15
Design and properties of fly ash, ground granulated blast furnace slag, silica fume and metakaolin geopolymeric based masonry blocks
Abstract
This chapter illustrates the reaction mechanism, the choice of materials used in making geopolymers and proportioning in brief. Later, the methods to be adopted to prepare mortar samples, determination of the optimum dry density, casting of mortar specimens and testing the same are illustrated. The development of compressive strength with various parameters is analyzed using the experimental data generated. This was done to formulate a phenomenological model to arrive at the combinations of the ingredients to produce geopolymer blocks to meet the strength development desired. The validity of phenomenological models was examined with an independent set of experimental data. It was found that the experimental values of compressive strength were in line with the predicted values.
Keywords
Compressive strength; Fly ash; GGBS; Masonry block; Metakoalin; Silica fume15.1. Introduction
Geopolymer is the name given to a wide range of alkali or silicate-activated aluminosilicate binders. Since the chemical reaction that takes place in this case is a polymerization process, Davidovits (1994, p. 9, 1999) coined the term ‘geopolymer’ to represent these binders. The dominant aluminosilicates are class F fly ash and ground granulated blast furnace slab (GGBS).
Provis (2014) reports that the chemistry of low calcium (“geopolymer”) and high calcium (blast furnace slag-derived) alkali-activated material differ from each other. He also reports that the underlying mechanisms of degradation in such materials may not be always the same for alkali-activated binders as for Portland cement-based binders. According to Chao, Sun, & Longtu (2010), there are two main models of alkali-activated cements. They are activation of slag and metakaolin. In the first case of alkali activation of GGBS, the Si + Ca system dominates. In the second case of a geopolymer made with metakaolin and fly ash, Si + Al dominate. The main drawback in this case is a very high water requirement, which in turn causes difficulties related to drying shrinkage and cracking of the material.
Geopolymer composites have a very small greenhouse footprint when compared to traditional cement composites. The study by Shi & Fernandez-Jiménez (2006) concludes that alkali-activated cements are a better matrix for solidification/stabilization of hazardous and radioactive wastes than ordinary Portland cement. Geopolymer concrete possesses excellent similar strength and appearance similar to conventional concrete made from Portland cement (Hardjito, Wallah, Sumajouw, & Rangan, 2004). It is also well-known that geopolymers possess excellent mechanical properties, fire resistance and acid resistance (Davidovits & Davidovits, 1988; Palomo, Macias, Blanco, & Puertas, 1992).
The choice of the source materials for making geopolymers depends on availability, cost, type of application, etc. Studies by Zhao, Ni, Wang, and Liu (2007) have confirmed the formation of mainly ettringite and calcium silicate hydrate gel in the activation of GGBS and class F fly ash pastes. According to Astutiningsih & Liu (2005), the strength of alkali-activated slag decreases as the water content increases.
According to Montes, Islam, Shi, Kupwade-Patil, & Allouche (2013), the materials prepared by geopolymerization of fly ash and GGBS offer considerable resistance to freeze-thaw action, sulfate attack, sulfuric acid attack and nitric acid attack compared to Portland cement products. Sakkas, Nomikos, Sofianos, & Panias (2014) report that the sodium-based geopolymer from slag would be an appropriate material for passive fire protection systems. In the absence of long-term durability of geopolymers, comparable in scale and longevity to Portland cement, well-established testing methods and research are essential to validate the laboratory trials. Van Deventer, Provis, & Duxson (2012) are of the opinion that colloid and interface science, gel chemistry, phase formation, reaction kinetics, transport phenomena, communication, particle packing and rheology play a salient role in the development of geopolymer technology.
A report by Dahmen & Muñoz (2014) indicates that geopolymerization of abundant minerals such as aluminosilicates has the capacity to radically transform traditional cement-based masonry products on a global scale.
It is possible to tailor the geopolymer material to attain the required strength and durability to optimize the cost. Given the correct mix design and formulation development, geopolymeric materials derived from fly ash and GGBS can exhibit superior chemical and mechanical properties over those of OPC composites. But no literature is reported so far about the logical mix proportioning of the geopolymer mix except those of Rangan (2008a, 2008b). Trial mix is essential for exact proportions of the concrete mix. As geopolymers are highly complex and yet relatively poorly understood, there are clearly many areas in which further work is required (Duxson, Provis, Lukey, & Van deventer, 2007). There are attempts to develop phenomenological models to reproportion geopolymer mortar and concrete (Radhakrishna, Madhava, Manjunath, & Venugopal, 2013; Radhakrishna & Udayashankar, 2008; Radhakrishna, Udayashankar, & Renuka Devi, 2010). Such models were reported for fly ash, as well as lime-based masonry blocks (Radhakrishna & Niranjan, 2013).
The vast majority of research conducted in the field of geopolymers has to date focused on manipulation of engineering properties, short-term durability and waste immobilization efficacy. The objective of this chapter is therefore to remedy this situation by developing methods to reproportion the geopolymer mortar under the framework of scientific laws rather than simply by empirical mix formulation. The possibility of developing phenomenological models to take care of this situation merits examination. Methods of accounting different parameters involved in strength development of fly ash and GGBS-based thermal cured/ambient cured geopolymer masonry blocks is the major outcome of this chapter.
