EFFECT OF ACTIVATOR AND WATER TO BINDER

RATIOS ON SETTING AND STRENGTH OF

GEOPOLYMER CONCRETE

Aimin Xu, ARRB Group Ltd, Australia

Ahmad Shayan, ARRB Group Ltd, Australia

ABSTRACT

The setting and compressive strength of geopolymer mortar and concrete made with fly ash as binder and blended fly ash and slag (ratio of fly ash to slag varied from 95:5 to 50:50) activated by two forms of activators, liquid activator composed of sodium hydroxide and water glass (liquid sodium silicate) and solid sodium meta-silicate anhydrous beads, were investigated. The compressive strength of mortars and concretes increases as the water content expressed as the water to geo-solid ratio decreases and the amount of activators expressed as alkalis to binder ratio increases. For the blended binder, the relation between strength and alkali in activator is better expressed as the mole ratio of total silica and alumina to total alkalis (including CaO). It was observed that if the ratio of alkali to silicate in the liquid activator was higher than certain values, the mix would set rapidly. The solid activator which had a high alkali to silicate ratio, resulted in rapid setting for mixes made from the fly ash-slag blended binder especially if the water content in mix was low. It is demonstrated that the setting problem of the geopolymer concrete made with the solid activator and low water to binder ratio can be resolved by using an appropriate mixing process.

INTRODUCTION

Geopolymer for concrete refers to a type of inorganic polymer synthesized from alkaline activation of industrial by-products such as blast-furnace slag (slag), fly ash (FA) or a mixture of them. The composition of geopolymer is a type of aluminosilicate hydrates which can be generally expressed as compound of Na2O-Al2O3-SiO2-H2O and CaO-Na2O-Al2O3-SiO2-H2O for FA and slag based geopolymer respectively1, 2. Research on the structure of geopolymers reveal that in geopolymer the atoms of Al penetrate the originally silicate lattice of fly ash producing a type of “chemically bonded ceramics” with the general formula Mn[-(Si-O)z -Al-

O]nwH2O, and an elevated temperature helps to establish an interconnected lattice of bonds in the geopolymer.3

With increased concerns on global warming, geopolymer has been considered as an environmental friendly alternative to ordinary Portland cement (OPC) based concrete4. Numerous studies on the engineering properties, e.g. the strength development, have been conducted worldwide5, 6, 7 and field applications have been in progress in Australia in the past decade8.

However, the mix designs of geopolymer concretes as presented in published papers were not clearly given in terms of alkali content, amount of activator and water to binder ratio, and how these factors influenced the property of concrete, due to the variations in composition of activators used in different research works. The most commonly used activators appeared to be sodium hydroxide (NaOH) solution of various molarity, water glass (WG) with different modulus (Ms, the mass ratio of silicate dioxide to sodium oxide), combination of NaOH solution and water glass, and solid sodium silicates. These activators vary widely in their alkali content and water content.

For example, the commonly presented composition of a liquid activator was expressed as the ratios of the water glass to the alkali solution9. This practice causes ambiguity in the magnitude

of the water to binder ratio as well as the alkali to raw material ratio, thereby generating uncertainty in analysing concrete properties, which was often ignored in the literature.

Moreover, the published research clearly demonstrated that slag-based geopolymers behaved satisfactorily in terms of strength development, but fly ash-based geopolymers cured at ambient temperature showed unacceptably long setting times and slow strength development. It is beneficial to use a binder with combined FA and slag, but it was not clear what an appropriate activator to binder ratio would be.

This study aims to investigate the strength development of mortar and concrete made from high fly ash-low slag binders to find an appropriate guideline for the mix proportion of geopolymer concrete. In this work, we studied parameters such as the ratio of water to geopolymer solids (the solid part of activator plus binder10), ratio of waterglass (Ms=2) to binder, and ratio of solid NaOH to waterglass. This makes the comparison of mix proportions of different geopolymers possible.

Preliminary tests showed that the setting time and strength development of geopolymer greatly depended on the composition of the raw materials and activator, e.g. pure FA based geopolymer would not harden after one day at room temperature. On the other hand, it has been reported that treatment at elevated temperature, e.g. at 40-80°C for 6 hours, is necessary for fly ash-based geopolymer concrete to achieve a strength comparable to or even exceed that of OPC concrete,3,11. Therefore, the effect of early age curing at elevated temperatures was also investigated.

EXPERIMENTAL DETAILS

Being a replacement for concrete, the geopolymer must meet the requirements of handling which primarily means a good workability and appropriate setting time for the fresh mix, and adequate strength development for the hardened concrete.

In this study, experimental work was conducted to investigate the above properties of geopolymer composed of fly ash or combination of fly ash and slag as binder, activated by alkaline activators (liquid activator and solid activator). Special attention was paid to the influence of the water content of the mix and alkaline activator to binder ratio on the strength gain. Mortars of various combinations were made and tested to obtain understanding of the geopolymer, and then concrete samples were made to verify the findings.

Materials

The materials used in this study were a fly ash and granulated blast furnace slag as binder, two types of activators, sand and coarse aggregate.

Aggregates

A concrete sand of modulus of fineness of 2.2, and crushed basaltic aggregate of size 7-14 mm were used.

Binder

A binder was a combination of an Australian fly ash (FA) supplied by Cement Australia, and a construction grade slag. The main chemical compositions of the FA and the slag are given in Table 1.

The ratios of FA to slag in the various binders were: 95:5, 92.5:7.5, 90:10, 80:20, 70:30, 60:40 and 50:50.

Table 1: Main composition of binder materials (% by mass)

Material SiO2 Al2O3 F2O3 MgO CaO Na2O K2O SO3 LOI

FA 51.5 27.6 11.8 1.3 2.2 0.4 0.6 0.2 0.3

Slag 32.0 13.0 0.4 4.9 41.5 0.2 0.4 5.8 1.8

Activators

Two types of activators were used: liquid activator and solid activator. The liquid activator was a combination of NaOH dissolved in water, and sodium silicate solution. NaOH pellet of purity 97% and Type D waterglass (Ms=2.0) were used. The solid activator was an anhydrous sodium meta-silicate, Na2SiO3. The compositions of the sodium silicates are given in Table 2.

Table 2: Composition of sodium silicates (% by mass)

Material SiO2 Na2O Water

Type D waterglass 29.4 14.7 55.9

Na2SiO3, anhydrous 47.6 51.5 0.9

The amount of water required for the mix, in addition to the water contained in the liquid activator, was determined by the required water to geopolymer solid (Geo-solid) ratio for the mortar or concrete mix. The “geopolymer solids” is the total amount of solid phase in geopolymer, including the binder (fly ash, slag, etc.) and the solid part in the sodium silicate and sodium hydroxide. The other two variables of liquid activator are the amount of waterglass (WG), which is determined by WG to binder ratio, and the amount of solid NaOH determined by

NaOH to water glass ratio. The specific gravity of 10 M NaOH solution (tested = 1.3) was used as a reference value for estimating the volume of actual NaOH solution in the mix by adjusting the difference in water volume according to the water to geo-solid ratio in the mix design.

For the solid activator, the variables are the water to geopolymer-solids ratio and activator to

binder ratio for the mix. The tested specific gravity of the activator is = 3.4.

Tests and Specimens

The main properties of mortar and concrete investigated were the workability and compressive strength at different ages. The setting times of mixes were determined when they were short enough to be measured during working hours.

To assess the workability of fresh mixes, the flow of mortar was determined according to ASTM C1437,12 and the slump and compressive strength of concrete was determined according to AS1012-3 d13 and AS1012-914, respectively.

Mortar cubes of size 50 x 50 x 50 mm, and concrete cylinders of 100 mm in diameter and 200 mm in length were fabricated for strength testing at different ages.

Mortar mixes

Mortar cubes were made with sand to binder ratios of 2.5, 2.0, 1.5 and 1.7, and various water to “geopolymer-solids” ratios. It was originally decided to use a sand to binder ratio of 2.5 for all mortar mixes, but test results showed that the mixes with low water to geopolymer-solids ratio was unworkable at this sand content. On the other hand, the sand to binder ratio for concrete of 40MPa Grade (or higher grade) was about 1.7 or lower. Therefore the sand to binder ratio of 1.7 was used for the later studies. Nevertheless, the results of mortars with high sand content are still useful and are presented here.

Concrete mixes

Concretes were made from selected binder compositions based on the results of mortar tests, in order to verify whether the trends shown by the mortars are also shown by the concretes. The basic concrete mix used contained a binder (FA, or combined FA and slag) content of 400 kg/m3, coarse aggregate 1100 kg/m3, and various amounts of activator and water, depending on the selected activator to binder ratio and water to geopolymer-solids ratio.

The values of normal consistency (water demand) tested for the slag and FA were 0.33 and 0.19 respectively, i.e., the slag absorbs more water than the FA. To compensate for the water absorption of the slag, higher water to geopolymer-solids ratios were used in the mix design than that for the pure FA based geopolymer.

The variations in the masses of activator and water in the mixes are associated with volume change which was adjusted by varying the amount of sand so that the volume of the unit mix was constant.

Amount of activator

Research on alkali activated slag by Kutti15 demonstrated that at Na2O to binder ratio of 5% (or less, depending on the type of slag) the mortars produced adequate strength. Hardjito and Rangan16 proposed that for a low lime FA-based geopolymer concrete mix the mole ratio of Na2O to SiO2 should be 0.095 – 0.120. This is approximately 5 to 6% of Na2O by mass of an FA that contains 50% Si2O. Considering that slag and FA have very different compositions (amount of SiO2, Al2O3 and CaO, etc.) while the requirement for alkali to activate them is similar, it can be said that the effect of alkali introduced into the mix is mainly to activate the raw material rather than being fully involved in the final structure. In this study, the amount of activators varied around 5% by binder mass.

Regarding the composition of water glass and concentration of NaOH solution, and the ratio between them, the values given in the literature varied widely, depending on the materials used. Mustafa Al Bakri et al9 studied Na2SiO3/NaOH ratios from 0.5 to 3 and concentration of NaOH solution from 6 M to 16 M, and concluded that the Na2SiO3/NaOH ratio of 2.5 and 12 M NaOH solution produced the highest strength. However, various waterglasses (due to supply) and NaOH solution molarities were also used by other researchers, e.g. Ms 2.7 waterglass and 8 M and 14 M NaOH solution by Rangan10 showed higher strength for the activator made with the 8 M NaOH solution.

The controversy was partly due to the effect of water to binder ratio, which played an essential role in the concrete strength development, but was not adequately analysed in the above investigations which were designed mainly based onthe ratio of water glass to NaOH solution.

In the current investigation, the mix proportions were designed by selection of water to geopolymer-solids ratio (W: Geo-solid = 0.175 to 0.215), amount of water glass to binder (WG: FA = 0.10 to 0.33) and ratio of water glass to solid NaOH (WG: NaOH = 2.5, 5.0, 8.1). Note: ratio of WG/NaOH= 8.1 is equivalent to the ratio of 2.5 for the WG to a 10 M NaOH solution.

Curing conditions

The mortar cubes made from fly ash-based geopolymers were cured at 38°C, 100% RH until age of one day. The cubes were then removed from moulds and placed in a fog-room at 23°C until strength testing.

For mortar or concrete made from blended (fly ash+slag)-based geopolymers, the specimens after compaction were stored in the fog-room (23 °C) until the age of one day when the specimens were removed from mould, and further cured in the fog-room until strength testing.

Three concrete mixes were tested for the effect of initial steam curing on the strength development. The specimens after initial setting were stored under steam curing conditions of

40°C, 60°C and 80°C. The rate of temperature increase for the steam curing was 20°C/hour, and the total curing time at the ultimate temperature of 40°C, 60°C and 80°C was 6 hours, 5 hours and 4 hours, respectively. The stream cured specimens were cooled down to 23°C within the chamber. After the initial steam curing, all specimens were cured at 23°C and 100% RH until strength testing.

RESULTS AND DISCUSSION

Geopolymer made with liquid activator

Geopolymer activated by the liquid activator showed a large variation in workability and strength gain depending on the amount of alkali in the activator and water in the mix. A small amount of Ca-rich material, e.g. cement, lime and slag, significantly enhanced the strength gain.

Workability and setting time of geopolymer with liquid activator

When properly proportioned, the geopolymer mixes made with the liquid activator had a higher workability than OPC mixes of the same water content, which could be attributed to the slipperiness of these mixes due to the presence of the alkaline solutions, i.e. the friction between particles especially the coarse aggregate grains was low due to the presence of alkaline liquid, which gave rise to a high flow or slump.

Preliminary experiments showed that FA-based geopolymer specimens made with the liquid activator would not set or harden even after one day, if the alkali content in the activator was lower than certain amounts. This phenomenon is also reported in the literature, e.g. by Hardjito et al17 and Cheng and Sarkar18. For the 100% FA-based geopolymer specimens to obtain enough strength, so that they could be demoulded at the age of one day, they needed to be stored at 38°C after the completion of casting.

It was also found that with the increase in NaOH content in the activator, the setting time became shorter. For a high alkali content liquid activator, e.g. water glass to solid NaOH ratio of 2.5, the mortar was found to set within minutes. This is in agreement with the observations of Mustafa Al Bakri et al9 that as NaOH molarity increased the unreacted FA amount decreased, and more crystalline phases were detected in the hydration products of the geopolymer.

Qualitative tests on the liquid activator, comprising waterglass (Ms=2) and 10 M NaOH solution, showed that, at the WG/ NaOH ratio of 2.63, some precipitation of solid phases occurred in the mixed activator the day after they were mixed. At the ratio of 1.58 the mixed activator solution solidified within 4.7 hours, and at the ratio of 1.05 the mixed activator solution formed a soft mass in 1.4 hours. The solidification of the activator may be one of reasons for the rapid setting observed for the mortars made with high alkali content activators.

The above observations indicate that the setting time of FA-based geopolymers activated by the liquid activator can be managed by changing the NaOH content in the activator, i.e. activator solutions with high NaOH to water glass ratios would accelerate the setting, and vice versa.

Strength of mortars

Table 3 shows selected data for some of the mortars. The effect of the amount of activator, presented as the mole ratio of total (silica + alumina) to alkalis, and the mass ratio of Na2O to binder on the 7-day strength of mortar are shown in Figure 1 and Figure 2, respectively.

Table 3: Mix proportions and test results for mortars activated by liquid activator

ID Slag: binder

Water: Geo-

solids

WG: NaOH

WG: binder

Na2O: binder

(Si+Al): Alkalis*

Flow (mm)

7-day strength

(MPa)

M1-1 0 0.215 2.5 0.165 0.075 10.1 105 42.8

M1-2 0 0.215 5.0 0.330 0.100 8.2 110 49.4

M1-3 0 0.255 2.5 0.165 0.075 10.1 132 14.5

M1-4 0 0.255 5.0 0.330 0.100 8.2 141 32.5

M2-1 0 0.215 2.5 0.165 0.075 10.1 120 26.3

M2-2 0 0.215 2.5 0.100 0.046 15.6 111 11.1

M2-3 0 0.175 2.5 0.165 0.075 10.1 103 45.5

M3-1 0 0.215 2.5 0.165 0.075 10.1 150 23.0

M3-2 0 0.215 2.5 0.100 0.046 15.6 149 9.1

M3-3 0 0.215 5.0 0.165 0.050 14.8 164 19.7

M3-4 0 .0215 5.0 0.100 0.030 22.6 149 2.5

M3-5 0 0.175 2.5 0.165 0.075 10.1 113 42.9

M3-6 0 0.175 2.5 0.100 0.046 15.6 106 22.4

M3-7 0 0.175 5.0 0.165 0.050 14.8 115 41.9

M3-8 0 0.175 5.0 0.100 0.030 22.6 103 11.3

M4-1 0 0.215 8.1 0.250 0.061 12.8 119 43.9

M4-2 0 0.215 8.1 0.330 0.080 10.1 120 50.3

M4-3 0 0.195 8.1 0.250 0.061 12.8 108 50.9

M4-4 0 0.195 8.1 0.330 0.080 10.1 112 61.6

M5-1 0.10 0.213 6.3 0.250 0.067 5.5 153 41.4

M5-2 0.10 0.230 8.1 0.182 0.044 6.4 152 20.3

Notes: * mole ratio of (SiO2+Al2O3)/(Na2O+CaO). Sand to binder ratio for M1, M2, M3, M4 and M5 was 2.5, 2.0, 1.5, 2.5 and 1.7 respectively. Mixes M1-M4 were initially cured at 38°C for 1 day, then at 23°C. M5 was cured at 23°C only.

Figure 1: Effect of amount of activator on 7-day strength of mortars.

Figures 1 shows that the increase in the amount of activator resulted in higher strength. It also shows that to achieve a reasonable strength, e.g. 20 MPa at 7 days (including the initial curing at 38°C), the mole ratio of the active ingredients in the FA-based geopolymer binder (silica and alumina) to alkali should be below 13. Similarly, the total amount of alkali as Na2O should be about 6% by binder mass. Incorporation of a small amount of slag, e.g., 10% slag by binder

mass, as in Mix M5, significantly enhanced the strength gain for mixes of the same water/binder ratio and activator contents. For example, as shown in Figure 1-left, the rate of strength gain for Mix M5-1, which had a water/binder ratio of 0.215, was similar to that of Mix M3-5 which had a water/ binder ratio of 0.175, whereas the latter would have developed much lower strength at the water/binder ratio of 0.215. The effect of slag incorporation could be due to the fact that Ca in the slag contributed to the microstructural development in the resulting geopolymer.

The effect of the water to geopolymer-solids ratio is demonstrated in Figure 2 for the FA-based geopolymer mortars with total (silica + alumina) to Na2O mole ratio of 10. It clearly shows that the lower water to geopolymer-solids ratio led to higher strength. Comparatively, at the same water to geopolymer-solids ratio of 0.215, Mixes M1 and M4 developed significantly higher strength than Mixes M2 and M3, which can be attributed to the higher content of water glass and the higher sand to binder ratio in M1 and M4, although other factors could also be involved.

Figure 2: Dependence of strength of mortar on water to geo-solid ratio.

Blended (FA + slag) geopolymer made with solid activator

It was reported in the literature that if a small portion of slag was blended into the FA based binder, the geopolymer would develop higher strength as well as could harden in a similar way as OPC. A study by Wang and Scrivener19 showed that the main hydration product of alkali activated slag is calcium silicate with low Ca/Si ratio (compared with cement hydration products), and some crystalline phases. These products fill up the pores as well as creating more bonds between particles, and are very beneficial to the strength development of geopolymer, which has a much lower water to binder ratio than OPC concrete.

This part of the study was focused on blended (FA+slag)-based binder, activated by solid activator (anhydrous sodium meta-silicate). Based on the results of geopolymer mortars made with liquid activator, the amount of activator to the blended binder was targeted such that the mass ratio of Na2O (present in the activator) to the raw material was 0.035 and 0.06 for the slag and FA, respectively.

Mortar and concrete mixes were made and their workability determined. The sand to binder ratio for mortars was 1.7, which was the same as that for the concrete. Both the mortar and concrete specimens were cured in the fog-room at 23°C until strength testing.

The mole ratio of SiO2/ Na2O of the solid activator was 0.953. As shown earlier in this paper, liquid activators of the same alkali content could cause rapid setting. Indeed, preliminary tests revealed that the mortar and concrete made with this activator set rapidly unless the water to geopolymer-solid ratio was very high.

Workability and setting time of geopolymer with solid activator

It is important that a concrete mix is workable during the time of casting and compaction. This seems to be a problem for the geopolymer made with the solid activator, the anhydrous sodium meta-silicate. Although the workability of both mortar and concrete mixes was very good when the mixing was completed, e.g. all concrete mixes shown in Table 5 had a slump of about 150 mm, the workability decreased with time and most of the mixes set within one hour.

It was also observed that during mixing, the mortar mixture appeared dry at the beginning, but it became plastic after mixing for several minutes. This feature can be utilised to mitigate the setting problem as described below.

To allow enough time for the compaction, the mixing procedure used was as follows: the raw material except the activator were mixed to a homogenous mixture. The activator was introduced into the mix only prior to the casting. In the first stage, water reducer or super-plasticizer can be used to enhance the workability to a point that the mix becomes plastic so that some mixing water remains for dissolving the activator.

Strength of mortar and concrete cured at 23°C

Mix proportions and strength of mortars and concretes (cured at 23°C and 100% RH) are presented in Table 4 and Table 5 respectively.

Table 4: Mortars made using blended (FA+slag) binder and solid activator

ID Slag: binder

Water: Geo-solid

Activator: binder

(SiO2+Al2O3): (Na2O+CaO)

Na2O: binder

7-day strength

(MPa)

M6 0.05 0.210 0.111 7.179 0.057 29.4

M7 0.05 0.193 0.111 7.179 0.057 25.8

M8 0.075 0.207 0.115 6.326 0.059 28.5

M9 0.10 0.243 0.087 6.284 0.045 14.8

M10 0.10 0.222 0.075 6.569 0.039 6.3

M11 0.10 0.243 0.110 5.817 0.056 23.2

M12 0.10 0.213 0.110 5.817 0.056 31.6

M13 0.20 0.245 0.109 4.120 0.056 49.8

M14 0.30 0.242 0.098 3.170 0.051 51.7

M15 0.30 0.276 0.085 3.241 0.044 51.2

M16 0.30 0.260 0.105 3.136 0.541 54.8

M17 0.40 0.280 0.093 2.502 0.048 52.5

M18 0.40 0.280 0.101 2.479 0.052 63.0

M19 0.50 0.295 0.088 2.025 0.045 47.3

M20 0.50 0.295 0.100 2.004 0.052 59.8

Table 5: Concrete made using blended (FA+slag) binder and solid activator

ID Slag: binder

Water: Geo-

solids

Activator: binder

Na2O: binder

Strength (MPa), 23°C 7d strength (MPa), initial steam cure

1 d 7 d 28 d 91 d 40° 60° 80°

C1 0.075 0.313 0.120 0.618 2.1 10.2 18.5 — 9.2 12.8 21.7

C2 0.075 0.247 0.120 0.618 4.5* 12.4 26.4 — 13.7 18.9 28.7

C3 0.075 0.224 0.120 0.618 6.3* 20.0 35.0 — 22.6 29.0 36.4

C4 0.075 0.208 0.114 0.587 5.0* 17.2 39.1 48.3 — — —

C5 0.15 0.218 0.111 0.618 9.9* 31.8 56.5 68.1 — — —

C6 0.50 0.400 0.081 0.618 6.5 30.2 38.1 — — — —

Note: * interpolated value according the strength tested at age of 1.9-day.

The strength of mortars and concretes made with a blend of FA and slag increased with the increase in slag content up to about 40%, and there were large variations in strength for the mixes of the same slag content, depending on the amount of activator (Figure 3).

The strength of the mortars as a function of the alkali content could roughly be separated into two regions: the mortars with slag content higher than 20% and those equal or lower than 10%. The former obtained higher strength than the latter. For the high slag binders, the strength slightly increases with the alkali content, whereas for the low slag binders, the strength significantly increases with the alkali content until the ratio of Na2O to binder is about 0.6. (Figure 4-left).

This can be attributed to the effect of calcium contained in the slag, which contributed to the strength as a result of calcium ions reacting with the activated silica and alumina to form calcium silicate and calcium aluminosilicate hydrates, which effectively act as bonding agents and also fill the pores which originated from the consumption of the mixing water.

Figure 3: Dependence of strength of mortar (cured at 23°C) on slag content.

Plotting the strength against the mole ratio of (silica + alumina) to the total alkali, i.e., (SiO2 + Al2O3)/ (Na2O + CaO) clearly indicates that an increase in the total alkali (i.e., smaller ratio) leads to higher strength, as shown in Figure 4-right. Figure 4 also shows that the strength is higher when the mole ratio (SiO2 + Al2O3)/ (Na2O + CaO) equals 4 (or 3), but further increase in total alkali content does not increase the strength. The trend shown in this figure also indicates that at the mole ratio of 5, the strength would be around 40 MPa, which may be considered adequate.

An adequate amount of alkalis is essential for the activation. For the FA-based binder, a mass ratio of Na2O/binder = 0.06 or slightly higher was required to obtain a good strength (e.g. 40 MPa at 7 days). The amount of effective alkali in the activator was corresponding to the (SiO2+Al2O3)/(Na2O) = 13. For the blended (FA + slag) binder, the Ca in slag contributes to the activation, i.e., the slag requires less alkali for the activation than does FA.

The effect of the water to geopolymer-solids ratio on the strength of these mortars (Figure 5) was somewhat masked by the effects of slag and alkali contents. Nevertheless, the trend of strength development shown in Figure 5 suggests that a decrease in the water to geopolymer-solids by 0.03 would result in a strength increase of about 10MPa.

Figure 4: Relationships between strength of mortar and alkali content.

Figure 5: Dependence of strength of mortar on the water to geo-solids ratio.

Similar to the behaviour of OPC-based concrete, the strength of geopolymer concrete also

increases with the curing age, as shown in Table 5 and Figure 6. The strength development

requirements for 40 MPa Grade concrete (e.g., VicRoads VR400/40), made with 400 kg/m3

binder, is represented by the dashed line in Figure 6. It is clear that the strength development

pattern of the blended (FA+slag)-based geopolymer concrete follows the same trend as that of

the OPC-based concrete.

One of the main features for the geopolymer binder is that, compared with OPC, it uses lower amounts of water. For example, the geopolymer mortar mix M1-1 has a water to geopolymer-solids ratio of 0.215 or water to binder ratio of 0.245, and geopolymer concrete C3 has a water to geopolymer-solids ratio of 0.224 or water to binder ratio of 0.25, whereas an OPC mortar and concrete of the same workability will require a water to cement ratio of about 0.4. As shown in Figure 2 and Figure 5, using high water to geopolymer-solids ratios (similar to that for OPC mortar) will result in unacceptable low strength for the geopolymers.

The effect of water to geopolymer-solids ratio on the strength of geopolymer concrete is demonstrated by Figure 7, which shows that the 28-day strength increased by about 20 MPa with the decrease in water to geopolymer-solids ratio from 0.31 to 0.21 (C1 to C4).

Figure 6: Strength development of geopolymer concrete compared to VicRoads requirements for VR400/40 concrete (dashed line)

Figure 7: Dependence of strength of concrete on water to geo-solid ratio.

Strength of concrete cured at elevated temperatures

After a brief initial steam curing at different temperatures, the geopolymer concretes achieved

significant strength gains. Figure 8 shows that In the temperature range of 23°C to 80°C, the

higher initial curing temperature led to higher strength gain. The 7-day strength of the concretes

subjected to 80°C steam curing was higher than that of their 23°C cured counterparts at age of

28 days (Table 5). This observation is in agreement with the finding that 38°C curing of the FA-

based geopolymer, made with liquid activator, accelerated the setting and strength gain.

It is likely that higher temperature promotes the dissolution of the glassy phases in FA and slag,

and enables the polymerisation to proceed faster and more fully.

Figure 8: Strength of concrete initially cured at elevated temperatures.

CONCLUSIONS

The following conclusions can be drawn from this work:

1. The strength of geopolymer is significantly influenced by the water content in the mix. The reduction in the water to geopolymer-solids ratio of 0.1 for concrete would result in a strength increase of about 20 MPa (28-day strength). A larger effect was shown for mortars.

2. The appropriate amount of activator for FA-based geopolymer, presented as equivalent Na2O, is about 6% by mass of FA. The mole ratio of total SiO2+Al2O3 to Na2O should be lower than 15:1. To obtain a satisfactory strength, the ratio should be 13:1 or lower.

3. A small proportion of slag blended with FA as binder (e.g., 5-10%) can significantly increase the strength of the geopolymer. This study showed that 30-40% slag was effective in strength development, particularly at ambient curing temperature.

4. The amount of activator required for activation of the blended (FA+slag) binder should consider the contribution of Ca to the strength gain. The ratio of (SiO2+Al2O3)/(Na2O+CaO) of around 5 would produce adequate strength.

5. The setting times of FA-based geopolymer activated by a liquid activator can be controlled by changing the NaOH to water glass ratio of the activator.

6. Geopolymer mixes made using anhydrous sodium silicate solid activator can quickly lose workability, and the handling of this type of geopolymer must consider this issue. This paper proposed a practical mixing process to overcome this problem

ACKNOWLEDGEMENT

The experimental work of this research was conducted under the AUTROROADS Project TS1835. The authors wish to express their gratitude to AUTROROADS for financing this research work.

REFERENCES

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AUTHOR BIOGRAPHY

Dr Aimin Xu is a senior research engineer at ARRB Group Ltd, Melbourne, Australia. He

received B.Sc. and PhD in civil engineering from Southeast University, China, and Chalmers

University of Technology, Sweden, respectively. His research areas include reinforced concrete

structure durability and various aspects of concrete technology.

Dr Ahmad Shayan is a Chief Research Scientist at ARRB Group, in the area of Durability of

Concrete Materials & Structures. His research interests include AAR, DEF, waste materials

utilisation in concrete, corrosion of steel reinforcement in concrete, application of CFRP for

confinement of AAR-induced expansion, utilisation of stainless steel in concrete under

aggressive environments, and Geopolymer concrete. He has written in excess of 200 papers

and 350 technical reports. He was chairman, Standards Australia Committee CE/12 for 10

years, and has served on IOC of many international conferences, and was chairman of the 10th

ICAAR, Melbourne, 1996. He is a foundation member of RILEM TC 191-ARP on accelerated

AAR testing. He won the prestigious Clunies Ross Medal in 2003 for his work on durability of

concrete structures. He also holds the position of Adjunct Professor at the Department of Civil

Engineering at both Swinburne University of technology and Monash University.

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• publish the work in printed format • publish the work in electronic format • publish the work online. The Author retains the right to use their work, illustrations (line art, photographs, figures, plates) and research data in their own future works The Author warrants that they are entitled to deal with the Intellectual Property Rights in the works submitted, including clearing all third party intellectual property rights and obtaining formal permission from their respective institutions or employers before submission, where necessary.