A New Building Material : Mixture of Rice Straw , Laterite and Cement D . Sow and A . Diokhane

The aim of this study is to improve the proprieties of the mixtures of rice straw and laterite. Thus, we conduct this study related to the mixtures of rice straw, laterite and cement for their use as building material. The lateritic formations represent the most abundant economical materials resources available in tropical and equatorial Africa. Rice straw is rich in cellulose. Consequently, few animals are able to use it as food. We have determined by experiments the mechanical and thermal properties of samples. The Interesting results obtained show that the integration of mixtures of rice straw, laterite and cement in building materials are great opportunities to reduce the cost of social housing. It also improves the thermal comfort.


INTRODUCTION
In previous studies, we are interested in the valorization of mixture of laterite with rice straw through its integration in construction material (Sow et al., 2014).
Thus, to improve the proprieties of these materials we interested in mixing them with Portland cement.
Portland cement is a hydraulic binder.It is the product obtained by reducing clinker powder consisting essentially of hydraulic calcium silicates and a small quantity of gypsum (hydrated calcium sulphate).Gypsum is a mineral used to delay and regulate the taking and hardening of cement (Norm EN 197-1, 2001;Norm AFNOR P 15-101-1, 2001).
The rice straw and the laterite are easily available in the open country (Demarquilly, 1987;BCEOM-CEBTP, 1975).
In Africa, earthen construction has become over the years an art: For example, cases-shells in Cameroon or the pyramids in Egypt.In these countries, finely chopped dried straw is a commonly used frame (Bakam et al., 2004).
The resistance to compression of materials is one of the properties used in the design of buildings.The thermal resistance of a material gives it a certain insulation depending on its thickness and this resistance is particularly inversely proportional to the coefficient of thermal conductivity.Thus, the main objective of this research is to optimize the properties of Portland cement mixed with materials that we have developed such as laterite mixed with straw rice for possible use in the building construction.

RESULTS AND DISCUSSION
Basic materials used: Cement: The cement that we used to improve our samples is CEM II/B-LL 32.5R cement from SOCOCIM in Rufisque, a town 20 km from Dakar, Senegal.
This cement is a hydraulic binder because it holds and hardens by combining with water.This reaction is called hydration.It corresponds to the chemical reaction in which water and cement combine to give rise to a solid mass.
Fillers are "secondary constituents" of cements, so they can never exceed 5% by mass in the cement composition.They are mineral materials, natural or artificial, which act by their granulometry on the Optimum (mixture consisting of 3% of rice straw and 97% of Montrolland laterite): This mixture is the most optimal mixture of rice straw and Montrolland laterite (Sow et al., 2014;Sow, 2014).Thus, this mixture has a compressive strength of 1.448 MPa and a thermal resistance of 0.268 m 2 K/W for a wall thickness of 20 cm.To make our sample, we have chosen the rice straw of variety Sahel 108, which is locally the most available.
We considered, also the fraction of this laterite passing the 5 mm sieve to make our samples.The straw comes from Dagana town located at 408 km from Dakar (Senegal).The laterite comes from Montrolland town located at 87 km from Dakar (Senegal).

Mechanical characterization:
Figure 1 shows the results of compression strength of cement mixed with the optimum.This Fig. 1 shows that the compressive strength increases, but slowly between 20% and 25% of cement in the mixture.This means the effect of cement on mechanical strength begins above 25%.
Thermal characterization: Figure 2 shows the results of thermal conductivity of cement mixed with optimum.This Fig. 2 shows that the thermal conductivity increases, according to the percentage of cement.It's due to the fact that the effect of cement on thermal conductivity begins above 20%.
Figure 3 shows the results of thermal resistance according to the percentage of cement mixed with the optimum for a wall thickness of 20 cm.This Fig. 3 shows that the thermal resistance decreases according to the percentage of cement.Furthermore, it is important to note from 40% the material becomes less economical.

THERMO-MECHANICAL RESULTS
To have in parallel the mechanical and thermal characteristics, we represented them in Fig. 4.This Fig. 4 shows that the most optimal mixture is composed of 30% of cement and 70% of optimum.This most optimal mixture has a thermal resistance of 1.739 m 2 K/W, for a wall thickness of 20 cm and a compressive strength of 0.979 MPa.
However, in order to justify this choice of optimal mixture, we can calculate the minimum mechanical strength we need to build a non-bearing wall with this material.
If we consider a 4 m high non-bearing wall built with this optimal composition of 1325 kg/m 3 density.The base of the wall must have at least 0.052 MPa.compression strength.This is much lower than the mechanical compression strength of the optimal formulation.This optimal mixture has a good thermal resistance and a mechanical resistance to the compression, which is above what is required for a filling wall of a building.
According to our previously published works, this one is simply additional data.

CONCLUSION
At the end of our study, we note that: • Proceeding to mixtures of cement with optimum, we were able to optimize the thermo-mechanical properties of materials.• The most optimal mixture is composed of 30% of cement and 70% of optimum (3% of rice straw and 97 % of Montrolland laterite).• The most optimal mixture has a compression strength of 0.979 MPa and a thermal resistance of 1.739 m 2 K/W.
Finally, the interesting results obtained allow to reduce the cost of social housing and to improve thermal comfort.

CONFLICT OF INTEREST
We certify and declare that there is no conflict of financial or relevant interest in relation to the abovementioned article.

Fig. 1 :
Fig. 1: Evolution of the compression strength according to the percentage of cement mixed with the optimum (laterite + straw)

Fig. 3 :
Fig. 3: Evolution of thermal resistance according to the percentage of cement mixed with the optimum for a wall thickness of 20 cm