A group of researchers recently published a paper in the journal Materials investigating the feasibility of using crushed macauba endocarp as a bioaggregate in a cement mixture.
Study: Bonding behavior of a bioaggregate embedded in a cement-based matrix. Image credit: Banana Republic images/Shutterstock.com
background
The development of non-conventional materials has facilitated the design of a wide range of new structural elements that can be exposed to different loads and environmental conditions.
For example, bio-aggregates, such as wood particles, bamboo and coffee husk, enable the preparation of lightweight bio-concrete, while artificial aggregates facilitate the production of concretes with improved strength and crack induction.
The advancement of cementitious composites mainly depends on the optimization of binder components, specifically the cement matrix-aggregate interface that affects all types of cementitious materials, from ultra-high performance to materials normal
The increase in the packing density of the interfacial transition zone (ITZ) between the aggregate and the cement matrix plays a crucial role in improving the durability of the material and providing the necessary mechanical properties.
Although several studies were conducted to evaluate the properties of ITZ between cement matrix and aggregate, standard procedures have not been adapted to investigate ITZ between bioaggregates and cement matrix to date.
Group 1: Investigation of Aggregate and Cement Matrix Interactions Using Image Analyses: Zoomed Views for (a) macaúba endocarp and (b) river sand embedded in cement paste. Image credit: Ferreira, SR et al., Materials
The study
In this study, researchers investigated the bond behavior between a cement-based matrix and a bioaggregate using experimental and numerical approaches to determine the feasibility of using the aggregate as a green building material.
The experimental evaluation of the bioaggregate consisted of image, chemical, physicochemical and mechanical analyses. Granitic rock, a mineral aggregate widely used as coarse aggregate in the civil construction industry in Brazil, was used as a reference.
Mauá/LafargeHolcim type V-ARI cement, bioaggregate derived from macaúba fruit, distilled water and coarse mineral aggregates/crushed granite were used as starting materials for this study. The cement was composed of calcium oxide, silicon dioxide, aluminum oxide, sulfur trioxide, titanium dioxide, potassium oxide and manganese oxide and had a density of 3.2 g/cm3 and a surface area of 420 m2 / kg.
Crushed macauba endocarp was used as bioaggregate. The process typically used to obtain crushed macauba endocarp involves removing the macauba pulp to obtain the endocarp, followed by high-speed centrifugation of the endocarp.
Bioaggregate: from Macaúba fruit to crushed endocarp. Image credit: Ferreira, SR et al., Materials
The American Society for Testing and Materials (ASTM C136) standard and a HORIBA LA-950V2 type laser particle analyzer were used to determine the particle size distribution of aggregates and cement.
The researchers also measured the density, evaluated the water absorption capacity and performed chemical analysis of the bioaggregates. A multi-channel semi-adiabatic calorimeter with a PIKO TC-08 recorder was used to perform calorimetric analyzes on hydrated cement pastes at 25 oC.
Scanning electron microscopy (SEM) and light and laser microscopy were performed to obtain an overview of the microstructure of the endocarp and the topography and roughness profile of the bioaggregates, respectively.
A V-Tomex-M was used for microcomputed tomography (microCT). The parameters selected for scanning each sample were a voltage of 100 kV, a current of 180 µA, 300 ms per projection exposure time, seven frames, a magnification of 10.43, a voxel size of 19 µm and one pixel size and a total of 1500 images.
Phoenix Datos software was used to perform three-dimensional (3D) reconstructions, in which a mathematical edge enhancement filter was used and beam-hardening correction and slice alignment were implemented to achieve better contrast between the pores and the rock matrix.
The software used was VG Studio Max v. 3.0 for two-dimensional (2D) and 3D viewing. The compressive strength test was performed on both the semi-ellipsoidal bioaggregate and the cement paste.
Finally, the bond between the macauba endocarp/granite rock and the cement-based matrix was evaluated using an adapted pull-out test. These tests were performed after the cement paste had cured for seven days.
A 3D linear analysis was performed using DIANA (Fite Element Displacement ANAlyzer) 10.5 software to simulate the mechanical behavior of the pullout tests. A macauba endocarp with a semi-ellipsoidal shell shape and constant thickness was used during the development of the numerical model.
An incremental-iterative solution procedure was used for the nonlinear analysis using the Secant method for the iterative procedure and displacement control for the incremental procedure.
Observations
The endocarp consisted of 52% hemicellulose and 39% lignin. Such high amounts of hemicellulose and lignin can make cement hydration difficult. Accelerators, such as calcium hydroxide, can be used to overcome this problem.
The density of macaúba endocarp was greater than one g/cm3, which can be classified as very heavy. However, the density was considerably lower than the density of conventional aggregates used in normal weight concrete.
The water absorption capacity of the bioaggregate was 9%, which was substantially lower compared to wood and other bioaggregates, resulting in lower shrinkage. In addition, the lower shrinkage reduced the dimensional variation, which facilitated good adhesion between the endocarp and the cement.
Simulated and experimental extraction curves. Image credit: Ferreira, SR et al., Materials
Although the bond between cement paste and crushed endocarp was lower compared to the bond between cement paste and crushed granite, the surface of crushed endocarp showed good compatibility with the cement matrix with a wider smoothing curve due to its hemispherical shape.
Complete debonding was observed between the crushed endocarp and the cement paste at a pullout load of approximately 175 N, which was 55% of the granite pullout load. Up to this load, a similar linear elastic branch was observed between granite and endocarp, indicating that lignocellulosic materials with lower shrinkage do not lead to premature debonding of the cement matrix.
The finite element model effectively simulated the pullout behavior up to 80% of the experiment and then deviated from the experimental results. Using the finite element model, parameters that could not be directly measured in physical experiments, such as the magnitudes of cohesion and tensile strength of the ITZ, were successfully determined.
In summary, the results of this study effectively demonstrated the feasibility of using crushed macauba endocarp as a bioaggregate in concrete.
source
Ferreira, SR, de Andrade, RGM, de Andrade, GM et al. Bonding behavior of a bioaggregate embedded in a cement-based matrix. Materials 2022. https://www.mdpi.com/1996-1944/15/17/6151
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