Geopolymers are inorganic aluminosilicate materials, which have been a great research interest as a material for sustainable development. However, they possess relatively low toughness properties similar to brittle solids. The limitation may be altered by fiber reinforcement to improve their strength and toughness. This research describes the synthesis of bamboo shaving (BS) reinforced geopolymer composites and the characterization of their mechanical properties. The effect of BS content (0–2 wt. %) on the physical and mechanical properties and microstructure of metakaolin based geopolymer paste were investigated. The workability, setting time, bulk density, apparent porosity, thermal conductivity, compressive strength, flexural strength, scanning electron microscopy (SEM), and X-ray diffraction (XRD) of geopolymer paste were determined. The results showed that the workability, setting time, density, and thermal conductivity decreased with the increasing of BS content. The BS content was proportional to the apparent porosity and a good linear relation was found between apparent porosity and BS content. The highest mechanical properties were achieved at an optimum BS content of 1.0 wt. %. The results of microstructural analysis revealed that BS act as inforcing phase in matrix, reducing cracks and making a dense geopolymer, which leads to favorable adhesion of the composites and produces a geopolymer composite with better mechanical properties than that of pure geopolymer. However, when the BS content exceeded 1.0 wt. %, interfacial bonding between BS and geopolymer matrix became less. XRD analysis showed that BS has little effect on the mineral composition of metakaolin-based geopolymer and no new phase is formed.
Geopolymers are now representing the most promising environmentally-friendly and sustainable alternative to ordinary Portland cement due to similar or even better bonding properties [
However, like Portland cement, geopolymers belong to quasi-brittle materials. The poor tensile and bending strengths can easily lead to catastrophic failure and represent the main drawback limiting the use of those materials in several applications [
Because of ever-increasing environment concern and the need to develop environment-friendly and energy-saving materials, natural fibers have been used as alternatives for steel or synthetic fibers as reinforcements in geopolymer composites in recent years. These include cotton, flax, jute, sisal, hemp, straw, pineapple leaf, wood and bamboo [
However, after reviewing the previously published findings, no reference has been made to use of BS as reinforcement in geopolymer matrix up to date. Although bamboo fibers have been used to strengthen geopolymer, the effect of BS on geopolymer is even not known, little information is available on physical and mechanical properties, as well as microstructure changes of geopolymer. Therefore, the present study is devoted to determing the workability, setting time, bulk density, apparent porosity, thermal conductivity, compression strength, flexural strength, and microstructure development of geopolymer with BS as reinforcement.
Metakaolin (MK, 1250 mesh fineness) was the solid aluminosilicate source in the preparation of geopolymer matrix, obtained from Inner Mongolia, China. The chemical compositions of MK are shown in
Composition | SiO2 | Al2O3 | Fe2O3 | TiO2 | CaO | K2O | MgO | P2O5 | Na2O | ZrO2 | LOI |
---|---|---|---|---|---|---|---|---|---|---|---|
MK | 50.13 | 42.55 | 3.41 | 2.40 | 0.45 | 0.35 | 0.17 | 0.15 | 0.15 | 0.11 | 0.13 |
The alkaline activator consisted of sodium silicate solution (water glass) and sodium hydroxide (NaOH). The water glass was purchased from Zhongfa Water Glass Factory in Foshan, China, which contained 26.5 wt. % SiO2 and 8.3 wt. % Na2O (modulus Ms. = 3.3). Analytically pure agent NaOH was obtained from Sinopharm Chemical Reagents Shanghai Co. Ltd., China. The modulus was adjusted to 1.7 by dissolving solid NaOH in the water glass. The alkali activator solution was premixed and left to rest for 24 h at ambient temperature prior to casting.
To determine the effect of the BS on properties of MK based geopolymer, 5 specimens of geopolymer composites reinforced with 0, 0.5, 1, 1.5 and 2 wt. % BS were designed and prepared by alkaline activation of MK in sodium silicate solution. The details of geopolymer mixtures are given in
Sample code | MK (g) | Alkaline activator (g) | BS content (wt. %) | |
---|---|---|---|---|
BS0 | 100 | 120 | 0 | |
BS0.5 | 100 | 120 | 0.5 | |
BS1 | 100 | 120 | 1.0 | |
BS1.5 | 100 | 120 | 1.5 | |
BS2 | 100 | 120 | 2.0 |
MK and BS were mixed for 2 min before contacting with alkaline liquid activator, and then the mixture was slowly added into the activator solution and mixed for 5 min. The fresh paste was rapidly poured into the molds and vibrated for 3 min on the vibration table, then cured at room temperature for 24 h. In order to prevent the evaporation of water, polyethylene film was used to cover the specimen during the setting and hardening process. After demolding, all specimens were subjected to further curing in a standard condition (22 ± 2°C, and 90 ± 5% relative humidity) up to acquired days for the research of properties and microstructure.
