1. Introduction
Geopolymers are considered low carbon-footprint binders because in the production phase the amount of CO
2 generated may amount to only 10% of the carbon dioxide emitted during Portland cement manufacturing [
1,
2]. In addition, the technological processes related to the production of geopolymers allow the use of solid waste from the industrial and energy sectors [
3]. As construction materials, geopolymers show high compressive strength, great fire resistance, thermal resistance and durability in many environments [
4,
5,
6]. Geopolymer binders are very often presented as alternatives to Portland cement-based materials. The available results mainly concern their chemical properties, mechanical properties, microstructure and the possibility of the potential application of the material. There is, however, little research regarding their rheological behavior. Rheological parameters are used to evaluate consistency, loss of workability, stability and pumpability [
7,
8]. Due to the very wide range of raw materials used in the geopolymer technology, both precursors (fly ash—FA, ground granulated blast furnace slag—GGBFS and metakaolin) and activators (sodium or potassium alkaline solution) indicate the need of complex analysis the rheology issues in geopolymer mixes. Knowledge about viscosity changes, yield strength and thixotropy as a function of time in combination with the values of the strength parameters that are obtained is extremely important especially in the context of intensively developing 3D printing technology using geopolymer material.
Fresh geopolymer paste is a solid–liquid suspension [
9]. The geopolymer mix at an early stage of the reaction is a three-part composite, assuming that the air content is neglected. The first component is the aluminosilicate precursor in the form of a solid phase (e.g., GGBFS, fly ash and metakaolin) [
10]. The second component is an alkaline solution containing alkalis and pre-existing silicate oligomers. The crystalline zeolite products are formed when pure hydroxide solution is used [
11] hence the use of sodium silicate solution is necessary to form amorphous geopolymeric structures. The third component is the product of the reaction between the aluminosilicate precursor and the alkaline activator. This component is the most complex and, in addition, heterogeneous even in the initial stage when the mix is well composed [
12,
13]. This is the most important phase affecting the mechanical properties of the geopolymer mix [
14]. Due to the complex composition of the geopolymer paste, it is difficult to unequivocally identify the mechanisms affecting the rheological properties of a geopolymer mix.
Favier et al. [
15] carried out a study during which each component was observed separately, and then, using scaling methods, conclusions regarding the physical and chemical properties of the geopolymeric paste were drawn. During the first stage, the geopolymer rheological properties at fresh state, when all the three components mixed together, were analyzed. The second step was to analyze the behavior of the liquid phase and solid phase separately and not taking into account the reaction product. In the last step the behavior of the reaction product was investigated. In the early stages of geopolymerization, dissolution and polymerization rates related to the H
2O/Na
2O: SiO
2/Na
2O ratios were of a major influence on the properties of the material. The mechanical properties at early age of geopolymer mixes seem not to be affected by the colloidal interactions between metakaolin grains. Al-rich gel is formed (Si/Al < 4.5) at the grain surface, within 15 min from the start of mixing. The authors of the study [
15] warn against generalizations regarding the results obtained pointing at the fact that the mechanisms responsible for the of aluminosilicate gel formation at the beginning of the process depend on the type of precursor and chemical composition of the activator. When using fly ashes and GGBFS, the presence of calcium can significantly alter the chemical structure of the geopolymer system and strengthen the interfacial transition zone.
Other study carried out by Favier et al. [
16] concerned the comparison between the rheology of metakaolin (MK) based geopolymer paste and the rheology of cement pastes. It was found that, from a macroscopic point of view, the geopolymer is a viscous fluid. It is characterized by very low yield stress and high viscosity. According to Flatt [
17], yield stress of cement mixes is controlled by plasticizers and superplasticizers. The use of admixtures common in cement concrete technology for MK-based geopolymers does not bring the desired effects due to the almost pure viscous behavior of the material. It is possible that it can be effective in the case of precursors with calcium content, such as fly ashes or GGBFS. The viscosity of a geopolymer material is a result of the viscosity of the alkaline silicate solution. Furthermore, the chemical reactions taking place during the setting process quickly modify the rheological behavior of geopolymers [
18].