15.2. Characteristics of geopolymer mortar
To get the maximum dry density and optimum moisture content, Standard Proctor's compaction test was conducted on fresh geopolymer mortar. The fluid media to prepare the mortar was water and various alkaline solutions of different molarities for comparison. The variation of the dry density with fluid-to-binder (f/b) ratio is shown in Figure 15.1. As in any of the particulate material with the interaction of water, the dry density increases and then decreases. At f/b ratio of 0.2, the mix would attain maximum dry density for all the fluids. It can be seen that compaction characteristics would be marginally affected by the fluid medium.
The fresh geopolymer mortar sample having f/b ratio of less than 0.10 would be in a very dry powdery form. It would be very difficult to compress the material to get the block of required size and density. Even after casting with great difficulty, the samples would crack immediately after casting without any cohesiveness. In the range of 0.150–0.225 fluid-to-binder ratio the mixture was in the desired wet condition and interaction of the particles was homogeneous. At this range of f/b ratio, the mortar would be in a three-phase system with solids, liquid and air content. Casting of the geopolymer mortar specimens was possible at this range. However, compaction effort would be essential to cast the cylindrical specimens of mortar. Normally, in the range of 0.225–0.250 clusters would form in the mortar mix. At the f/b ratio of 0.250 and above, the mix attains full saturation resulting in a two-phase system with solids and liquid without any air content. It would not be possible to cast blocks in this system. However, the consistency of the mortar mainly depends on the combination of the materials along with the f/b ratio. A specially fabricated “static compaction device” was used to prepare cylindrical specimens to test in a laboratory.
15.3. Static compaction device
A static compaction device can be used to cast the compressed cylindrical blocks of geopolymer mortar at laboratory. This device can be fixed to a platform as shown in Figure 15.2. The device essentially consists of a brass tubing of 38 mm internal diameter and 240 mm in length. Both the ends of the cylinder would be threaded to fit cylinder caps. These caps are machined to have suitable length of external bosses. Two piston rods of 25 mm diameter work from both ends in the bosses provided in the cylinder covers. The diameter of the piston rod governs the structural strength and the external bosses of the cylinder caps offer lateral rigidity to the piston rod. A number of threads per unit length of the piston rod control the sensitivity of the device. The position of the locknuts on the piston rods controls the final length of the sample to 76 mm. The piston head with a sliding fit in the cylinder is mounted on the ball bearing rigidly fixed to the piston rod. This prevents the rotational motion of the piston rod being transferred to the material. The compacted sample can be removed by using “sample extruder”. This device provides an easy way of casting geopolymer mortar compressed blocks by manual operation.
A sample cylinder prepared using a static compaction device is shown in Figure 15.3. The ratio of height-to-diameter was maintained at 2. This type of cylindrical specimen was used for the laboratory study. However, the compressive strength was also correlated with the actual masonry block/brick used in practice.
15.4. Strength development with degree of saturation
The blocks with a constant density were cured at a temperature of 60 °C and above normally for 24 h (Radakrishna et al., 2013). The samples were left in open air after thermal curing. The variation of the unconfined compressive strength of the cylinders is shown in Figure 15.4 for three different ages. It can be noticed that the strength increases with f/b ratio and then decreases. This contradicts the trend of strength development according to Abrams' law (1918), p. 20.
In the case of cement mortar/concrete, strength decreases as the air content increases at saturated condition. In the case of partially saturated compressed mortar cylinders, the degrees of saturation at various dry density values were calculated and indicated in Figure 15.4. In the case of a partially saturated system, the material would be in a three-phase system – solids, liquid and air. Based on this system, to calculate the degree of saturation, the following basic relation of soil mechanics (Eqn (15.1)) was used.
(15.1)
where,
γ = Density in g/cc
γw = Density of water in g/cc
G = Specific gravity of the material used
W = Water content in percent
Sd = Degree of saturation in percent
From Eqn (15.1), it can observed that as the water content (W) increases, the degree of saturation (Sd) and air content in the mortar increases. For different f/b ratios, degree of saturation is shown in Figure 15.4. It can be seen that as the f/b increases the degree of saturation (air content) gradually increases. It can be seen that initially, as the f/b ratio increases from 0.10 to 0.20, strength increases (which is contrary to Abrams' Law). However from f/b ratio 0.20 to 0.25 (i.e. at more or less the same saturation level of 55.6% and 55.9%), the strength drops down as per Abrams' law. This suggests that when degree of saturation is maintained constant, the effects of f/b ratio get truly reflected in the strength development. Hence the study of strength development of compressed blocks at a constant degree of saturation can be made.
To maintain a constant degree of saturation (specified constant air content) in the compacted state, adjustment of dry density would be essential by varying water content using Eqn (15.1). If the water content (W) is known, the f/b ratio of the mortar can be calculated. This can be carried out by precalculations as is done in gravimetric and volumetric calculations in particulate material such as soils.
The proportioning of thermally cured and ambient cured geopolymer mortar is discussed in this chapter under two headings:
• Thermal cured geopolymer blocks
• Ambient cured geopolymer blocks
The masonry block used for the study is shown in Figure 15.5. The dimensions of the block are 200 × 110 × 60 mm and cured either in an oven or in open air as indicated.
15.5. Thermal cured geopolymer blocks
A typical series of geopolymer mortar blocks having constant binder-to-aggregate ratio and other parameters are shown in Table 15.1.
