Experimental Investigation on the Development of Geopolymer Paving Blocks by Using Romanian Local Raw Materials

For the world economic system cement and concrete are indispensable elements for the construction industry. Demand for concrete, hence for cement, is constantly growing, especially in highly developed countries, which means that alternative binders are urgently required to meet the needs of millions of people, without compromising the CO2 levels of the atmosphere. Although environmental issues are currently not sufficiently convincing to create a sufficiently high demand for the production of geopolymer technology, this is expected to increase with the imposition of rules on carbon dioxide emissions and their effects. Preliminary alkali-activated geopolymer concrete mixtures were developed based on a rigorous trial-and-error process in order to establish the mechanical properties of the geopolymer paving blocks. The aim of this paper is to present results regarding the technology of development and optimisation for the production of geopolymer paving blocks, their mechanical properties and implementation possibilities in accordance with the intended scope of use. The study results indicate that alkali-activated geopolymer paving blocks have excellent mechanical properties, by reference to OPC paving blocks, making them suitable for practical applications.


Introduction
Worldwide, concrete is the second most used material and it is considered the most versatile, durable and reliable building material in civil engineering industry. Portland cement production is one of the most important carbon dioxide generators and global demand in the industry for its production as binder for concrete is rising constantly, generating environmental associated problems which are well known and start on being carefully monitored. Cement production requires large quantities of liquid fuel (between 60 and 130 kg / tonne) and the electricity amount required to produce on tonne is approximately 110 KWh [1]. In terms of environmental pollution, measurements have shown that for each tonne of cement that is produced, the industry releases into the atmosphere around 1.1 tonnes of CO2, generated by the combustion and calcareous calcination methods [2]. World cement production indicates an obvious yearly increase, with more than 4.2 billion tonnes being produced and delivered to the industry only in 2014 [3].
A particular procedure in which the solidification the fly ash powder could be achieved is represented by its alkaline activation. When it is mixed with a certain type of alkaline activator, it creates a new binding material, with mechanical properties similar to those of Ordinary Portland Cement (OPC) concrete. Based on their properties, this type of new materials can be considered as an alternative to OPC traditional concrete. Alkali-activated geopolymers are on-going developing materials, their research being strongly connected to the worldwide need of global CO2 emissions reduction. Characterised by excellent mechanical properties and resilience in aggressive environments, these materials represent an opportunity for both the environment and the engineering fields of novelty as a reliable alternative to the traditional technology [4][5][6].
In order to produce alkali-activated geopolymer concrete, fly ash can be used as main raw, binding material. Therefore, careful consideration is needed when choosing this type of material to obtain the geopolymer binder. Depending on the type of coal that is burnt, the chemical properties of the fly ash can be very different and this has to be carefully considered when fly ash is used as raw material so that the geopolymerisation process can be fully generated [7]. The two main constituents of the geopolymer binders are the alumino-silicate elements, which represent the solid part, and the alkaline activator, which represents the liquid part [8].
Paving blocks have been used in the construction industry for many years, being considered appropriate for outdoor applications. These units turned out to be cheap to produce and present exceptionally high mechanical properties, making them useful for their intended purpose: commercial, industrial or residential areas, plazas, parking areas, bus stops, etc. In addition to being economical, interlocking concrete paving blocks are also broadly obtainable in water-permeable designs, which have additional ecological benefits [9].
Although the durability of alkali-activated geopolymer materials is not exactly extensively discussed and analysed, there are numerous studies regarding the performance of these materials in terms of sustainability [10][11][12][13]. Experimental results showed excellent resistance to aging, freeze-thaw, but also to carbonation [14].
The two main barriers to the introduction of new materials in the construction industry [15] are: 1) the need for standards for the testing methods, the evaluation and admissibility conditions, given that their development and introduction takes a long time and it is a gradual process; 2) the durability performance of such materials, taking into account the fact that they must meet certain requirements, they must withstand for a long, relevant period, the in situ (put in work), specific durability challenges, and for new materials this is not quantified yet.
The aim of this paper is to present results regarding the development technology and optimisation approach for the production of geopolymer paving blocks, by using Romanian local raw materials, their mechanical properties and implementation possibilities, in accordance with the intended scope of use. Based on the results obtained for alkali-activated geopolymer concrete [16], research directions on the material applicability were identified.

