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The potential application of alkali-activated material (AAM) as an alternative binder in concrete to reduce the environmental impact of cement production has now been established. However, as the production and availability of the primarily utilized waste materials, such as fly Ash and blast furnace slag, decrease, it is necessary to identify alternative materials. One such material is clay, which contains aluminosilicates and is abundantly available across the world. However, the reactivity of untreated low-grade clay can be low. Calcination can be used to activate clay, but this can consume significant energy. To address this issue, this paper reports the investigation of two calcination methodologies, utilizing low-temperature and high-temperature regimes of different durations, namely 24 h heating at 120 °C and 5 h at 750 °C and, and the results are compared with those of the mechanical performance of the AAM produced with untreated low-grade clay. The investigation used two alkali dosages, 10% and 15%, with an alkali modulus varying from 1.0 to 1.75. An increase in strength was observed with calcination of the clay at both 120 and 750 °C compared to untreated clay. Specimens with a dosage of 10% showed enhanced performance compared to those with 15%, with Alkali Modulus (AM) of 1.0 giving the optimal strength at 28 days for both dosages. The strengths achieved were in the range 10 to 20 MPa, suitable for use as concrete masonry brick. The conversion of Al (IV) is identified as the primary factor for the observed increase in strength. [ABSTRACT FROM AUTHOR]

An investigation has been conducted to identify the reaction mechanism and compressive strength of geopolymer made from brown coal fly ash from two locations at Yallourn power station in Australia. The Yallourn-1 (Y1) ash has completed the storage period within the pond, whereas Yallourn-2 (Y2) ash has been stored in the pond for a short period of time. Compressive strength of Y1 geopolymer increased from 12.63 MPa (1.83 ksi) at 7 days to 13.62 MPa (1.98 ksi) at 28 days and 15.90 MPa (2.31 ksi) by 90 days, whereas Y2 achieved 13.26 MPa (1.92 ksi) at 7 days but reduced in strength to 8.55 MPa (1.24 ksi) by 28 days, followed by an increase in strength to 14.00 MPa (2.03 ksi) by 90 days. The low strengths observed is attributed to the low of alumina content of the Yallourn ash, coupled with the higher unburnt carbon content. [ABSTRACT FROM AUTHOR]

A comprehensive experimental study has been conducted to investigate the geopolymerisation and compressive strength development of mortar made from brown coal fly ash from two separate locations in the storage ponds of an Australian power plant. The specimens gave similar compressive strengths but had significantly different material and performance characteristics despite being from the same storage location. The Loy Yang‒A (LYA) geopolymer mortar demonstrated an approx. 30% strength increase while Loy Yang‒B (LYB) gave an approx. 18% strength drop over the period from 7 and 90 d, though both geopolymer mortars initially achieved a similar 28-d strength of approx. 23 MPa. The LYA ash had almost double the alumina content compared to LYB and a higher proportion of AlVI compared to the LYB. The lower alumina content coupled with the low quantity of AlVI in the ash and its lower conversion to AlVI during geopolymerisation is identified as the primary reason for the reduction in strength observed in the LYB geopolymer. The increase of Q4(3Al) during geopolymerisation and some conversion to Q4(4Al) coordination over time resulted in the increase in the compressive strength observed in the LYA mortar. This strength increase of LYA mortar is further correlated with an increase in Quartz phases coupled with a reduction in the Moganite phase. Formation of sodium carbonate due to atmospheric carbonation of unreacted sodium hydroxide in Loy Yang geopolymer additionally contributed to the strength development of LYA geopolymer. [ABSTRACT FROM AUTHOR]

