In this paper, class-C fly ash (FA) and ground granulated blast-furnace slag (GGBS)-based geopolymer activated in NaOH and NaOH + Na2SiO3 was studied regarding setting time, compressive strength, porosity, microstructure, and formation of crystalline phases. When comparing the effects of alkali type on the FA and GGBS geopolymer composites, results revealed that NaOH has a lesser effect in developing strength and denser microstructure than does NaOH + Na2SiO3, since the addition of Na2SiO3 provides the silica source to develop more compact structure. Incorporation of Na2SiO3 reduced the crystallinity and the paste was more amorphous compared to NaOH activated pastes. The class-C FA and GGBS blends resulted in prolonged setting time, reduced strength, and loose matrix with the increase in fly ash content. The un-reactivity of calcium in blends was observed with increasing fly ash content, leading to strength loss. It is evident from XRD patterns that calcium in fly ash did not contribute in forming C-S-H bond, but formation of crystalline calcite was observed. Furthermore, XRD analyses revealed that the reduction in fly ash leads to the reduction in crystallinity, and SEM micrographs showed the unreactive fly ash particles, which hinder the formation of a denser matrix.

https://www.mdpi.com/1996-1944/13/1/59/pdf

Strength and Microstructure of Class-C Fly Ash and GGBS Blend Geopolymer Activated in NaOH & NaOH + Na2SiO3.

The potential of applying geopolymerization to a wide range of solid industrial waste and by-products is of great interest. In this research, the physical and mechanical properties of fly ash (FA)-based geopolymer concrete (GC), compared to those of cement concrete (CC), were studied. Three GCs with different content of FA and three appropriate CCs were designed, prepared, tested and evaluated. The results were compared with the requirements of Standards EN 206-1 and EN 1992-1-1. It was shown that in some cases minor adjustments of the regulations are needed, while in other cases complete revision is required. GC indicated competitive compressive strength compared to CC, tensile strength within the limits specified by Eurocode 2 for CC and modulus of elasticity about 50% less than that of CC. The ratio of binder (FA) to aggregates seems to have a significant effect on the properties of GC. The concrete with 750 kg/m3 FA seems to be the best choice taking into consideration both engineering and environmental criteria.

https://www.mdpi.com/2075-5309/11/5/178/pdf

Physical and Mechanical Properties of Fly Ash Based Geopolymer Concrete Compared to Conventional Concrete

One way to promote sustainability, energy conservation and environmental protection is to produce construction materials by the activation of aluminosilicate precursors with alkalis. Alkali activated materials produced with raw materials containing low calcium such as pumice harden in aggressive conditions. Therefore, determining the optimum curing conditions are of great importance for the evaluation of these materials. The curing conditions of pumice-based alkali activated materials has been reported by a few studies to a limited extend. This paper succinctly discusses the different curing conditions of pumice-based alkali activated composites incorporating ordinary Portland cement (OPC). The precursor raw material consists of 80% pumice and 20% OPC or %100 pumice. The prepared 50 × 50 × 50 mm cubic paste specimens were subjected to different heat-curing temperatures (20 °C, 40 °C, 60 °C, 80 °C, 100 °C and 120 °C) for 24, 48, 72 and 96 h and aging effect (3-28-90 days) was studied. Compressive strength , UPV , dry density, porosity and total water absorption tests were performed after curing processes. Furthermore, SEM/EDX images of the samples were taken to investigate the effect of curing conditions. The results show that OPC addition has increased the compressive strength under all curing conditions. The highest compressive strength was obtained by curing OPC added samples at 100 °C for 72 h. However, as the heat-curing temperature increased samples gained much of strength with less curing duration. Satisfactory compressive strength results can be obtained from samples cured under ambient conditions and especially containing OPC (63 MPa). The specimens containing OPC with the highest compressive strength are also the specimens with the highest density (1.89 g/cm 3) due to dense geopolymeric structure. At the same time, as the compressive strength increases the water absorption and porosity values decrease due to the reduced voids. Consequently, experimental findings revealed that the curing conditions influence the mechanical and physical properties of pumice-based alkali activated composites incorporating OPC.

Influence of curing conditions on pumice-based alkali activated composites incorporating Portland cement

Compressive strength optimization of metakaolin‐based geopolymer by central composite design

Approximately 2.78 Mt of coal fly ash is produced in the Philippines, with a low utilization rate. Using fly ash-based geopolymer for construction will lessen the load sent to landfills and will result in lower GHG emissions compared to OPC. It is necessary to characterize the fly ash and optimize the geopolymer components to determine if it can replace OPC for in situ applications. The activator-to-precursor ratio, the water-to-solids ratio, and the sodium hydroxide-to-sodium silicate ratio were optimized using a randomized I-optimal design from the experimental results of 21 runs with five replicates, for a total of 105 specimens of 50 mm × 50 mm × 50 mm paste cubes. The engineering properties chosen as the optimization responses were the unconfined compressive strength (UCS), the initial setting time, and the final setting time. The samples were also ambient-cured with the outdoor temperature ranging from 30 °C to 35 °C and relative humidity of 50% ± 10% to simulate the on-site environment. Runs with high unconfined compressive strength (UCS) and short setting times were observed to have a low water-to-solids (W/S) ratio. All runs with a UCS greater than 20 MPa had a W/S ratio of 0.2, and the runs with the lowest UCS had a W/S of 0.4. The initial setting time for design mixes with a W/S ratio of 0.2 ranged from 8 to 105 min. Meanwhile, five out of seven design mixes with a W/S ratio of 0.4 took longer than 1440 min to set. Specimens with an alkali activator ratio (NaOH/WG) of 0.5 (1:2) and 0.4 (1:2.5) also had significantly lower setting times than those with an alkali activator ratio of 1. The RSM model was verified through confirmatory tests. The results of the confirmatory tests are agreeable, with deviations from the expected UCS ranging from 0 to 38.12%. The generated model is a reliable reference to estimate the UCS and setting time of low-calcium FA geopolymer paste for in situ applications.

Investigation on the Compressive Strength and Time of Setting of Low-Calcium Fly Ash Geopolymer Paste Using Response Surface Methodology.

In general, fly ash is often used as a binder in geopolymer mixture. In this paper, seven samples of fly ash were collected from five Indonesian power plants. The variation of fly ash may depend upon many factors. However, there is no standardization of fly ash used for geopolymer binder. This experiment was conducted to study and categorize fly ashes both by assessing their physical and chemical characters. All fly ashes were activated with alkali activator to prepare geopolymer paste. Ratio of fly ash to the alkali activator was kept constant at 65 : 35. Compression test was conducted at three, seven, 14, 21, 28, and 56 days to identify the strength development of each sample. The result revealed that the content of CaO in fly ash affected the setting time and early strength development. In three days paste with CaO content more than 20% gain 54% of its 28-day strength. It is caused by both hydration and polymerization reactions. A range of Si to Al ratio contributed to the strength.

/pdf/factors-influencing-strength-and-setting-time-of-fly-ash-1tymbblh1q.pdf

Adhitya Leonard Wijaya

Papers

Factors influencing strength and setting time of fly ash based-geopolymer paste