Materials and methods
This chapter summarizes the applied experimental methodology, involving the raw materials, mix design, moulding, curing and also the testing methods for the mechanical properties' specific evaluation of the developed alkali-activated geopolymer paving blocks.
By using the full information on the production of the alkali-activated geopolymer concrete, as well as on the basis of the obtained results [6,16], a "trial and error" experimental research was built, with the aim of producing building geopolymer materials (geopolymer paving blocks). The study of technology transfer to the industry, by enhancing the legal framework, which sets out the certification of products for the construction market, was also considered Geopolymer fly ash-based paving blocks were manufactured by the use of class F fly ash, aggregates and alkaline solution (sodium silicate + sodium hydroxide).

Fly ash
Low calcium-fly ash from a power plant in Romania was used as main raw material for the production of the alkali-activated geopolymer paving blocks. Its chemical composition was established through X-ray fluorescence methods (Table 1). As seen in Table 1, the CaO content of the fly ash was less than 10%  3 and SiO2 + Al2O3 + Fe2O3 > 70%, SO3 < 5% and L.O.I. < 6%. The chemical composition of the fly ash used in this study can be classified as Class F fly ash, according to ASTM requirements [17,18].

Alkaline activator
A combination between sodium silicate solution (Na2SiO3 solution) and sodium hydroxide solution (NaOH solution) was chosen as alkaline activator for the production of the alkali-activated geopolymer paving blocks. The Na2SiO3 solution was commercialy purchased and had the folowing chemical composition: SiO2=30%, Na2O=14% and H2O=56%. Sodium hydroxide pearls were used to produce the NaOH solution, which was set to 10M, therefore 400g were dissolved in water, for one litre of solution. Based on the molecural weight of NaOH, in order to achieve the desired molar concentration of the solution 40g x 10 = 400g of NaOH flakes were dissolved in one liter of water.

Aggregates
Natural aggregates, granular class 0/4 mm (S) and 4/8 mm (CA) (sand and coarse aggregates) were used in this study to produce alkali-activated fly ash-based geopolymer concrete paving blocks.

Mix design, moulding and curing treatment
In order to study the mechanical properties of the alkali-activated geopolymer paving blocks, the following preliminary mixtures have been produced, considering the following parameters as constants: AA/FA ratio, Na 2 SiO 3 /NaOH solution ratio and S/CA ratio ( Table 2). The technology of producing alkali-activated geopolymer paving blocks (Figure 1) was based on the principles already set out for the production of alkali-activated geopolymer concrete [6,15]. Mixing of the materials was performed at constant temperature of (20±2)C and the following the technological steps were applied: (1) Mixing of the sand and the coarse aggregates (0/4 mm and 4/8 mm) at low speed for 30 seconds; (2) The addition of the necessary amount of fly ash and then the dry raw materials (fly ash + aggregates) were mixing together, for 30 seconds; (3) The alkaline activator addition, for90 seconds to the homogenous dry mixture; (4) After adding the total alkaline activator quantity, the mixing was continued at low speed for another 3 minutes;  (5) Casting into polypropylene moulds, with the corresponding 5 minutes vibration and starting of the heat treatment (70C / 24 hours); (6) After demoulding, the geopolymer paving blocks were stored in the climatic chamber at the temperature T (20±1)°C and relative humidity RH (60±5)%, and the tests for their mechanical properties were determined at the age of 7 days.  1. Alkali-activated geopolymer paving blocks production.