The results of studies into geopolymer bonding mechanisms [
19] show that the molar ratio (
MR) of the alkaline solution plays a key role in accelerating the geopolymer hardening process. When solutions with a lower
MR value are used, the course of the condensation reaction between Si and Al oligomers is faster, the setting time of the material is shortened and, as a result, the open time for 3D concrete printing is very short. In the case of materials used for 3D printing, it is very important to optimize the delay time between layers to ensure sufficient water content in the contact area. In the case of geopolymer mortars, the degree of cross-linking of the material mainly depends on an irreversible chemical reaction. The use of an alkaline solution with lower reactivity reduces the degree of cross-linking of the material and affects the overall strength of the resulting material.
Panda et al. [
20] researched the effect of ground granulated blast furnace slag (GGBFS) or silica fume (SF) on the fresh and hardened properties of fly ash-based geopolymer mixes for 3D concrete printing. As in the case of OPC (Ordinary Portland Cement) -based mixes [
21], an increase in the yield stress of geopolymers with time was observed, which is a result of the physical interaction of the particles and the chemical activation of the binder. In the case of mixes containing GGBFS, a clear increase in yield stress was observed after 20 min of mixing, which was most probably caused by accelerated setting of the mixes by the activation of GGBFS [
22]. The reactivity of fly ash based geopolymers with GGBFS is much higher in ambient conditions. Furthermore, the study results indicate a limited effect of GGBFS on the properties of fresh geopolymer pastes and a clear influence on the increase of the early age compressive strength. Moreover, the inclusion of GGBFS in the binder composition contributes to the development of a homogenous microstructure and the formation of a stronger 3-dimensional network. The addition of GGBFS does not significantly improve the rheology of the mix, but reduces the workable time of the paste. Due to the requirements regarding the setting time of the mix and the strength of the material, the amount of GGBFS used must be carefully controlled.
Other studies of geopolymer mixes [
21,
23] show that the rheology of geopolymers is strongly dependent on the viscous nature of the alkaline solution used, not with a particle interaction. The rheology of fresh geopolymer pastes can be controlled by dissolution and geopolymerization in the suspension system. In the case of geopolymer binders, the molar ratio of the alkaline solution used in the material synthesis process determines the ion dissolution and polymerization of fresh pastes, therefore it is the main factor affecting the rheology of the fresh geopolymer pastes.
The results of the study [
24] show that a low content of sodium hydroxide decreases the fluidity of fresh geopolymer pastes. The fluidity of geopolymer pastes gradually increases together with the amount of NaOH added. High alkali content in geopolymer pastes reduces yield stress and plastic viscosity. The increase in the Si/Na ratio of the alkali activator reduces the yield stress of fresh pastes. The rate of development of yield stress increases with a decrease in the Si/Na ratio [
25].
Based on the literature review, it can be concluded that the problem of the rheology of geopolymer composites is quite complex. Apart from the alkaline solution, the main factor shaping the properties of the geopolymer binder is the type of precursor used. Considering the possibility of setting material at ambient temperature and high strength values, it seems beneficial to use a blended FA-GGBFS binder. The use of this type of binder extends the application possibilities of geopolymer composites. The aim of the study was to expand knowledge in the field of rheology of blended FA/GGBFS based geopolymer mortars. Three types of mortars were prepared in which binders FA were replaced by 10 wt%, 30 wt% and 50 wt% of GGBFS. The consistency of fresh mortar was determined and the rheological properties of the mixtures were investigated using a rheometer. The kinetics of strength development and the porosity after 28 days were determined. The research results can be used to develop geopolymer technology, in particular in terms of application possibilities. One of the possible directions of application of geopolymer binders is 3D printing.
