This paper investigates the porosity and the mechanical strength of an Autoclaved Clayey Cellular Concrete (ACCC) with the binder produced with 75 wt% kaolinite clay and 25 wt% Portland cement. Aluminum powder was used as foaming agent, from 0.2 wt% to 0.8 wt%, producing specimens with different porosities. The results show that the specimens with higher content of aluminum presented pore coalescence, which can explain the lower porosity of these samples. The porosities obtained with the aluminum contents used in the study were high (approximately 80%), what accounts for the low mechanical strength of the investigated cellular concretes (maximum of 0.62 MPa). Nevertheless, comparing the results obtained in this study to the ones for low temperature clayey aerated concrete with similar compositions, it can be observed that autoclaving is effective for increasing the material mechanical strength.
Cellular concrete is a hardened Portland cement slurry that has been aerated prior to setting to give a homogeneous void or cell structure containing 50–80 vol% or more of air bubbles, void spaces, and capillary porosity [
There are many possible ways to produce cellular concrete. Different compositions together with different curing methods can be used in order to obtain variable final properties, such as density, mechanical strength and thermal and acoustic conductivity [
Final properties of a cellular concrete largely depend on its porosity, which can be modified by changing the foaming agent type and content. A very porous material will present excellent thermal and acoustic insulation properties, due to its high amount of entrained air. However, these properties are achieved to the detriment of mechanical strength, which decreases as the pore volume increases. Therefore, characterization of porosity and its influence on the mechanical strength of cellular concrete is a very important factor to be analyzed in the production of this kind of material.
Some investigations [
A kaolinitic clayey-based cellular concrete composition was investigated by Goual et al. [
This study aims particularly at investigating the porosity and mechanical strength of an Autoclaved Clayey Cellular Concrete (ACCC) and compares the results with those presented by CCC with similar compositions, reported in the literature.
The clay used was composed of 98.3% kaolinite (Caulina Minérios, Brazil) and the cement was a standard high initial strength Portland cement (CP V-ARI-RS, Votorantim, Brazil). The aluminum powder was Stanlux Flake CL 4010 (Aldoro, Brazil) with an average particle size of 16
The Autoclaved Clayey Cellular Concrete (ACCC) was obtained through the aeration of an aqueous paste of a kaolinitic clay and Portland cement using aluminum powder as the foaming agent. Aluminum powder reacts with the caustic solution that evolves during the hydration reaction to form hydrogen gas bubbles [
Four compositions were analyzed, differing in the amount of aluminum powder used. The amounts, calculated in terms of weight percentage of dry clay-cement materials, were the following: 75 wt% clay, 25 wt% cement, 65 wt% water, and polycarboxylate-based superplasticizer (0.8 wt%, weight percentage of solids in relation to dry materials), to which amounts of 0.2 wt% to 0.8 wt% of aluminum powder were added in increments of 0.2%. The samples in this study were denoted by A2 for 0.2, A4 for 0.4, A6 for 0.6 and A8 for 0.8 wt% of aluminum.
In this experiment, clay and cement were dry-mixed in a planetary axis mixer at low speed for 2 minutes. Water was then gradually added while mixing continued at low speed for another 2 minutes. A short stop of 1.5 minutes on mixing was done in order to scrape the material sticked on the walls of the mixing container. The mixture was then homogenized at low speed for 1 minute, followed by a 2 minutes period at high speed. The superplasticizer was added while the mixer was maintained stopped for 30 seconds. After the addition of this additive the paste was mixed at low speed for 1 minute and subsequently stopped for another 30 seconds for the addition of the aluminum powder, which was mixed and homogenized into the paste for 1 minute at low speed.
Eight specimens of each mix were cast into cylindrical (50
The microstructures and phases of ACCC were compared to those of a low-temperature clayey cellular concrete, cured at ambient temperature in a humid room at 25
The
In order to measure the
For measuring
In order to determine the mechanical strength of the specimens, five cylindrical samples (50
The products in this work were characterized by X-ray diffractometry (XRD, Phillips, model Xpert, The Netherlands), and scanning electron microscopy (SEM, Phillips, The Netherlands).