The various parameters considered for thermal cured compressed blocks include the following:
• Molarity of alkaline activator: 8, 10, 12, 14 M
• Age of the specimen: 1, 3 and 7 days
• Fine aggregate: Sand and quarry dust
• Curing conditions: in oven – wrapped and unwrapped
The compressive strength development in geopolymer mortar for various parameters is discussed in this section.
15.5.1. Strength variation at initial constant degree of saturation
From the laboratory trials, it can be observed that it is possible to cast the compressed blocks at the degree of saturation of 44% without any practical difficulties. This degree of saturation can be maintained constant for thermal cured blocks. In Figure 15.6 the strength variation is shown with f/b ratio at a constant degree of saturation of 44%. The molarities of the alkaline activator solutions were 6 and 8.
Table 15.1
Mix proportions of thermal cured geopolymer compressed blocks
| Sl no | Series ID | Fine aggregate | Age (days) | Molarity | W/UW |
| 1 | 10M-Sand-W-7 Days | Sand | 7 | 10 | W |
| 2 | 10M-Sand-W-7 Days | Sand | 7 | 10 | W |
| 3 | 10M-Sand-W-7 Days | Sand | 7 | 10 | W |
| 4 | 10M-Sand-W-7 Days | Sand | 7 | 10 | W |
| 5 | 12M-Sand-W-3 Days | Sand | 3 | 12 | W |
| 6 | 12M-Sand-W-3 Days | Sand | 3 | 12 | W |
| 7 | 12M-Sand-W-3 Days | Sand | 3 | 12 | W |
| 8 | 12M-Sand-W-3 Days | Sand | 3 | 12 | W |
| 9 | 14M-Sand-W-1 Day | Sand | 1 | 14 | W |
| 10 | 14M-Sand-W-1 Day | Sand | 1 | 14 | W |
| 11 | 14M-Sand-W-1 Day | Sand | 1 | 14 | W |
| 12 | 14M-Sand-W-1 Day | Sand | 1 | 14 | W |
| 13 | 14M-QD-W-7 Days | Quarry dust | 7 | 14 | W |
| 14 | 14M-QD-W-7 Days | Quarry dust | 7 | 14 | W |
| 15 | 14M-QD-W-7 Days | Quarry dust | 7 | 14 | W |
| 16 | 14M-QD-W-7 Days | Quarry dust | 7 | 14 | W |
| 17 | 14M-QD-UW-3 Days | Quarry dust | 3 | 14 | UW |
M, molarity; QD, quarry dust; W, wrapped; UW, unwrapped.
The strength development, particularly at a low f/b ratio, despite the degree of saturation being constant, did not reflect the strength levels as per Abrams' law. But distinctly, with an alkaline solution of molarity 10 the strength levels would be practically in tune with f/b ratios as shown in Figure 15.7. Owing to low molarity (concentration of alkaline salts) in a low f/b ratio (volume of fluid), the strength developed would be less. This is also another factor that influences strength development. With an increase in molarity, the concentration of alkaline salts would be adequate enough to push the strength levels to be in tune with Abrams' law as expected. Alkaline fluid of molarity of 10 and beyond with a constant degree of saturation could impart strength development in accordance with f/b ratios. Similar observations have been made by Chindaprasirt, Jaturapitakkul, Chalee, & Rattanasak (2009) for geopolymer concrete.
15.5.2. Effect of fine aggregate on strength
After identifying the role of degree of saturation, the influence of fine aggregate on strength development of fly ash-based geopolymer mortars merits examination. Apart from conventional fine aggregate (sand), the possibility of using other marginal aggregates need to be explored for sustainable development. The alternative fine aggregate quarry dust was also used. A typical variation of the compressive strength with f/b ratio is shown in Figure 15.8 for sand and quarry dust as fine aggregate for 14 M.
The strength development in the case of quarry dust as aggregate was marginally higher compared to natural river sand having the same fineness modulus. It can be noticed that the strength development with age would not be affected in any noticeable way with change of fine aggregate from sand to quarry dust. This is in order since both sand and quarry dust are noninteracting particulate materials with water. They are basically quartz with change in specific surface. Due to the fine size of particles in quarry dust, there would have been better particle packing in the mortar.
15.5.3. Strength development with/without loss of moisture
Fly ash-based geopolymer mortar blocks are normally cured in an oven at an elevated temperature. At a temperature of 60 °C, it can be realized that there would be a loss of moisture that would affect optimal strength development. The loss of moisture at low fluid content would be more crucial. This loss can be considerably prevented by wrapping them with aluminum foils. But from a practical viewpoint, this may not be desirable since the production of compressed blocks would be hampered. The purpose of this study was to assess the strength development in unwrapped and wrapped conditions. The compressed blocks were tightly wrapped in aluminum foil with the least air trapped in between them before keeping in the oven. The strength data of such samples after curing is shown in Figure 15.9.
The comparison of data in Figure 15.9 indicates that owing to prevention of loss of moisture the strength developed would be higher. This increase can be seen at all ages to the extent of 20–25% and can be attributed to greater reaction of salts with fly ash to promote higher strength development.
The compressive strength with f/b ratio of all the series having a molarity of 10 and above is shown (all together) in Figure 15.10 at a constant degree of saturation of 44%. The compressive strength with binder-to-fluid (b/f) ratio is linear as shown all together in Figure 15.11.