2. Materials and Methods
2.1. Materials
Siliceous fly ash (FA) produced by the Połaniec power plant, Poland, and ground granulated blast furnace slag (GGBFS) obtained from Ekocem, Poland were used in the study. The main components of this fly ash are SiO
2—52 wt% and Al
2O
3—28 wt%. In addition, the FA Połaniec contains Fe
2O
3 and a small amount of MgO, K
2O, TiO
2 and Na
2O. The total content of silicondioxide, alumina and iron (III) oxide exceeds 70 wt%. Due to its specific chemical composition, Połaniec fly ash is classified as a siliceous ash and based on the chemical analysis and according to EN 450-1:2012, this FA can be classified as type II. According to ASTM C618, the FA belongs to class F. The specific gravity of the fly ash and ground granulated blast furnace slag (GGBFS) used was 2.1 g/cm
3 and 2.9 g/cm
3, respectively. Siliceous sand (0/2 mm) was used to prepare the geopolymer mortars. The chemical composition of the FA and GGBFS can be found in
Table 1 and
Table 2 respectively.
To prepare the solution, sodium silicate solution (Chempur, Poland, Na2O = 11.1 wt%, SiO2 = 27.9 wt%, H2O = 61.0 wt%), sodium hydroxide pellets and water were used together. The prepared sodium alkaline solution was characterized by a molar ratio MR = 1.7. Preparation of the alkaline solution started from dissolution of NaOH pellets in water. In the next step NaOH solution was mixed with sodium silicate solution and extra water. The resulting solution was stored for 24 h prior to use.
In order to determine the effect of GGBFS on the behavior of the geopolymer mix, the content of the additive was varied. Mortars containing 10 wt%, 30 wt% and 50 wt% replacement of FA by GGBFS were analyzed. Geopolymer mortars were signed G10, G30 and G50 where the numerical value corresponds to the mass percentage content of GGBFS in binder composition. The compositions of all mortars are given in detail in
Table 3. In this study the molar ratio of alkaline solution (
MR = 1.7), sand to binder, alkaline solution to binder (FA+GGBFS) and water to binder (FA+GGBFS) ratios were set as constants. The values are shown in the
Table 4. In this study no plasticizers or superplasticizers were used.
To obtain the same volume of the mortar with a different mass of GGBFS in the composition. The content of other components has been corrected. FA and GGBFS differ in density.
The mixing of all mortars was carried out in accordance with the procedure optimized for the geopolymer by Davidovits [
26], as follows.
- (I)
Preparation of an alkaline solution—24 h before starting the mixing. The total amount of water required in the mortar composition was added directly to the alkaline solution. Due to the exothermal character of the reaction, water evaporation was prevented during the cooling down of the alkaline solution and the container was enclosed with a lid.
- (II)
Paste preparation. FA with an alkaline solution was mixed first for 10 min, and then GGBFS was introduced and mixed for another 3 min.
- (III)
Incorporation of aggregate. Eventually, quartz sand sized 0/2 mm with a specific mass of 2.64 g/cm3 was added and the mortar was mixed for 2 min.
2.2. Methodology
Immediately after mixing fresh geopolymer mortar samples were put into rheometer and the first measurement was performed. The start of measurement procedure was set at 200 ± 20 s after finishing of the mixing process. Secondly, the consistency was determined (by the flow table method, following EN 1015-3 procedure). The test was set at 10 min after finishing the mixing process. Third and fourth step were rheological measurement repetition, after 15 and 25 min, respectively. Finally, the 40 mm × 40 mm × 160 mm samples were formed for hardened state tests. Mortars set at ambient temperature (T = 18 ± 2 °C, RH = 75%) in plastic molds with a lid. Additional heating was not applied. After removed from the molds, samples were stored in the same ambient conditions.
A prototype RMB-2 rotational rheometer with a helix impeller was used to determine the rheological properties [
27]. It is a rheometer with a layout corresponding to the mKII setup by Tattersall and Banifill [
28] with a driven external cylinder and measurement of the torque of the probe axis on a rigid beam. The external cylinder capacity was approximately 4.5 dm
3.