Figure
Apparent density and porosity of ACCC as a function of aluminum powder content.
The analysis of ACCC samples fractured surfaces revealed that mixtures A6 and A8 presented pores with a nonuniform shape, which were larger than those observed for mixtures A2 and A4, suggesting the coalescence of the pores of mixtures with higher Al content. This could explain the nonexpected results for density and porosity. Figures
Fracture surfaces of samples with (a) the lowest (0.2 wt%), and (b) the highest (0.8 wt%) aluminum content.
When pores join together, they acquire a higher volume and tend to escape from the material [
A factor that can explain the pore coalescence occurred to these samples is the high reactivity of the Al powder used in this study. As showed in Figure
Possible solutions for this problem would be to minimize the amount of superplasticizer used in order to increase the viscosity of the cementitious paste so that the escape of the hydrogen bubbles would be detained, and to optimize the amount of Al power used (Stanlux Flake CL4010), since a lower amount of this powder is enough to produce high porosities.
The mechanical strength of the ACCC samples was consistent with the density and porosity results, that is, higher porosities resulted in lower mechanical strength, what can be observed in Figure
ACCC compressive strength as a function of aluminum content. The vertical bars represent the standard deviation of the measured values.
The different mechanical compressive strength values presented by the samples analyzed in this study are a consequence of their cellular morphology. In samples A2 and A4, due to the higher quantity and homogeneous distribution of their pores, the interpore struts are thinner, conferring a lower strength to the material. However, in samples A6 and A8, though the pores are larger due to coalescence, the struts are thicker, what resulted in higher compression strength. Even so, the obtained values were very low, achieving a maximum of 0.62 MPa for the composition A8 (78.23% porosity).
According to Gibson and Ashby [
Through this equation the values of mechanical strength that CCC samples would present if they had same porosities as the ACCC samples analyzed in this study were estimated based on the data provided in the literature [
Compressive strength of the ACCC samples investigated in this study and the estimated values based on the literature for CCC [
It can be observed that the mechanical strength values estimated for CCC samples are significantly smaller than those measured for the ACCC with same relative densities. This allows the conclusion that autoclaving is effective on strengthening a clayey-based cellular concrete, since for an equal porosity value, the compressive strength of ACCC is substantially higher than that of CCC, which is cured at room temperature.
As the higher compressive strength analyzed in this study was 0.62 MPa for the sample with 0.8 wt% Al, it becomes clear that a reduction on the amount of the Al powder used is necessary, in order to obtain samples with lower porosities and consequently higher mechanical strength to meet the specifications. However, if the purpose is to use a material with similar porosity as the ones here analyzed, some change in composition should be investigated.
The crystalline phases of ACCC were compared to those of a clayey cellular concrete cured at
XRD diffratograms of samples cured at low temperature (
SEM images of samples cured in (a) at low temperature (
This study aimed at investigating the influence of the autoclave curing on the mechanical strength of a clayey-based cellular concrete. Porosity and mechanical strength of samples with different compositions, differing only on the Al content, were analyzed and the results were compared to the literature for clayey cellular concrete cured at room temperature (CCC).
According to the results, the following could be concluded. The samples with 0.6 to 0.8% aluminum powder produced so much hydrogen that the pores no longer remained discrete, that is, the pores coalesced to such an extent the excess hydrogen could escape the matrix. This means that there is likely to be an optimum aluminum addition level in the range 0.4 to 0.6% to produce maximum porosity. The mechanical strength of the ACCC samples varied according to its porosity, that is, it increased as the porosity decreased. However, the obtained values were very low, achieving a maximum of 0.62 MPa for the sample with the lower porosity (78.23%). By extrapolating the data presented in the literature [ Lower amounts of Al powder should be used in order to reduce the porosity and increase the mechanical strength. Another possibility would be an alteration on the solid phase composition of the ACCC here investigated.
The authors are thankful to Mr. Zanon and Mr. Nogara from Celucon (Criciúma, SC, Brazil) for providing the aluminum powder used in this study, as well as permitting them to use their autoclave for our experimental work.