In summary, the analysis of all test results of thermal cured fly ash-based geopolymer mortar blocks shows that strength development would influence basically by f/b ratio. Unlike in a saturated (two-phase) system, degree of saturation was another variable parameter in compressed blocks, which has been maintained constant in all the series of experiments. The trend remains the same in all the cases. Similar trends can be seen in the case of cement compressed blocks as reported by Prasad, Narasimhulu, Nagaraj, Naidu, and Ifthakaruddin (2005).
15.5.4. Proportioning of geopolymers
Proportioning of geopolymer composites is scarcely reported in the open literature. The available literature focuses more on strength, durability and performance of geopolymer composites. Also, the research on partially saturated geopolymer mortar is rare. However, Radhakrishna et al. (2008) and Radhakrishna et al. (2010, 2013) have reported that heat-cured geopolymers can be reproportioned by generalized Abrams' (Abram, 1918, p. 20; Nagaraj & Banu, 1996, 1999) and Bolomey's laws (1927). As per the reported research, by using a single input parameter in the model, the b/f ratio for any other desired strength can be calculated using the phenomenological model.
15.5.5. Development of phenomenological model – thermal cured blocks
A rational, rapid and simple method to arrive at the combination of ingredients to realize a specific value of strength development at a required age is desirable. It is due to the following situations that may arise during handling of large volumes of waste materials.
1. The strength requirements and the age at which this is required vary depending upon end usage. As such, to arrive at the required f/b ratio, simple procedures are needed.
2. As the density, molarity of solution and curing conditions (thermal and ambient conditions) could vary, it is rather difficult to arrive at the required f/b ratio with minimum trials.
3. The batches of these materials may vary from time to time, which needs required control to recheck the mix proportions with minimum test data and computations.
In a wider context, if the method advanced has a rational basis it lends additional support and confidence to employ the same in practice.
In the case of OPC composites, water-to-cement ratio rules the strength development when other factors are kept constant (Nagaraj & Banu, 1996). In a similar manner, within the range of values of b/f ratio considered in this investigation inverse of f/b ratio of 0.2 (b/f ratio of 5.0) can be considered as a reference for normalization. This chosen value of b/f ratio of the compressed blocks is arbitrary. There is no other significance. The resulting functional relation by regression analysis is as shown in Eqn (15.2) and Figure 15.12 having R2 value of 0.98. This is a phenomenological model for assessment of thermal cured geopolymer mortar blocks.
(15.2)
where S is strength for which b/f ratio is to be calculated.
S@b/f=5 is strength of mixture at f/b ratio of 0.2 (inverse = 5.0);
b/f is inverse of f/b ratio;
0.1833 and 0.0747 are constants.
15.5.6. Validation of the proposed model
To use this relation (Eqn (15.2)) for a given set of materials, the strength developed at a specified age for a b/f ratio of 5.0 needs to be determined. Using this as an input parameter in the equation, the b/f ratio for any other desired strength can be calculated using the phenomenological model. Using the calculated b/f ratio, all other ingredients in the mortar mix can be determined. Hence the mix proportions for the required strength can be arrived at.
To carry out this exercise, a series of experimental data with different conditions were considered. This was an independent set of data which is not a part of data analyzed in the formulation of the phenomenological model. Binder-to-fluid ratio (b/f) is an independent variable in each of the sets.
In the same range of b/f ratio, the strength developed in each set varies due to variation of other parameters. From each of these sets, the compressive strength at reference b/f ratio can be taken into consideration in the denominator of the left-hand side of phenomenological model (Eqn (15.2)). The strength developed at other f/b ratios can be calculated. The comparisons with experimental values are as shown in Table 15.2. There is very close match between the experimental and predicted values reinforcing the applicability of this model practically.
15.6. Ambient cured geopolymer blocks
It is a well-established fact that class F fly ash can be activated with thermal input with higher molarity of alkaline solution. However, it would not be an economical option to produce compressed blocks with fly ash alone as binder. In view of this, strength development of compressed blocks can be studied by the addition of GGBS and other binders.
The mix details are shown in Table 15.3. The blocks were cured in ambient conditions (around 24 °C) and prepared using different binders as indicated in the table.
In this section, strength development of geopolymer compressed blocks at ambient temperature is discussed considering various parameters. The parameters considered for the study are as follows:
• Age of the sample: 1, 3, 7, 14, 28, 56, 90, 120 and 180 days.
• Fly ash: FA1, FA2, FA3 and FA4.
• Alkaline activator: Sodium hydroxide and potassium hydroxide.
• Ratio of binder-to-aggregate: 1:1, 1:2 and 1:3.
• Degree of saturation: 40% and 60%.
• Molarity of alkaline solution: 8, 10, 12 and 14 M.
• Fine aggregate: Sand, quarry dust and pond ash.
• Temperature: 25, 30, 40, 50, 60, 70 and 80 °C.
• Binder: fly ash, GGBFS, silica fume and metakaolin.
• Sample size and shape: PB1, PB2, PB3, CB1, CB2 and CB3.
