Geopolymer Composites with Recycled Binders

At the end of 80’s the idea of sustainable development has been published by the World Commission on Environment and Development in the report “Our Common Future” [1]. According to this report humanity has the ability to make development sustainable to ensure that it meets the needs of the present without compromising the ability of future generations to meet their own needs. That leads to wise using natural resources and recycling of materials as much as it is possible. But being sustainable should also include relatively low emission of greenhouse gases. According to European Green Deal strategy signed in 2019 the European Union has set ambitious targets to reduce greenhouse gas emissions by 55% by 2030 and achieve carbon neutrality by 2050 [2]. In order to meet these targets, the construction industry needs to find alternative materials that are more sustainable. Concrete is being used in the largest amounts in whole world, except water, what leads to increasing demands for natural resources and creates large emission of greenhouse gases [3, 4]. But production of the most important constituent of concrete—clinker, leads to very high emission of carbon dioxide. For each ton of clinker about 830 kg of CO₂ is being emitted to the atmosphere. It is about 5–8% of total industrial emission of CO₂ [5, 6]. The cement industry is responsible for a significant amount of CO₂ emissions, which is a major contributor to climate change. The process of production of clinker is still being optimized to reduce the carbon foot print but it can’t reach much lower values, due to the emission of CO₂ related to the decarbonation of lime process which leads according to the stoichiometry of reaction to the emission of 600 tons of CO₂ for each ton of calcinated lime [7]. Development in the area of cement and concrete production aims to produce concretes with successively larger amounts of recycled materials and by-products. Other way of reducing the emission of CO₂ related to the production of clinker, is by decreasing its demand in cement and concrete by producing low clinker cements and concretes or by promoting other types of binders [6]. One of the alternatives of using clinker are geopolymers. Geopolymers are made by combining an aluminosilicate material with an alkaline activator solution, which then forms a solid material. Geopolymers have several advantages over traditional cement, including lower carbon emissions, higher durability, and better resistance to fire and chemicals [8]. Geopolymers might be also used in waste management. They can be used to immobilize hazardous waste, reducing the risk of contamination and pollution. Additionally, they can be used to create new materials from various types of inorganic industrial wastes and by-products. Geopolymers have the potential to revolutionize several industries and contribute to achieving the goals of the European Green Deal. They are sustainable, durable, and versatile materials that can be used in construction industry [9‐11]. One of the main by-product successfully used in production of geopolymers is silicerous fly ash [12]. Nowadays high dement for good quality fly ash especially from cement and concrete industry and also decreasing production of this by-product, cause the lack of this constituent on the market. That is the reason why a new constituents for geopolymers which might replace a part of fly ash, should be applied. Using waste as a binder for geopolymers is a new direction, but one that is necessary, particularly to find ways to reuse it.

This paper presents results of tests of geopolymers in which a part of silicerous fly ash (FA) was replaced by a various types of building wastes such as ceramic waste (CW), recycled cement mortar (RCM), and also fly ash–slag mix (FAS). Aim of this article is to show that this type of waste materials might be successfully used in some conditions to replace a part of fly ash in geopolymer.

The fly ash (FA) that was used met the requirements of the standard EN 450–1:2012. Ceramic fines (CW) were made from grinding ceramic hollow bricks damaged during transport or production. Recycled cement mortar (RCM) was obtained after sieving the fraction < 4 mm resulting from the crushing of concrete rubble. The fly ash–slag mixture (FAS) is post-production waste that was generated as an unavoidable by-product of energy production in conventional coal-fired power plants. All additives, before use, were milled to the dust fraction < 0.063 mm.

The alkaline activator used to prepare the samples was an aqueous solution of sodium silicate and sodium hydroxide of an 8 M concentration. The mass ratio of sodium silicate (Na₂SiO₃) to sodium hydroxide (NaOH) was 2.5. In addition, standard sand 0/2 mm was used. Twelve research series were planned in the experiment, in which 25% of FA was replaced with individual additives in the form of an ash-slag mixture, ceramic dust and recycled mortar. After forming a sample of geopolymer mortar with dimensions of 40 mm x 40 mm x 160 mm, it was heated together with the mould for 24 h at temperatures of 65ºC, 75ºC and 85ºC, respectively, and then, after demolding, it was left in laboratory conditions for 14 days. The experiment plan and the composition of additive-modified geopolymer composite mixtures is shown in Table 1.

