The influence of rice husk ash addition on the properties of metakaolin-based geopolymers

This paper investigates the replacement of metakaolin (MK) with rice husk ash (RHA) in the production of alkali-activated binders or geopolymers. The influence of the RHA addition on compressive and flexural strength, as well as water absorption and apparent porosity were determined, in terms of the percentage of RHA in the mixture and molar ratios of the mixes. Fourier Transform Infrared (FTIR) spectroscopy and Energy Dispersive spectroscopy (EDS) were carried out to assess the changes in the microstructure of the geopolymer matrices with the RHA addition. Results have shown that RHA may be a supplementary precursor for geopolymers. The composition of the geopolymer matrices containing 0-40% RHA is very similar, which indicates that the additional Si provided by RHA is not incorporated to the geopolymer matrix. In addition, geopolymers with RHA content higher than 40% present a plastic behavior, characterized by extremely low strength and high deformation, which can be attributed to the formation of silica gel in formulations containing variable Si/Al ratio

Properties and performance of SI-rich geopolymer binder systems

This paper examines specific roles of various constituent oxides on the hydrolysis and condensation reactions that underpin the properties and performance of high silica geopolymer binder systems. Geopolymer systems formulated to high Si/Al ratios provide an ideal system for this form of analysis given their potential for mainstream engineering applications. For this study, specific emphasis was placed on the roles of silica and alkali species present in the feedstock material and their impact on mechanical properties such as early strength development. It is observed, that high silica geopolymers with SiO2/Al2O3 > 15 can be synthesized as compared to conventional geopolymers which generally have SiO 2/Al2O3 = 2-4. Prior to curing, such high-Si mixtures display a more viscous consistency than conventional geopolymers and have a lower pH after setting. The relative high initial strength gains of the systems are complemented with good bonding characteristics. The overall performance trends of these silica-rich systems are explored and discussed in this Paper

Durability and performance characteristics of recycled aggregate concrete

In recent years, the recycling of concrete to produce aggregates suitable for nonstructural concrete applications is emerging as a commercially viable and technically feasible operation. In this paper, we report on performance and durability tests carried out to determine the fresh and hardened properties of concrete made with commercially produced coarse recycled concrete aggregate and natural fine sand. Test results indicate that the difference between the characteristics of fresh and hardened recycled aggregate concrete and natural aggregate concrete is relatively narrower than reported for laboratory-crushed recycled aggregate concrete mixes. For concrete without blast furnace slag, having similar volumetric mix proportions and workability, there was no difference at the 5% significance level in concrete compressive and tensile strengths of recycled concrete and control normal concrete made from natural basalt aggregate and fine sand. Water absorption rates and carbonation of recycled concrete and reference concrete were comparable. However, the abrasion loss of recycled aggregate concrete made with ordinary Portland cement increased by about 12% compared to normal concrete, while the corresponding drying shrinkage was about 25% higher at one year. Long-term performance results generated from field case studies involving the construction of a 120 m long bicycle/footpath with a 40 m recycled aggregate concrete (RC) segment are discussed. The paper further discusses observed fresh and hardened concrete properties of both recycled and conventional aggregates, and examines specific implications for satisfactory performance of the recycled product for recreational and non-structural construction applications

Development of guide specifications for recycled aggregates in concrete construction

Recycling of construction materials is growing along with the demand for recycled materials. However, this growth is often constrained by specifiers` insufficient knowledge of material performance, low awareness of benefits, and perceived risks. Until recently, in Australia, the use of crushed concrete derived from building demolition has been restricted to granular subbase layers in road pavement construction and drainage, or excavation fill applications. However, improved crusher technologies, rubble screening and aggregate washing, and tighter regulation of the recycling industry, have contributed to significant improvements in the quality of recycled concrete products. Consumer acceptability of recycled materials, however, largely depends on products being technically suitable, cost competitive and meeting environmental impact requirements. This paper discusses the proposed Guide specification document which will cover existing technical information and potential uses of recycled concrete and masonry waste into a structured format, required for specification guidelines. This includes information on recycled material properties and performance, and examines technical and market considerations of construction and demolition (C a. D) waste recycling, based on field trials of premix recycled concrete. Different sources of aggregate batches are assessed in relation to existing Australian Standards for natural aggregates, in particular AS 1141, to establish conformance of recycled products. The proposed Guide specification document will provide engineers with product specification information and the tools required for conventional design with graded recycled C a. D waste material