This study reports the effect of heat curing at 120 °C on the geopolymeric reaction and strength evolution in brown coal fly ash based geopolymer mortar and concrete. Moreover, an examination of this temperature profile of large size geopolymer concrete specimens is also reported. The specimen temperature and size were observed to influence the conversion from the glassy (amorphous) phases to the crystalline phases and the microstructure development of the geopolymer. The temperature profile could be divided into three principal stages which correlated well with the proposed reaction mechanism for class F fly ash geopolymers. The geopolymerisation progressed more rapidly for the mortar specimens than the concrete specimens with 12 to 14 h providing an optimum curing time for the 50 mm mortar cubes and 24 h being the optimum time for the 100 mm concrete cubes. The 50 mm and 100 mm concrete specimens' compressive strengths in excess of 30 MPa could be obtained at 7 days. The structural integrity was not achieved at the center of 200 mm and 300 mm concrete specimens following 24 h curing at 120 °C. Hence, the optimal curing time required to achieve the best compressive strength for brown coal geopolymer was identified as being dependent on the specimen size. [ABSTRACT FROM AUTHOR]

This study evaluates the influence of the elemental distribution in the fly ash particles and their impact on phase formation and compressive strength of five low-calcium fly ash geopolymers. The degree of geopolymerization in each geopolymer system was assessed by FT-IR and solid state 27Al MAS-NMR analysis. The corresponding pore volume changes were investigated by mercury intrusion porosimetry (MIP). The uniformity of the distribution of SiO2 and Al2O3 in the fly ash was observed to directly influence the dissolution of the amorphous surface layer in the initial geopolymerization process and control aluminosilicate gel precipitation and gel-phase creation. The results showed that the higher the uniformity of distribution (coupled with the stable conversion of aluminium from octahedral to tetrahedral coordination), the higher the aluminium amalgamation with silicates. The result of this is the production of a three-dimensional (3-D) polysialatesiloxo (Si-O-Al-O-Si) polymeric gel structure with high rigidity and stability, which in turn results in higher compressive strength. It was also observed that an increase of meso-porosity in geopolymer phase formation coupled with a cumulative pore volume below 1000 nm (3.937 x 10-5 in.) is a good indicator of the degree of geopolymerization. [ABSTRACT FROM AUTHOR]

Geopolymer is a sustainable construction material produced by the activation of fly ash using a high concentration alkali to initiate a polymerisation reaction. A key parameter in determining the potential adoption of geopolymer concrete in the construction industry is the long term durability of the material. To determine the durability characteristics a detailed investigation of the permeation properties of four different fly ash geopolymer concretes was carried out up to one year of age. An improvement in the durability properties is observed for all geopolymer concretes with time. This is attributed to an on-going geopolymerization which results in continuing gel formation leading to a more densely packed microstructure, with an associated reduction in meso-pores and macro-pores. The packing density coupled, with the pore size distribution, were observed to determine the permeation and diffusion characteristics of the concrete. The increased in meso-pores represents the increase in the gel of the matrix and in turn this affect the increase of water absorption. On the other hand, a high quantity of macro-pores leads to an increase in the water and air permeability of geopolymer concrete. A large quantity of coarse particles in fly ash results in an uneven gel distribution which reduces pore-filling ability, while the presence of a high quantity of CaO was observed to contribute to a densely packed microstructure. Notably the initial chloride diffusion coefficients are analogous to those observed in Portland and blended cement concretes and also decrease with the age in a similar manner. [ABSTRACT FROM AUTHOR]

A major challenge in the specification of geopolymer mix designs is the variability in the fly ash used and the impact of that variability on the performance of the geopolymer produced. The factors affecting the performance of geopolymers made from a total of five chemically and physically distinct fly ashes are reported. The key factor identified as influencing the strength was the workability, with a flow in the range between 110 ± 5% and 140 ± 5% required for optimal performance. In this flow range, the strength of geopolymer is governed by the specific surface area of precursor fly ash coupled with the quantity of the 10 μm and 20 μm particles. In addition a negative zeta potential of the fly ash was identified as assisting gel formation with the smaller the negative zeta potential of the geopolymer product the more gel formation and high compressive strength observed. [ABSTRACT FROM AUTHOR]