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NaOH activation of soils is an affordable and promising way to improve mechanical properties of earthen bricks. If for well-activated geopolymers, the hard polymeric network limits the influence of water on mechanical properties, for the weakly activated one, as non-calcined raw clayey soils, the influence of water on these properties would be more critical. This work aims to determine the effect of sodium hydroxide concentration on the drying kinetics of bricks made with raw clayey soils, and to model this kinetics. The results show that the drying kinetics is governed by the diffusion of water due to the absence of free water. The drying duration increases linearly with the increasing of NaOH content, while the volumetric shrinkage decreases, probably thanks to the reduction of the material porosity during the formation of the zeolitic structures. Besides, the drying duration is strongly and negatively correlated with the initial drying rate ( −0.97) and bricks did not show visible cracks. Among the five parametric models tested, the Khazaei’s model is the best in terms of all statistical criteria considered. For all models used, the coefficient of determination is ranged from 0.993 to 0.999, and the evolution of the models’ parameters is in accordance with that of the drying kinetics observed.

Access to decent housing remains a significant challenge for many people, especially in developing countries [

The geopolymer compressive strength depends notably on the nature of the aluminosilicate source, the type and concentration of the alkaline activator used, and the duration and the temperature of the curing [

Two raw clayey soils were used for the manufacture of bricks. The first one was obtained by crushing termite Cubitermes spp. mounds (CMS) collected in the savanna around NGO (2˚29'14"S; 15˚45'20"E). They are at most 50 cm high and 30 cm large and are usually used to build floors in traditional houses or as rural road pavement. The crushed soil was sieved and only grains smaller than 2 mm were retained to manufacture bricks. The characteristics of this soil have been published [

The second soil used is the lateritic soil (LS) from Mouyami (4˚27'34"S; 14˚42'48"E). It has been sieved to retain only the grains smaller than 2 mm. Its

Soil characteristics | LS | CMS |
---|---|---|

Clay (%) | 31 | 25 |

Silt (%) | 19 | 25 |

Sand (%) | 50 | 50 |

W_{L} (%) | 27.9 | 11.6 |

W_{p} (%) | 9.9 | 2.1 |

PI (%) | 18 | 9.5 |

ω_{omc} (%) | 10 | 15.2 |

γ (t/m^{3}) | 2.1 | 1.72 |

OM (%) | 0.54 | 5.00 |

geotechnical characteristics have been determined as described in [

For the stabilization of the bricks, five alkaline solutions were prepared (1.5%; 2.5%; 5%; 7.5% and 10% by weight) by dissolving sodium hydroxide pellets with a purity of 98% in distilled water. These NaOH solutions were used instead of the usual tap water for the manufacture of the bricks, at the content of the optimum moisture content determined through the Proctor test. The bricks have been molded by using a mechanical press with a pressure of 6 MPa. A whitish powder appeared on the surface of the stabilized bricks for 7.5% and 10% NaOH solutions. This powder is probably due to the reaction of sodium hydroxide with the air carbonic gas [

The volumetric shrinkage rate of bricks at the instant t, λ(t), is equal to the difference between the initial volume V(0) and the volume at the instant t, V(t) divided by the initial volume.

The monitoring of the drying kinetics of the bricks was carried out as described in [

Five parametric models have been tested to simulate the drying kinetics of the bricks: the Page-Avrami model [^{2}, the root mean square of the error (RMSE) and the Akaike’s Information Criterion (AIC). The models’ expressions are reported in

The evaporable water content of the brick at the time t (the reduced mass) M_{r}(t) is calculated according to the formula:

M r ( t ) = M ( 0 ) − M ( t ) M ( 0 ) − M f

where M(t) is the brick’s mass at the time t and M_{f} the mass at the end of the drying.