Table 15.2
A typical comparison of experimental and predicted compressive strengths
| f/b ratio | b/f ratio | Series ID | ES (MPa) | PS (MPa) | ES/PS |
| 0.150 | 6.67 | 10M-Sand-W-7 Days | 18.3 | 18.29 | 1.00 |
| 0.175 | 5.71 | 10M-Sand-W-7 Days | 15.88 | 15.81 | 1.00 |
| 0.200 | 5.00 | 10M-Sand-W-7 Days | 14.1 | 14.1 | 1.00 |
| 0.225 | 4.44 | 10M-Sand-W-7 Days | 12.2 | 12.52 | 0.97 |
| 0.15 | 6.67 | 12M-Sand-W-3 Days | 19.25 | 18.8 | 1.02 |
| 0.175 | 5.71 | 12M-Sand-W-3 Days | 16.19 | 16.25 | 1.00 |
| 0.200 | 5.00 | 12M-Sand-W-3 Days | 14.5 | 14.5 | 1.00 |
| 0.225 | 4.44 | 12M-Sand-W-3 Days | 12.85 | 12.88 | 1.00 |
| 0.150 | 6.67 | 14M-Sand-W-1 Day | 19.75 | 19.43 | 1.02 |
| 0.175 | 5.71 | 14M-Sand-W-1 Day | 17.1 | 16.79 | 1.02 |
| 0.200 | 5.00 | 14M-Sand-W-1 Day | 14.98 | 14.98 | 1.00 |
| 0.225 | 4.44 | 14M-Sand-W-1 Day | 13.2 | 13.3 | 0.99 |
| 0.150 | 6.67 | 14M-QD-W-7 Days | 27.01 | 28.2 | 0.96 |
| 0.175 | 5.71 | 14M-QD-W-7 Days | 24.06 | 24.38 | 0.99 |
| 0.200 | 5.00 | 14M-QD-W-7 Days | 21.75 | 21.75 | 1.00 |
| 0.225 | 4.44 | 14M-QD-W-7 Days | 19.25 | 19.32 | 1.00 |
| 0.150 | 6.67 | 14M-QD-UW-3 Days | 21.98 | 21.82 | 1.01 |
| 0.175 | 5.71 | 14M-QD-UW-3 Days | 19.37 | 18.86 | 1.03 |
| 0.200 | 5.00 | 14M-QD-UW-3 Days | 16.82 | 16.82 | 1.00 |
| 0.225 | 4.44 | 14M-QD-UW-3 Days | 15.04 | 14.95 | 1.01 |
ES, experimental strength; PS, predicted strength.
15.6.1. Strength development with age
The pattern of strength development of the geopolymer blocks with age for different f/b ratios is shown in Figure 15.13. The strength of compressed blocks increases with age and its development would rapidly increase during the early age compared to the later age. Ambient cured geopolymer blocks attain strength with age, owing to the continuous formation of CSH gel since GGBS was used as part of the binder. It is interesting to note that ambient cured blocks would develop strength of more than 1 MPa within 24 h. This strength would be sufficient to handle the blocks for the purpose of transportation. A minimum strength of 3 MPa can be achieved at the age of 7 days. This is again advantageous as the blocks can be used for masonry works early.
Table 15.3
Typical mix proportions of ambient cured geopolymer compressed blocks
| Series ID | Binder-to-aggregate ratio | Fly ash type | Binder composition | Degree of saturation (%) | Alkaline activator with molarity | Curing temperature (°C) | Fine aggregate |
| ABS1 | 1:1 | FA1 | FA:GGBFS = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS2 | 1:2 | FA1 | FA:GGBFS = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS3 | 1:3 | FA1 | FA:GGBFS = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS4 | 1:2 | FA1 | FA:GGBFS = 1:1 | 40 | NaOH 8 M | Ambient | Sand |
| ABS5 | 1:2 | FA1 | FA:GGBFS = 1:1 | 40 | KOH, 8 M | Ambient | Sand |
| ABS6 | 1:2 | FA1 | FA:GGBFS = 1:1 | 40 | NaOH 10 M | Ambient | Sand |
| ABS7 | 1:2 | FA1 | FA:GGBFS = 1:1 | 40 | NaOH 12 M | Ambient | Sand |
| ABS8 | 1:2 | FA4 | FA:GGBFS = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS9 | 1:2 | FA3 | FA:GGBFS = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS10 | 1:2 | FA2 | FA:GGBFS = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS11 | 1:2 | FA2 | FA:GGBFS = 1:1 | 40 | NaOH 14 M | Ambient | QD |
| ABS12 | 1:2 | FA2 | FA:GGBFS = 1:1 | 60 | NaOH 14 M | Ambient | Sand |
| ABS13 | 1:2 | – | GGBFS: MTK = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS14 | 1:2 | – | GGBFS:SF = 1:1 | 40 | NaOH 14 M | Ambient | Sand |
| ABS15 | 1:1 | FA1 | FA:GGBFS = 2:3 | 60 | NaOH 12 M | Ambient | Sand |
| ABS16 | 1:2 | FA1 | FA:GBFS = 1:1 | 40 | NaOH 14 M | 30–80 °C | Sand |
| ABS17 | 1:2 | FA1 | FA:SF = 1:1 | 40 | Na(OH)2, 14 M | Ambient | Sand |
| ABS18 (size and shape) | 1:2 | FA1 | FA:SF = 1:1 | 40 | Na(OH)2, 14 M | Ambient | Sand |
| ABS19 | 1:2 | FA1 | FA:GGBFS = 1:1 | 40 | Na(OH)2, 14 M | Ambient | Pond ash |
QD, quarry dust; MTK, metakaolin; SF, silica fume.