Testing methods
Based on the results regarding the possibility of developing such construction elements [6,15], the research initially focused on preliminary evaluation of the physical, mechanical and durability characteristics of the obtained products, for the study pf their potential, different areas of applicability: (1) Weathering resistancetotal water absorption; (2) Weathering resistancefreeze-thaw resistance with de-icing salt;

Results and discussions
As mentioned in the literature, alkali-activated fly ash-based geopolymer materials properties are notably influenced by many important parameters. The physical and chemical properties of the main raw material (fly ash) used as precursor, the chemical properties of the solution used as alkaline activator and the mix-design ratios, are the most important parameters that affect the mechanical properties of the material [1,4,6,10]. The 7 days compressive strength of the alkali-activated geopolymer paste (32,6 N/mm 2 ) [16], produced using the same materials and the same mix-design ratios, was consider relevant as testing age for the evaluation of the above nominated parameters of the alkali-activated geopolymer paving blocks.

Weathering resistance
Weathering resistance for paving blocks is represented by the total water absorption test (according to Annex E from EN 1338:2004 / AC:2006) or the freeze-thaw resistance (according to Annex D from EN 1338:2004 / AC:2006). The recommendations related to the weathering resistance considered necessary to ensure the durability of the element, are determined by well-established criteria.
Total water absorption. Total water absorption of the alkali-activated geopolymer paving blocks has been determined by submerging the test samples in water, at constant temperature of (20±5)°C, until they reached constant mass. The minimum immersion period of the samples was 3 days. After reaching the saturated state constant mass, they were weighed (M1). Subsequently, the paving blocks were placed in the oven and dried at temperature of (105±5)°C, to reach the constant mass (M2). The Wa (water absorption) of each paving block was expressed according to the standard, as a percentage by mass, acc. to the equation (1) [19].
(1) 3.1.2. Freeze-thaw resistance with de-icing salt. The geopolymer paving blocks test specimens were preconditioned and then subjected to 28 freeze-thaw cycles, while their surface was covered with a 3% NaCl solution. After carrying out the 28 cycles, the exfoliated material was collected from the surface of the specimens (M) and the mass lost per unit area was calculated, acc to the eqation (2) [18]. (2)

Böhme abrasion test
For the determination of this parameter 70 x 70 mm cubes were used, (Figure 2), cut from the geopolymer paving blocks and placed on the Böhme abrasive disc (Figure 3). Before the test and after every four cycles, the test specimens were weighted. Test specimens were tested for 16 cycles, each consisting of 22 full rotations of the abrasion track. After the end of the 16 cycles (Figure 4), abrasion was calculated [18].

Unpolished slip resistance (USVR)
Unpolished slip resistance of the geopolymer paving bocks has been determined by using a test equipment consisting of a friction pendulum ( Figure 5), (Figure 6). At the pendulum swing (Figure 6), the force between the cursor and the test surface was measured by using a graduated scale. Prior the start of the test the geopolymer paving blocks were immersed in water at (20±2)°C for 30 minutes, according to the standard methodology requirements.

Tensile splitting strength
In order to determine the tensile splitting strength, the alkali-activated geopolymer paving bocks were placed in the testing equipment, in a special device for the tensile splitting strength test (Figure 7), according to the specifications of the standard. The splitting load was applied progressively, until the paving blocks were broken (Figure 8).
According to EN 1338:2004 / AC:2006, the tensile splitting strength T of the paving blocks should not be lower than 3,6 MPa, and the breaking load per length unit should not be lower than 250 N/mm [18]. Results obtained on geopolymer paving blocks show that the breaking load per length unit was 410 N/mm, and the tensile splitting strength was 3,6 MPa, which qualifies them as fulfilling the admissibility conditions specified by the assimilated standard.

Mechanical properties of the alkali-activated geopolymer paving blocks
Alkali-activated geopolymer paving blocks have been subjected to tests for mechanical and durability performance evaluation by assimilating the methodology and admissibility conditions as well specified by standard EN 1338:2004 / AC: 2006. The obtained results on the nominated parameters confirmed their ability of the products to comply with this standard (Table 3), demonstrating the possibility of their use as paving elements in the construction materials industry Moreover, by optimizing the mixtures, improved performance can be achieved, and consequently the extension of the applicability interval within the specification of the EN 1338:2004 / AC: 2006.