Relationships between composition, structure and strength of inorganic polymers, part 2: fly ash-derived inorganic polymers

This article is the second in a two-part series and discusses inorganic polymers derived from fly ash. Part 1 [1] concerns inorganic polymers derived from a metakaolin precursor. For this study, 15 fly ash-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four component oxides (Na2O, SiO2, Al2O3 and H2O) and two alkali cations (Na and K) on the microstructure and compressive strengths were assessed. Similar to metakaolin-derived inorganic polymers, it was observed that high-strength fly ash inorganic polymers were related to low porosity and a dense, fine-grained microstructure. Such structures were characteristic of formulations with high silica mole fractions (SiO2/Al2O3 ~3.9) and low water contents, as well as those with high alkali and low alumina contents. For the latter, not only was a characteristic slower strength development with increasing alkali content observed, but there was also a limit of alkali concentration (Na2O/Al2O3 ~1) beyond which the strength deteriorated. Furthermore, SEM micrographs disclose that the fly ash precursor dissolves more readily in the sodium-based system compared to the potassium equivalent. The interrelation between microstructures of the respective formulations and their strength development are discussed. It is observed that the charge-balancing role of the alkali cations in the fly ash formulations may be dominant compared to initial alkali dissolution reaction of the aluminosilicate fly ash particles, which is partly responsible for initial strength development

Relationships between composition, structure and strength of inorganic polymers, part I: metakaolin-derived inorganic polymers

"Inorganic polymers", or geopolymers, are novel synthetic binders produced by reactions between alkali silicate solutions and solid aluminosilicates. In Part 1 of this study, 12 metakaolin-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four most important oxide components of inorganic polymers (Na2O, SiO2, Al2O3 and H2O) was assessed by electron microscopy and by strength testing. Additionally, the effect of the type of alkali cation was determined. In general, the results followed expected trends and there were clear correlations between composition, microstructure and strength. It was found that high strength was related to low porosity and a dense, fine grained microstructure. Such a structure was found in inorganic polymers with high alkali contents (Na2O/Al2O3 = 1.2) and low water contents (H2O/Al2O3 = 12). High silica and low alumina contents (SiO2/Al2O3 = 3.5-3.8) also produced this structure, however, there was a limit beyond which the strength deteriorated. In relation to the effect of alkali cations, sodium was found to give higher resin strength than potassium. The results of the study further confirm that the selection of precursor raw materials remains a critical factor to initial strength development. The relationship between different resin formulations and resulting microstructures are discussed

Bayer process waste stream as potential feedstock material for geopolymer binder system

Abstract not available

Dissolution processes, hydrolysis and condensation reactions during geopolymer synthesis: part II. high Si/Al ratio systems

The mechanisms of speciation of aluminate and silicate phases during dissolution and condensation stages of alumino-silicate geopolymer reactions characterised by Si/Al ‡ 3, have been investigated and the results compared to predictions of the partial charge model. Solid-state nuclear magnetic resonance (NMR) traces indicate that free [Al(OH)4]– species, present in lower silicate formulations such as Si/Al £ 1, do not occur in the present systems, suggesting that the condensation reaction between [Al(OH)4]– and silicate species is fairly quick and is consumed as soon as it is formed. This observation is also consistent with both calorimetric measurements and model predictions, as the condensation time increased exponentially with increased Si/Al ratio in the geopolymeric phase, indicating again that the high content of Al species in the gel phase greatly enhanced the condensation rate. The experimental observations suggest that the condensation process in these systems occurs in two stages: (a) quick condensation between aluminate and silicate species; followed by (b) a slow condensation stage solely involving silicate species

Small footprint aluminosilicate matrix: refractory hybrid materials

This study investigates the effects of alumina, titania, boron nitride and silicon carbide additions on low energy (typical cure < 90oC) alkali reactive aluminosilicate matrix material properties as potential small environmental footprint refractory materials. The structure - property relationships of the aluminosilicate matrix - refractory hybrid materials were characterized for thermal performance. Electron microscopy complemented with X-ray diffraction and FTIR revealed the different reaction mechanisms occurring within the hybrid aluminosilicate matrix - refractory systems. Alumina and silicon carbide additions were found to react with the aluminosilicate matrix to a greater extent than boron nitride and titania. Thermogravimetric and differential thermal analysis indicate that thermal behaviour is predominantly dictated by water loss from the aluminosilicate matrix, with refractory additions playing a minor role. The reactivity of the refractory addition towards the aluminosilicate matrix influenced sample microstructure and thermal performance