The drying kinetics curves of earthen bricks made with Cubitermes spp. mound soil and lateritic soil activated with hydroxide sodium solutions are reported in

These curves did not show the constant drying rate phase (a straight line at the start of the curve), but a steady variation of the drying rate, that is, there is almost no free water in the bricks. In other words, these drying kinetics are governed by the diffusion of moisture from the interior to the surface of bricks [

Model | M r ( t ) | Parameters | References |
---|---|---|---|

Page-Avrami | exp ( − k t n ) | k, n | [ |

Diffusion | a exp ( − k t ) + ( 1 − a ) exp ( − k b t ) | a, k, b | [ |

Khazaei | 1 − a ( 1 − exp ( − t / T ) ) − b t | a, T, b | [ |

Peleg | 1 − t / ( a + b t ) | a, b | [ |

Yong | c t − μ exp ( − t / T ) | c, µ, T | [ |

bricks with the 10% NaOH solution. For the means values and by linear regression, the following relationship was found between the drying duration (D) and the NaOH content (α):

D = 24 + 0.8 α − 0.04 α 2 ; R 2 = 0.91

This lengthening of the drying duration by the NaOH activation could be explained by the tendency of NaOH to form compounds with water, and/or by the formation of the zeolitic structure between the initial soil grains [

As for the drying kinetics, there is no significant difference between the volumetric shrinkage rate of Cubitermes spp. soil bricks and that of lateritic soil bricks (

The shrinkage decrease observed here is lower than that induced by the addition of lime [

The reduced water contents predicted by the models are represented as functions of those measured in

the mean values of the coefficient of determination (R^{2}), the root mean square error (RMSE), and the Aike’s information criterion (AIC) for all models tested are also reported on these figures. For each model and any drying kinetics curve, the coefficient of determination is greater than 0.992, and the RMSE is less than 0.03. Therefore, all these models could be considered suitable to simulate this drying kinetics. However, the Khazaei model has better results than the others (

The values of the parameter n (the exponent of the time) of the Page-Avrami model and those of the khazaei’s model (_{2} reaction. Indeed, as we have already reported in the material and methods part, for the bricks activated with 7.5 and 10% NaOH solution, a whitish powder appeared on the surface of the bricks.

CMS bricks | LS bricks | |||||
---|---|---|---|---|---|---|

Rank | R^{2} | X^{2} | AIC | R^{2} | X^{2} | AIC |

1 | Kharzaei | Kharzaei | Kharzaei | Kharzaei | Kharzaei | Kharzaei |

2 | Diffusion | Diffusion | Avrami | Diffusion | Diffusion | Diffusion |

3 | Avrami | Avrami | Diffusion | Avrami | Avrami | Avrami |

4 | Yong | Yong | Yong | Peleg | Peleg | Peleg |

5 | Peleg | Peleg | Peleg | Yong | Yong | Yong |

Page-Avrami | Diffusion | Yong | Khazaei | Peleg | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

NaOH (%) | k | n | a | b | k | c | T | u | a | b | n | T | a | b |

0 | 0.28 | 0.92 | 0.8 | 3.3 | 0.21 | 0.94 | 4.40 | 0.01 | 1.02 | −0.001 | 0.90 | 4.2 | 2.7 | 0.87 |

1.5 | 0.22 | 0.97 | 1 | 4.5 | 0.21 | 0.98 | 4.90 | 0.01 | 1.03 | −0.001 | 0.95 | 4.9 | 3.5 | 0.84 |

2.5 | 0.17 | 1.04 | −13 | 1 | 0.14 | 1.03 | 5.40 | −0.01 | 1.02 | −0.001 | 1.03 | 5.8 | 4.4 | 0.80 |

5 | 0.15 | 1.06 | 0 | 22 | 0.01 | 1.04 | 5.89 | −0.01 | 0.96 | 0.002 | 1.06 | 6 | 5 | 0.79 |

7.5 | 0.12 | 1.08 | −5 | 1.1 | 0.10 | 1.04 | 6.50 | −0.01 | 0.98 | 0.001 | 1.05 | 7 | 5.9 | 0.76 |