The flow test for workability and the setting time were carried according to the Chinese standard GB/T 8077-2000 and GB/T 1346-2011, respectively. Both in these two tests, three specimens were tested and an average of measurements was taken.
The bulk density (Db) and apparent porosity (Pa) were determined according to ASTM C20 and calculated using the
where m0 is the weight of the dried sample; m1 is the weight of the sample saturated in air; and m2 is the weight of the sample suspended in water.
The thermal conductivity test was performed on the 200 × 200 × 40 mm3 cubes according to ASTM D518. All specimens were first moist-cured for 28 days, and then air-dried for 14 days followed by oven drying at 100 ± 2°C for 24 h.
The strength tests were undertaken based on the Chinese standard GB/T 17671-1999. Cubic specimens were used in compressive strength tests (40 × 40 × 40 mm3) and flexural strength tests (40 × 40 × 160 mm3). The tests were performed by using an electronic universal testing machine (MWD-50, Jinan, China) with a loading speed of 2.4 kN/s and 50 N/s for the compressive and flexural tests, respectively. Six and three specimens were measured for compressive strength and flexural strength tests, respectively, and the average calculated.
The changes in microstructure and morphological of geopolymer composites as well as the nature of interactions between BS and matrix were studied using a TESCAN field emission scanning electronic microscopy (SEM) at an accelerating voltage of 10 kV for photomicrographs.
The X-ray diffraction (XRD) analysis was determined by an EmpyreanX-ray diffractometer (PANalytical, Netherlands) equipped with monochromatic Cu-Kα radiation at 36 kV and 20 mA. A typical scan is from 10 to 80° with a scan speed of 0.02°/s.
In order to check the physical consistency of fresh geopolymer mix with and without BS, the workability of geopolymer composites blended with different BS content was measured, the flow diameter measured results were shown in
The test results of setting time were shown in
The effect of BS content on bulk density of geopolymer composites is shown in
The addition of BS increases the apparent porosity, but not significantly. The reason is that the metakaolin based geopolymer itself has certain porosity, and the porosity brought by BS acts as the internal pores of the original geopolymer.
As shown in
Compressive strength tests of geopolymer specimens were performed on each mixture at 3, 7, and 28 days of curing. The results were shown in
However, the compressive strengths of BS1.5 and BS2 decease slightly. The reason for the reduction in compressive strength with the addition of higher content of BS may be attributed to these BS agglomeration together and leaving voids in the matrix. Other reasons for the reduction may be that BS is easy to absorb water and it has absorbed too much water, reducing the geopolymer around the BS enough water required for geopolymerization, which in turn decreased the adhesion between BS and the matrix.
The change trend of the compressive strength in this work is in agreement with the research results by other researchers. For example, Alomayri et al. [
The results of flexural strength were presented in
However, further increasing of the BS content induces poor workability and the non-homogeneity within the matrix such that agglomerations are formed which degrade the interfacial bonding between BS and the matrix. These flaws may act as stress concentrators to cause the reductions in flexural strength. Similar trend was also reported by other researchers. For instance, the research reported by Alomayri et al. [
In this composite system, geopolymer played a binder role and BS a reinforcing role. The bonding between BS and matrix is primarily important to obtain a strong geopolymer and serves as the proof of BS reinforced geopolymer. Therefore, the influence mechanism of BS on the properties of geopolymer composites was explored by SEM to identify the fracture surface and internal microstructure.
The morphological changes of reference geopolymer (BS0) and geopolymer specimens with addition of 1% and 2% BS (BS1 and BS2) after 28 days of curing were selected to assess the effects of BS and were presented in
However, higher BS content will result in the aggregation of BS and increase the pores of geopolymer matrix. Compared with BS2 (
On the basis of the results of strength as tested above, it can be concluded that mechanical properties of geopolymer are closely related to the micro cracks development and the interfacial adhesion between BS and geopolymer matrix.
In order to study the effect of BS on the crystalline phase of geopolymer paste, BS0, BS1, and BS2 were selected to be characterized by XRD. The XRD patterns of the geopolymer specimens are shown in
The experimental results from this study are summarized below:
(1) BS has a cellular porous structure according to the SEM. Therefore, BS addition decreases the workability, setting time, and bulk density of geopolymer, and a good linear relation was found between bulk density and BS content.
(2) The apparent porosity of BS reinforced geopolymer increases with the BS content. This results in the decrease in thermal conductivity of material and thus improves the thermal performance.
(3) The compressive strength and flexural strength of geopolymer composites were significantly improved with the use of BS and 1% BS content provided the optimum improvement.
(4) The analysis results of mechanical properties are consistent with those of microstructure. The good mechanical properties of composites are closely related to their dense structure and good interfacial adhesion. It is believed that BS is feasible to be used as reinforcing component in geopolymers.