 20 bars the autoclave treatment increases the compressive strength of bricks made from lime. Usually the optimum content of lime in the production of lightweight concrete ranges from 10% to 15% by weight of binder.



The Effect of Curing Duration.

The relationships between curing duration with the compressive strength specimens is presented in Fig.3. The specimens were cured at the same steam pressure of 14 bars for two, three and four hours. Specimens cured with the autoclave for four hours produced the best performance. This is according to research conducted by Zhao. Increasing the duration of autoclaving produces higher compressive strength. The longer curing time will improve the mechanical properties of lightweight concrete. For example, as illustrated in Fig.3, compressive strength of 0.6 MPa is shown by specimen P10-1 cured at two hours. When the autoclaving duration was prolonged to three hours then the strength increased to 0.7 MPa. When the time was extended to four hours, the strength increased to 1.3 MPa. The effect of curing duration showed more effect on the compressive strength than the effect of lime ocntent.

It is known that calcium silicate hydration changes into solution. When the curing time is prolonged, this solution freely moves between hydration products leads to diffusion in solution between the particles. Formation of hydration product increases since the crystalline in layers between the particles also increases. This process will generate strength of lightweight concrete.

The test results were also consistent with research conducted by Hanecka (1997). Autoclaving significantly increases the compressive strength due to high temperature. The pressure generated stable voids. The strength is obtained mainly depends in pressure and expected duration of autoclaving.



Comparison with Non-Autocclaved Specimens.

Non-autoclaved specimens were compressed at the age of 14, 28 and 56 days. The strength of specimens increased since the hydration reaction slowly takes place. Under room curing condition, quartz filler is less reactive. Fig 4 shows that the specimen P10-1 has the lowest strength of 0.1 MPa at 14 days. The results were then compared with the autoclaved-specimens. This proves that the pressure system at 14 bars is required in the process of lightweight concrete. The strength increased as the pressure of steam was increased. It is illustrated in Fig.5 that steam pressure at the higher temperature (approximately 180 centigrade at 14 bars) initiates the increase of concrete strength. It might be due to the reaction of quartz with cement paste. Yazici et al (2013) reported that quartz source under autoclaving generated a formation of tobermorite. Tobermorite belongs to a wider family of cement minerals that contributes higher mechanical performance. It appears when the steam pressure reaches 10 bars, the non-autoclaved curing system remains a non-reacted silica source in the mixture. Thus, it indicates that the requirement of steam pressure curing plays an important role in the factor improving the compressive strength.

Fig. 6 shows the temprerature of curing influence the density of lightweight concrete. Because of drying shrinkage occurred, density of non-autoclave specimens decreased. Specimens containing more lime showed less density than others due to lower specific gravity of lime. As the temperature was increased during autoclaving, crystallization occurred in autoclave-specimens. This process decreases drying shrinkage and increases denser pores with stable void shape. It can be explained that  the density of autoclaved-specimen is slightly higher than the density of the non-autoclaved specimens.



Relation between Curing Time and Compressive Stength.

It has been determined that by providing a certain steam pressure in the autoclave curing will produce a higher compressive strength. Further optimization of curing duration was obtained by using only pressure that produced the highest mechanical property. Steam pressure was kept constant at 14 bars with different treatment times ranging from 2,4,6,8and 10 hours. Fig.7 shows a relation between curing duration and compressive strength. The highest strength of 2.4 MPa is shown by specimen P-10 cured for six hours. The increased compressive strength under steam pressure is probably contributed by the effect pozzolanic reaction and the hydration of cement. However, extended autoclaving period may cause the strength decrease. The optimum compressive strength can be achieved in a shorter curing duration at higher temperatures to generate the hydration of calcium silicates and pozzolanic activity. It is believed that longer exposure steam pressure causes the formation of excessive crystalline calcium silicate. It will induce the strength reduction.



Density of Autoclaved-Specimens.

Relation between strength and density is presented in Fig.8. All specimens were cured at steam pressure of 14 bars. The data was taken at seven days after casting. As expected, the strength was the function of density. The strength improves with the increase of density. Denser and stable pore shape generated the increase of strength. The density of specimens with minimum strength of 2 MPa was 0.82 g/cm3.



CONCLUSIONS

Lime content reduced the density of lightweight concrete, but in excessive usage, it induced to decrease the strength. It is certainly recommended that the optimum lime content in the mixture is 10-15% by binder weight. The length of standard curing in room temperature can be achieved in a very short period by autoclave curing. However, prolonged exposure curing will influence the strength. In this study, the optimum curing period is six hours with steam pressure at 14 bars. It was revealed that the density was the function of compressive strength.