10 | 0.08 | 1.18 | −29 | 1 | 0.07 | 1.09 | 7.15 | −0.02 | 1.07 | −0.001 | 1.10 | 9.1 | 7.8 | 0.67 |

Page-Avrami | Diffusion | Yong | Khazaei | Peleg | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

NaOH (%) | k | n | a | b | k | c | T | u | a | b | n | T | a | b |

0 | 0.39 | 0.76 | 0.7 | 8.3 | 0.19 | 0.78 | 5.30 | 0.04 | 1.1 | −0.003 | 0.67 | 4.7 | 2.7 | 0.87 |

1.5 | 0.25 | 0.84 | 0.8 | 5.4 | 0.16 | 0.87 | 6.00 | 0.03 | 1.0 | 0.000 | 0.80 | 4.9 | 3.5 | 0.84 |

2.5 | 0.22 | 0.88 | 0.9 | 7 | 0.14 | 0.91 | 6.76 | 0.02 | 1.1 | −0.002 | 0.80 | 6.9 | 4.4 | 0.80 |

5 | 0.14 | 1.00 | −0.1 | 54 | 0.01 | 0.98 | 7.35 | 0.01 | 1.4 | −0.007 | 0.87 | 10.9 | 5 | 0.79 |

7.5 | 0.09 | 1.14 | −14 | 1.1 | 0.07 | 1.06 | 7.78 | −0.01 | 1.1 | −0.001 | 1.04 | 9.7 | 5.9 | 0.76 |

10 | 0.05 | 1.28 | −29 | 1.1 | 0.05 | 1.12 | 8.42 | −0.03 | 1.0 | 0.003 | 1.19 | 10.7 | 7.8 | 0.67 |

The values of the parameter k of the Page-Avrami model, those of 1/T of the Khazaei’s model, and the inverse of the parameter a of the Peleg’s model decrease when the NaOH content increases. This result is consistent with the fact that the first two parameters are directly related to the time after which 63.2% of water is evaporated, and the third with the initial drying speed.

The values of the parameter K of the diffusion model obtained here are larger than those obtained earlier [

The objective of this work was to study the effect of NaOH activation on the drying kinetics of earthen bricks and to test five parametric models on this kinetics.

The results obtained show that:

1) There is not a drying rate constant phase during the drying, and therefore the drying kinetics is governed mainly by the diffusion of water from the interior to the surface of the brick. The drying duration increases almost linearly with the NaOH content. It varies from about 23 days for non-activated bricks to 28 days for activated bricks with a 10% NaOH solution. This increase could be explained by the formation of the zeolite structure that reduces the soil porosity, the NaOH-H_{2}O affinity, and the NaOH-CO_{2} reaction;

2) NaOH activation reduces the volumetric shrinkage rate of bricks for the soils studied. It varies from about 5% for non-activated bricks to about 3% for the activated bricks with 10% NaOH solution. The reduction of the porosity due to the formation of a zeolitic structure could explain this shrinkage decrease. Manufactured bricks did not show visible cracks;

3) The coefficients of determination of the five models range from 0.993 to 0.999. The Kazaei’s model is the best of all. The parameter n of Avrami-Page and Khazaei models is about equal to 1, in accordance with the absence of free water in the bricks. This parameter increases slightly with the NaOH content. The evolution of the parameters k of the Page-Avrami and diffusion models, that of the parameters T and a of the Kharzaei and Peleg models, respectively, are in agreement with the variation of the drying duration due to the NaOH content.

The authors declare no conflicts of interest regarding the publication of this paper.

Bouyila, S., Elenga, R.G., Ahouet, L., Ngoulou, M. and Konda, S. (2019) NaOH Activation of Raw Soils: Effect of NaOH Content on the Drying Kinetic and Its Modelling. Geomaterials, 9, 55-66. https://doi.org/10.4236/gm.2019.92005