The equipment can be programmed for any speed–time procedure including the frequency of the data collecting during the experiment. The typical descending ramp technique was used, with a speed range from 50 to 2 rpm (0.84 ÷ 0.03 (1/s)), 11 steps, with a repetition of the highest speed at the end. The initial maximum speed was held for 20 s to obtain a proper breakdown of the structure and the following steps were held for 6 s. The flow curves were plotted in the M (Nm)—ω (1/s) units.
The physical (density) and mechanical (compressive and flexural tensile strength) properties of hardened mortars were tested over time to monitor the evolution of those properties during curing. Flexural tensile strength was performed on 40 mm × 40 mm × 160 mm specimen in three-point bedning and constant load rate of 50 N/s. The remaining mortar prisms were used for the determination of compressive strength on a hydraulic press according to the standard procedure for cement mortar with a constant load rate 2400 N/s. The tests were carried out at the age of 2, 14 and 28 days.
Furthermore, after 28 days pore size distribution was measured with mercury intrusion porosimetry (MIP). Using MIP open pores between 3.5 nm and 500 μm were detected [
29] the pore size distribution and total cumulative porosity was determined and presented.
4. Conclusions
This study investigated fresh state rheological properties of blended FA-GGBFS geopolymer mortars in the time frame from 0–25 min. The impact of replacement of the FA by GGBFS in the amount of 10 wt%, 30 wt% and 50 wt% on the flow behavior in the fresh state and the mechanical performances of the hardened material were examined.
Based on the conducted research, it can be stated that the content of GGBFS in the range up to 50 wt% did not significantly alter the rheology of geopolymer mortars within the time rage from 0 to 25 min The increase in GGBFS content was accompanied by a decrease in consistency (i.e., an increase in yield stress), but the differences between the successive levels analyzed were not very significant.
The mortars tested were considered as multi-phase systems due to the presence of paste and filler in the form of fine aggregate. The analysis of the rheology of such mixes is particularly difficult, as the properties of the mixes are determined by different mechanisms and phenomena. In the case of the mortars under consideration, a relatively high content of paste can have a decisive effect on the rheological properties of the mixes. During the fresh mix tests, a consistency loss was perceived in two steps. The first step was occurring between 5 and 20 min. The drop inconsistency was caused mainly by the increase in viscosity in the low-shear zone. The second decrease around 35 min after mixing was observed and the consistency drop was probably associated to a yield value rise.
The analyses conducted indicate a certain analogy between the behavior of the geopolymer mortars to that of a pseudoplastic liquid representing shear thinning behavior. The flow curves obtained were highly non-linear, following roughly the Herschel–Bulkley model with n < 1. The relatively high paste to sand ratio and the presence of alkalis in the mix most probably have a decisive effect on the rheological properties of the tested materials, i.e., high viscosity, low yield value response in the low-shear zone. The observed shear thinning behavior results probably from the occurrence of reversible mortar aeration process in the mid-shear zone. In the high shear zone high but stable aeration occurs, resulting in the change of rheological response to high yield and low viscosity.
Tests of mechanical properties show a clear increase in strength together with an increase in the amount of GGBFS. After 28 days the fc values had reached 36.0 MPa, 49.5 MPa and 67.5 MPa respectively for G10, G30 and G50. The higher GGBFS contents give a more compact structure of the material and the result is higher apparent densities and better mechanical properties. The total porosity measured with the MIP method have shown that the porosity of the fly ash geopolymer mortar with higher GGBFS addition gave a denser matrix with a total open porosity of 14%. The dominant diameter of the pores was located in the macropores region between 2050 and 3130 nm for all tested FA-GGBFS blended geopolymers mortars. The sand presence had a significant impact on porosity and pore-size distribution when comparing to the literature where geopolymer pastes were investigated.
The presented research results of rheology of geopolymer mortars can form the basis for further development works for use geopolymer mortars with blended FA-GGBFS binder in additive manufacturing (3D printing technology).