15.6.2. Strength development with type of fly ash
Fly ash may vary in characteristics based on the source. It may become necessary to study strength development for various fly ash samples procured from different sources. This is required to assess a particular fly ash for use in making geopolymer compressed blocks. The strength development pattern of the blocks with different fly ash samples is shown in Figure 15.14.
The order of the fly ash samples with fineness is FA1, FA2, FA3 and FA4. The strength developed would be proportional to the fineness of the ash used. FA1 is finest among all the fly ash samples and FA4 is the coarsest. Fineness plays a significant role in pozzolanic action in concrete incorporating fly ash. The same is true in the case of geopolymer blocks.
15.6.3. Strength with alkaline solution
Different alkaline solutions can be used to activate the binders in developing geopolymers. Among all the activators, sodium hydroxide and potassium hydroxide are the popular choice because of their easy availability. However, the latter is more expensive. The strength development for both the activators is shown in Figure 15.15.
The use of potassium hydroxide as an activator increases the strength of the compressed blocks compared to sodium hydroxide with the same molarity. Since potassium hydroxide is expensive, its use would not be economical, but economy can be compromised for strength.
15.6.4. Strength development with binder-to-aggregate ratio
The binder-to-aggregate ratio plays a role in the development of geopolymer compressed blocks. As in the case of OPC mortars, in geopolymers it is to be considered as a parameter to influence strength.
It can be noticed from Figure 15.16 that as the binder-to-aggregate ratio increases, strength also increases and vice versa. This parameter helps in economizing the product cost. At the lower f/b ratio of 0.15, the variation of strength with binder-to-aggregate ratio would be marginal.
15.6.5. Strength development with degree of saturation
Since dry mortar would be partially saturated material, the degree of saturation affects strength. It may not be possible to cast the mortar blocks for the degree of saturation below 40% and above 60% due to practical difficulties. The strength development of the compressed blocks with two degrees of saturations – 40% and 60% – is shown in Figure 15.17.
Degree of saturation (Sd) is the index of air content present in a three-phase material like geopolymer mortar. It is well-known that as the air content increases the strength decreases. This is true even in the case of geopolymer compressed blocks. The blocks with 40% degree of saturation would possess higher strength compared to the blocks with a higher degree of saturation of 60%. It should be noted that as the degree of saturation increases, the mortar becomes too wet and cannot be used to cast blocks. On the other hand, if the degree of saturation is less, the effort required to cast the samples becomes too high. Hydraulic machines can be used to cast the samples at lower degree of saturation.
15.6.6. Strength development with molarity of alkaline solution
Molarity (M) of the alkaline solution represents the concentration of the salts in the solution. As molarity increases, the basic material required to prepare the solution increases. Molarity plays a major role in strength development.
Strength developed is always proportional to the molarity of the alkaline solution used. As the molarity of the solution increases, the formation of aluminosilicates and CSH gel increases (Figure 15.18). This results in higher strength. It is to be noted that at molarity less than 8, the strength developed would be much less and hence may not be considered for practical application. The solution of higher molarity of more than 14 cannot be prepared easily. As the molarity increases beyond 14, more salts would be deposited, and it becomes extremely difficult to maintain a homogeneous solution.
15.6.7. Strength development with fine aggregate
Natural river sand is turning scarce and hence should be preserved. Efforts are being made in the construction industry to use material alternate to sand. Hence, the alternatives such as quarry dust and pond ash as fine aggregate can be used.
The use of quarry dust and pond ash with the same fineness moduli as fine aggregates strengthens the blocks marginally higher (Figure 15.19). This reinforces the possibility of using both pond ash and quarry dust in place of sand.
15.6.8. Strength development with binder
The properties of binders vary with the source. The strength development in the compressed blocks depends on the properties of the binder. Different binders can be used to prepare geopolymer composites apart from fly ash. The base materials used should be rich in alumina and silica. For this, a combination of three binders – GGBS, silica fume and metakaolin with/without fly ash – can be considered. The variation of strength with these binders is shown in Figure 15.20.
It is interesting to note that, with the use of silica fume as part of binder, strength development would be almost four times that of other combinations. At the age of 7 days and f/b ratio of 0.15, the strength would be more than 23 MPa. The reason for higher strength is due to the higher silica content and its higher surface area. If the strength requirement is more, silica fume can be recommended as binder in making the geopolymer compressed blocks. The finished block would be very light in color (creamy white) compared to other blocks. These blocks can be used for architectural purposes. But the cost of silica fume is to be considered before using it as binder in making the blocks. The strength developed with GGBS as binder would be marginally higher compared to metakaolin. It is advantageous to use GGBS in place of metakaolin, which is expensive.
The compressive strength of ambient cured blocks in this study varies in the range 1–30 MPa for different parameters. This would cover a wide range of strength. Based on the end use, the raw materials and methods are to be selected to get the required strength. Blocks having any strength in this range can be prepared at ambient conditions without any special curing.
15.6.9. Development of phenomenological model – ambient cured blocks
It is interesting to note from the voluminous experimental data that at a constant degree of saturation, f/b ratio alone determines the strength development with all other parameters remaining unchanged. This was observed in thermal cured compressed blocks as discussed in the previous section. Keeping this in mind, another phenomenological model can be developed as that of thermal cured blocks.