The flexural and compressive strength tests were performed according to EN 196–1: 2016. The water absorption test was executed by determining the percentage increase in the weight of the specimens when they saturated with water in relation to the weight of the specimen in the dry state. The volume density in a dry state and in a saturated state were determined based on EN 1015–10:1999. The microstructural analysis of geopolymers were performed using scanning electron microscopy (SEM) model Sigma 500 VP produced by Zeiss. Analysis in microareas were performed using Energy Dispersive X-Ray Analysis detector (EDX) model Ultim Max 40 produced by Oxford. Samples for SEM examinations were prepared by cutting a slice about 3 mm thick, from the inner section of geopolimer sample. The samples were dried in oven in temperature of 40 ℃ for 24 h. Than they were filled with epoxy resin in vacuum chamber. The next day the samples were polished to receive a smooth surface. Samples were prepared in same way as described in previous publications [13].

Table 2 contains physical and mechanical properties of geopolymer composites.

Based on the obtained results, it was found that the highest flexural strength was obtained by the samples of the 6_25%FAS_85 series, annealed at 85 ℃ (6.62 MPa) and it was about 32% higher compared to the 3_100%FA_85 series containing only FA. The lowest flexural strengths were shown by the series containing recycled cement mortar. In the presence of each of the additives, an increase in flexural strength was observed with an increase in the curing temperature from 65 ℃ to 85 ℃, but in the presence of FA, FAS and CW it was insignificant, while in composites with recycled mortar, it was even 80%.

As was observed before, the highest compressive strength results were obtained for the 6_25%FAS_85 series with ash and slag mix, annealed at 85 ℃ (26.85 MPa) and they were 15% higher compared to the series containing 100% FA (3_100%FA_85). The lowest compressive strength in the experiment was obtained for the series containing ceramic fines (CW), but it was in the presence of this additive that the highest and equal to 64% increase in compressive strength was observed with increasing the curing temperature from 65ºC to 85ºC. A similar increase in compressive strength (by 63%) resulting from the increase in temperature was recorded for the series with the RCM recycling mortar, in this case, the strength results were only up to 13% lower compared to those obtained for the composite with 100% FA_65. The highest Strength Activity Indices (SAI) above 1.1 were obtained for the series with FAS. It should be noted that the chemical composition of FAS is the closest to the chemical composition of FA, but it contains more aluminium and calcium oxides, as well as large amounts of silicon oxide, which are the basis for the construction of geopolymer composites forming a C-A-S-H gel [14].

The obtained results show that an increase in the curing temperature from 65 ℃ to 85 ℃ generally causes a decrease in bulk density for each composite with additives, both in the dry and saturated state. Taking into account the geopolymer composites heated at 85 ℃, the highest bulk density in the dry and saturated state was obtained by composites in the 25%FAS series, respectively 2.03 and 2.14 g/cm³, and the lowest in the 100% FA series, 1.84 and 1.98 g/cm³. Analyzing the obtained results of tests on the weight absorption of composites, it can be concluded that the increase in the temperature of maturation of geopolymers was accompanied by an increase in the absorption of composites. Curing temperature has a significant effect on the properties of geopolymers because it affects specimen setting and hardening. Synthesized products are very sensitive to experimental conditions. The percentage of water absorption increased after curing for a certain period of time at higher temperature. Prolonged curing at higher temperatures can break down the granular structure of geopolymer mixture. This results in dehydration and excessive shrinkage due to contraction of the gel, which does not transform into a more semi-crystalline form [15, 16]. The highest water absorption of 7.4% of the mass was obtained for the 100%FA_85 series, and the lowest 3.8% of the mass for the 25%CW_65 series. This may be due to production of more compacted specimens. Fine particles are capable to fill the vacancies and produce more densified specimens.

The aim of microstructural analysis was to examine the differences between the microstructure of composites with the same composition but cured in different temperature. SEM examinations might also help to discover the causes of observed increase of mechanical properties with simultaneously decrease of volume density. Figures 1 and 2 present microstructure of geopolymers containing 25% of CW and cured at 65 ℃ and 85 ℃, respectively.

Microstructure of analysed series 7_25%CW_65 and 9_25%CW_85 was porous at about the same level. The main difference observed between those two samples was in the shape of the C-A-S-H gel which seems to be more dense in the serie 9_25%CW_85 than in 7_25%CW_65. Also the transition zone between ceramic grains and the geopolymer gel was different. In the sample cured in lower temperature the transition zone was porous and discontinuous, but in the sample cured in higher temperature the transition was more sealed and had a better contact with the ceramic grain. The better formation and more sealed transition zone in higher temperatures and different microstructure of C-A-S-H gel might be the cause of better mechanical properties obtained by those geopolymers.

Based on the results of the conducted research, the following conclusions can be observed: