Thermally Activated Bentonite As a Supplementary Cementitious Material – A Review

Bentonite is a mixture of clay and non-clay minerals. Montmorillonite clay mineral is a dominant mineral in bentonite. Silica and alumina bond in the crystal structure of montmorillonite. Therefore, they cannot contribute to the pozzolanic reaction. Heat treatment of bentonite leads to the destruction of the crystal structure of montmorillonite and converting silica and alumina to reactive phases. Thermally activated bentonite (TAB) is a relatively low reactive pozzolan when used as partial replacement of Portland cement modifies both fresh and hardened properties of cement paste, mortar, and concrete. The most desired effects of TAB are: improve segregation resistance, reduce the rate of strength gain, and enhance concrete durability against sulfates, chlorides, and acids, in addition to economic and ecological beneficiations. This paper provides information related to heat treatment of bentonite clays and montmorillonite minerals, and their effects on the paste, mortar, and concrete when used as a partial replacement of Portland cement.


INTRODUCTION
The primary sources of supplementary cementitious materials (SCMs) are: naturally formed materials such as volcanic materials and diatomaceous earth, and by-products or wastes from some industries such as fly ash, silica fume, and blast furnace slag. Unfortunately, these sources are available in limited areas. Moreover, the increasing demand for SCMs necessitates investigation for additional sources. The clays and shale represent the successful additional sources due to their widespread and their richness in the oxides of silicon, aluminum, and iron. These oxides are often present combined in the crystal structure of clay minerals.Therefore, they exhibit low chemical reactivity. Heat treatment of clay leads to the destruction of the crystal structure of clay minerals, hence producing reactive phases of silica, alumina, and iron oxide. Clay minerals differ in chemical and mineral composition, which results in their different response to thermal activation [1]. Bentonite is impure clay consisting mainly from montmorillonite mineral [2]. The utilization of thermally activated bentonite (TAB) in concrete production is still uncommon, despite the thermally activated montmorillonite (TAM) has been successfully used in important projects such as Flaming Gorge Dam (completion in 1963) on the Green River in the USA [3].

THERMAL ACTIVATION OF BENTONITE
Heating leads to subsequent transformations in the structures of minerals that form bentonite. Montmorillonite mineral is the dominant mineral in bentonite. Therefore, the structural transformations in montmorillonite mineral representthe bentonite changes due to heating. Montmorillonite and bentonite pass into four successive stages during heating [4,5]: 1. Dehydration stage: loss of uncombined water (adsorbed and interlayered). The temperatures requirecompleting this stage varies according to initial moisture content, size of clay particles, type of cations fill the interlayer spaces and the presence of defects in mineral structure [6]. The temperatures recorded for dehydration completion were: 100 -150 °C [5], 150°C [7], 160°C for sodium montmorillonite and 180°C for calcium montmorillonite [8], and 200°C [1,9]. 2. Dehydroxylation stage: loss of hydroxyl group from the mineral structure. The most of OH group binds in aluminum octahedral coordinate (Al [VI] ) sheet. Therefore, the dehydroxylation leads to convert Al [VI] to 5-fold coordinate (Al [V] ) and tetrahedral coordinate (Al [IV] ) [1,4]. The structural transformation in octahedral sheet leads to loss the crystallinity of 1:1 clay minerals like kaolinite, but the 2:1 clay minerals like montmorillonite conserve their crystallinity despite the transformations in octahedral sheet [1]. The temperatures reported for the initiation and completion of the dehydroxylation ranged from 400 to 800 °C [1,2,[4][5][6][7][8]10].The factors that influenced the dehydroxylation temperature are: the type of exchangeable ions and chemical composition of mineral [5,6] especially the presence of iron in the mineral structure [6]. 3. Amorphization stage: partially or completely decomposition of crystalline structure and formation of new amorphous and/or cryptocrystalline reactive phases. This stage represents the goal of the thermal activation process; the optimum temperature required to achieving this aim was determined precisely in each experiment but there is no specific degree. According to Aljubury [2], the structure of Iraqi calcium bentonite is completely collapsed at 700°C while higher degrees were reported: 800°C [4,7], 830°C [8,10], and 900°C [5,6]. It is not certain what causes the differences in temperatures required for amorphization, but the differences may be attributed to the calcination program e.g: heating rate, holding period, and cooling rate. The types of montmorillonite and impurities have little effect on amorphization temperature [10]. 4. Recrystallization stage: formation of new crystalline inert phases due to high temperature effect; recrystallization was noticed at 920°C [7,10], 1050°C [6], and (1000 -1100) °C [4]. Thermal activation program followed in each study illustrate in Table 1.

POZZOLANICITY OF TAM AND TAB
The pozzolanic reactivity of TAM and TAB were investigated either with lime or with Portland cement. The proofs of the pozzolanic reaction include the decline of reactants and the formation of reaction products.
Taher [11] studied the chemical reaction between TAM and lime; the mix of 80% TAM at 1000°C and 20% lime, cured under 10-atmosphere pressure for 24 hours, was developing compressive strength of 55MPa. The reaction products detected by scanning electron microscope (SEM) were mainly C-S-H and C-S-A-H. In addition to the formation of C-S-H, the formation of C 4 AH x was also observed due to lime and TAM reaction [8,10].
The reactivity of TAM with lime depends on activation temperature. Moreover, the structure of produced C-S-(A)-H was strongly affected by the activation temperature, the incorporation of Al in the structure of C-S-H proportions to the reactivity degree [4]. Figure 1 illustrates the effect of activation temperature on the percentage of reacted montmorillonite, and Al/Si in the produced C-S-(A)-H. The pozzolanic reaction was also observed with Portland cement; the replacement of Portland cement by TAB at different percentages leads to gradual consumption of Ca(OH) 2 as shown in Figure 2 [12]. Fernandez et al. [1] stated that TAM, at 600°C, seems to be effective in depleting Ca(OH) 2 at 30% substitution of Portland cement in both cement paste and mortar. Grag and Skibsted [9] reported 36.8% consumption of TAM due to pozzolanic reaction in the paste consists of 30% TAM at 800°C and 70% white cement, using 0.5 water to powder ratio at 365 days age.

4.1.Fresh properties of cement paste
Darweesh and Nagieb [12] studied cement substitution by TAB at (3,6,9,12, and 15%) in cement paste; the study concluded that TAB incorporation causes linearly increase in water demand to achieve standard consistency of the paste, the stiffening effect was attributed to fineness, absorption, and pozzolanicity of TAB. The same results were reported at high replacement levels (30, 50, and 70%) [3]; but Trümer et al. [9] mentioned that TAB at 900°C and 30% substitution reduceswater demand slightly.The conflicting results indicate the importance of chemical and mineral composition, and thermal activation program in the behavior of TAB. On the other hand, the retardation effect on both initial [9,12] and final setting time was observed [3,9,12]; the extension in setting time proportioned to TAB content [12]. Both higher water demand [3,12] and lower reactivity of TAB [3,9,12] cause the retardation effect. Significant accelerated effect of initial setting time was noticed at 70% substitution due to apparent quicker absorption of TAB [3].

Hardened properties of cement paste
Cement paste with standard consistency shows increases in compressive strength and bulk density, and decreases in porosity due to cement substitution by TAB at 12% or less. This is attributed to densifying microstructure by pozzolanic reaction and filling effect of bentonite particles. The reverse effect was observed at 15% replacement due to higher water demand [12].

Fresh properties of cement mortar
Laidani et al. [7] studied the fresh properties of self-compacted mortars containing Ca-bentonite activated at 800°C for 3 hours and Na-bentonite activated at 800°C for 4 hours. The replacement percentages were (5, 10, 15, 20, 25, and 30%) for each type of TAB. The results obtained from the mini-slump flow test and V-funnel flow test are illustrated in Figure3. It is obvious that the bentonite type plays an important role in the flow behavior of mortar. In general, the incorporation of TAB into self-compacted mortar leads to increase in the dosage of superplasticizer admixture to maintain flow characteristics.

Hardened properties of cement mortar
TAM characterized by its low reactivity in the comparison with activated kaolinite [1].Therefore, the cement replacement by TAM or TAB leads to reduction of the rate of strength gain, but there is wide dissimilarity in the results of compressive strength of mortars that contain TAM or TAB. This is attributed to differences in chemical and mineral composition of both montmorillonite and bentonite types in addition to activation program and replacement level.
At 20% cement replacement, the strength activity indexes at 7 and 28 days were reported at different activation temperatures: 70 and 85% respectively at 500°C [13], 93 and 94% respectively at 800°C [14], and 101 and 104% respectively at 150°C [15]. At early ages of mortars (28 days or less), and replacement levels more than 20%, significant reductions in compressive strength were reported and the reduction increases as the replacement increases [3,9,14,15]; but the reduction in compressive strength disappears at later ages (56 -90 days) or is reversed into an increase at 30% replacement [3,9].
Bond strength with brick masonry of mortar containing 25% TAB at 150°C is similar to that of reference mortar [15].
TAB at 900°C and 30% replacement modifiesthe microstructure of mortar by reduction of overall porosity and refinement of the pore structure by increase of pore fraction less than 10 nm, as shown in Figure 4 [9].
Because of pozzolanic reaction and pores refinement, TAB improves the durability of cement mortar against alkali-silica reaction at 30% replacement [9] and sulfates attack at 20% replacement [14]. The depth of carbonation of mortar contains 30% TAB is twice the carbonation depth of reference mortar [9].
At elevated temperature (600-1000°C), the performance of mortars contain TAB, represented by mass loss and residual strength is slightly better than control mortar [7].

Fresh properties of concrete
Portland cement replacement by TAB at (3 -21%) leads to reduction of the slump value of concrete; the reduction in slump is proportionally linearly with TAB content, for instance, 50mm slump for control concrete reduces to 35mm for concrete at 21% replacement [16]. Fresh density of concrete decrease as TAB content increase, for example, fresh density of 2482 kg/m 3 for control concrete decreases to 2275 kg/m 3 at 21% replacement [16].
Fresh properties of self-compacted concrete containing TAB were investigated by Laidani et al. [18]; the study concluded the following results: • The increase in the percentage of replaced cement by TAB decreases the flowability of concrete, which requires an increase in superplasticizer dosage to achieve the targeted slump. • TAB incorporation leads to an increase in concrete viscosity.
• TAB increases the passing ability of concrete.
• Decreases the tendency to segregation as TAB content increase.
At the same conditions, the reduction in modulus of rupture is about half the reduction of compressive strength. In reinforced concrete beam, at 25% replacement, a reduction of 30% in cracking load and reduction of 20% in ultimate flexural strength were reported; these reductions associated with an increase in fracture deflection by 70% which enhance the ductility of flexural members [15].
Water absorption decreases as TAB content increase, for example, 21% replacement leads to reduces absorption by about 30% [16]. TAB enhances concrete durability by: • Increase the resistance to acids attack (sulfuric acid and hydrochloric acid) at (3 -21%) replacement [16], • significant reductions in chloride-ion penetration reach to 64% at 15% replacement [18] • Reduction of chloride migration coefficients by 66% at 30% replacement [9]. On the other hand, TAB reduces the resistance of concrete to the frost action at 30% replacement [9].

BENEFITS OF TAB AS AN SCM
There are common benefits forutilization ofthe pozzolanic materials as a partial replacement for Portland cement in the production of concrete, and the foremost of these benefits are the sustainability aspects. Each pozzolanic material has intrinsic properties that distinguish it from other materials and makes it more suitable for specific uses in concrete. The most popularcalcined clay as a pozzolanic material is the metakaolin (MK). Therefore, it is useful to compare TAB to MK to highlight the potential specific uses of TAB.
MK characterized by its high pozzolanic reactivity which leads to increase both the total heat of hydration and its generation rate [19,20], while TAM and TAB characterized by their lower