When the compressive strength was generalized with reference to strength at b/f ratio of 5.0, the model Eqn (15.3) can be obtained.
(15.3)
The above model resembles Eqn (15.2) developed for thermal cured blocks in the previous section of this chapter with marginal variation in the constants. This strongly implies that the strength development in thermal cured and ambient cured blocks follows the same trend. This model is shown graphically in Figure 15.21 with R2 value of 0.94. The data used to develop the model was not part of the data used for the prediction of the strength. The compressive strengths of thermal cured blocks can be predicted using Eqn (15.3) and of ambient cured blocks using Eqn (15.2) and vice versa with minimum error.
15.6.10. Validation of the model
To use the relation Eqn (15.3), for a given set of materials, the strength developed at a specified age for a b/f ratio of 5.0 (or f/b = 0.2) is to be determined experimentally. Using this as an input parameter in the equation, the b/f ratio of any other desired strength can be calculated using the phenomenological model.
A separate series of experimental data was generated to examine the predictions made using the phenomenological model. The strength developed at other f/b ratios is calculated and tabulated in Table 15.4 for comparison with experimental values. The correlation between experimental and predicted strength values is shown in Figure 15.22. There is a close match between the experimental and predicted values reinforcing the applicability of the phenomenological model. With more data being generated for a still wider range of b/f ratio the scope of this phenomenological model can further be enhanced.
15.6.11. Strength development with size and shape
The majority of the compressed blocks were either cylinders of 38 mm diameter and 76 mm height or the blocks of size 200 × 110 × 60 mm, which were prepared with manual compression. To correlate the compressive strength of these blocks, a series of blocks were prepared using a motorized hydraulic compressed machine. The shape and size of the blocks selected were such that they could be directly used in the field once prepared. For speedier production and commercial usage, the blocks should be made using a motorized hydraulic compressed machine, which is currently in use to make cement blocks. Compressed blocks of PB1, PB2, PB3 and CB1 were made by employing the motorized hydraulic machine and of CB2 and CB3 with a manually operated device. Size and shapes of the blocks are shown in Table 15.5 and Figure 15.23. The results of the compressive strength test of various blocks are shown in Figures 15.24 and 15.25. It can be seen from these figures that strength development is the same for different sizes and shapes of the blocks and pavers. There is no effect of scale and size of the blocks on the strength development. This encourages the use of these blocks as masonry blocks and pavers.
Table 15.4
Comparison of strength data using Eqn (15.3)
| Series ID | Fluid-to-binder ratio | Binder-to-fluid ratio | Predicted value (MPa) | Experimental value (MPa) | Error (%) |
| ABS1 – 1D | 0.15 | 6.66 | 4.43 | 4.26 | −3.90 |
| 0.175 | 5.714 | 3.75 | 3.78 | 0.88 | |
| 0.2 | 5 | 3.23 | 3.32 | 2.60 | |
| 0.225 | 4.44 | 2.83 | 2.75 | −2.96 | |
| 0.25 | 4 | 2.52 | 2.5 | −0.61 | |
| ABS2 – 3D | 0.15 | 6.66 | 5.04 | 4.76 | −5.87 |
| 0.175 | 5.714 | 4.27 | 4.26 | −0.14 | |
| 0.2 | 5 | 3.68 | 3.78 | 2.60 | |
| 0.225 | 4.44 | 3.22 | 3.32 | 2.90 | |
| 0.25 | 4 | 2.86 | 2.75 | −4.14 | |
| ABS3 – 7D | 0.15 | 6.66 | 6.01 | 5.78 | −4.03 |
| 0.175 | 5.714 | 5.09 | 5.15 | 1.17 | |
| 0.2 | 5 | 4.39 | 4.51 | 2.60 | |
| 0.225 | 4.44 | 3.85 | 3.92 | 1.88 | |
| 0.25 | 4 | 3.42 | 3.32 | −2.91 | |
| ABS4 – 1D | 0.15 | 6.66 | 2.68 | 2.92 | 8.23 |
| 0.175 | 5.714 | 2.27 | 2.44 | 7.04 | |
| 0.2 | 5 | 1.96 | 2.01 | 2.60 | |
| 0.225 | 4.44 | 1.71 | 1.6 | −7.14 | |
| 0.25 | 4 | 1.52 | 1.5 | −1.52 | |
| ABS5 – 3D | 0.15 | 6.66 | 5.39 | 5.2 | −3.58 |
| 0.175 | 5.714 | 4.56 | 4.62 | 1.32 | |
| 0.2 | 5 | 3.93 | 4.04 | 2.60 | |
| 0.225 | 4.44 | 3.45 | 3.49 | 1.28 | |
| 0.25 | 4 | 3.06 | 2.95 | −3.75 | |
| ABS6 – 7D | 0.15 | 6.66 | 5.44 | 5.62 | 3.21 |
| 0.175 | 5.714 | 4.60 | 4.76 | 3.27 | |
| 0.2 | 5 | 3.97 | 4.08 | 2.60 | |
| 0.225 | 4.44 | 3.48 | 3.49 | 0.30 | |
| 0.25 | 4 | 3.09 | 2.92 | −5.86 | |
| Table Continued | |||||
| Series ID | Fluid-to-binder ratio | Binder-to-fluid ratio | Predicted value (MPa) | Experimental value (MPa) | Error (%) |
| ABS7 – 1D | 0.15 | 6.66 | 3.20 | 3.2 | 0.01 |
| 0.175 | 5.714 | 2.71 | 2.79 | 2.92 | |
| 0.2 | 5 | 2.34 | 2.4 | 2.60 | |
| 0.225 | 4.44 | 2.05 | 2.02 | −1.32 | |
| 0.25 | 4 | 1.82 | 1.8 | −1.01 | |
| ABS8 – 3D | 0.15 | 6.66 | 3.81 | 3.78 | −0.87 |
| 0.175 | 5.714 | 3.23 | 3.27 | 1.30 | |
| 0.2 | 5 | 2.79 | 2.86 | 2.60 | |
| 0.225 | 4.44 | 2.44 | 2.52 | 3.21 | |
| 0.25 | 4 | 2.17 | 2.15 | −0.78 | |
| ABS9 – 7D | 0.15 | 6.66 | 5.52 | 5.36 | −2.98 |
| 0.175 | 5.714 | 4.67 | 4.76 | 1.85 | |
| 0.2 | 5 | 4.03 | 4.14 | 2.60 | |
| 0.225 | 4.44 | 3.53 | 3.57 | 1.10 | |
| 0.25 | 4 | 3.14 | 3.07 | −2.16 | |
| ABS10 – 1D | 0.15 | 6.66 | 3.51 | 3.66 | 4.20 |
| 0.175 | 5.714 | 2.97 | 3.12 | 4.87 | |
| 0.2 | 5 | 2.56 | 2.63 | 2.60 | |
| 0.225 | 4.44 | 2.24 | 2.24 | −0.13 | |
| 0.25 | 4 | 1.99 | 1.87 | −6.55 | |
| ABS11 – 3D | 0.15 | 6.66 | 4.17 | 4.25 | 1.81 |
| 0.175 | 5.714 | 3.53 | 3.74 | 5.56 | |
| 0.2 | 5 | 3.05 | 3.13 | 2.60 | |
| 0.225 | 4.44 | 2.67 | 2.82 | 5.34 | |
| 0.25 | 4 | 2.37 | 2.2 | −7.79 | |
| ABS12 – 7D | 0.15 | 6.66 | 4.57 | 4.24 | −7.85 |
| 0.175 | 5.714 | 3.87 | 3.82 | −1.33 | |
| 0.2 | 5 | 3.34 | 3.43 | 2.60 | |
| 0.225 | 4.44 | 2.93 | 3.02 | 3.14 | |
| 0.25 | 4 | 2.60 | 2.7 | 3.76 | |
| Table Continued | |||||
| Series ID | Fluid-to-binder ratio | Binder-to-fluid ratio | Predicted value (MPa) | Experimental value (MPa) | Error (%) |
| ABS13 – 1D | 0.15 | 6.66 | 1.69 | 1.77 | 4.34 |
| 0.175 | 5.714 | 1.43 | 1.44 | 0.47 | |
| 0.2 | 5 | 1.24 | 1.27 | 2.60 | |
| 0.225 | 4.44 | 1.08 | 1.15 | 5.82 | |
| 0.25 | 4 | 0.96 | 1.01 | 4.74 | |
| ABS14 – 3D | 0.15 | 6.66 | 20.97 | 20.22 | −3.72 |
| 0.175 | 5.714 | 17.75 | 17.25 | −2.91 | |
| 0.2 | 5 | 15.32 | 15.73 | 2.60 | |
| 0.225 | 4.44 | 13.41 | 14.64 | 8.37 | |
| 0.25 | 4 | 11.92 | 12.5 | 4.66 |
15.7. Conclusions and future trends
It was noted that, apart from f/b ratio, air content in compressed blocks (degree of saturation) also affects strength development with age. If the air content is maintained constant then the strength development is in accordance with Abrams' law for the specified range of f/b ratio. Silica fume can be used as one of the components of binder if the strength required is high and also in situations where architectural aesthetic appeal is called for. Phenomenological models can be developed to reproportion the materials for making geopolymer compressed masonry blocks. The phenomenological model developed can be used for a given set of materials and conditions. If there is any change in the properties of materials or conditions, a fresh reference value of strength at f/b ratio of 0.2 (Sf/b = 0.2) shall be generated to make use of the model. A wide range of reference strength data would be useful to proportion the materials for the given strength at ambient conditions. The masonry blocks can be made using locally available materials, which are rich in silica and alumina.
Further research about geopolymer mortar masonry blocks is needed in order to clarify several aspects that current knowledge does not, such as the following:
1. The f/b ratio can be further decreased for better strength and use of appropriate plasticizers need to be invented to compensate the workability without compromising the economy.
2. Various combinations of locally available aluminosilicates as base materials need to be investigated.
3. The use of silica fume as a base material needs to be further investigated without compromising autogenous shrinkage, compressive strength, color and economy. The surface finish and color of the blocks may avoid plastering. The attractive light color of the blocks may influence the architects to use them as facade elements.
4. Long-term field studies need to be carried out to establish endurance of the blocks.
5. The commercialization of geopolymer masonry blocks may be possible only if the properties are comparatively better than the conventional masonry units available in the market without compromising the economy.
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