**1. Introduction**

The inordinate rise in the presence of greenhouse gases in the atmosphere is creating a pressing need to lower CO2 emissions by, among other methods, reducing the proportion of clinker in cement [1–3]. The Portland cement industry is presently deemed to account for around 7.4% of the close to 2.9 Gt of carbon dioxide emitted annually worldwide (value for 2016) [4]. In light of such facts, the industry is assessing the measures that could be taken to ensure zero emissions by 2050. Under review are the processes involved in clinker, cement and concrete production, construction procedures and cement-based material carbonation during and after service life [5]. As the mitigation technologies presently in place are believed to be insufficient to hit the net zero carbon target by 2050, innovative measures are called for, including carbon dioxide capture, utilization and storage (CCUS) [6] and flameless mineral calcination systems. One new proposal for the latter, the tube-in-tube helical method, features use in concentrated solar power plants [7].

In the past, clinker content has been replaced with industrial waste such as fly ash, slag or silica fume with no adverse effect on concrete mechanical performance or durability [8]. With the abatement of the likewise carbon-intensive source industries, however, the availability of those substitute mineral additions is beginning to wane. Although the contribution of coal power plant-fueled energy to total consumption declined in Spain from 20.2% in 1990 to 9.8% in 2017 thanks to the growing use of alternative energies [9], coal continued to supply 38.5% of world demand in 2018. Concerns about greenhouse

**Citation:** Andrade, C.; Martínez-Serrano, A.; Sanjuán, M.Á.; Tenorio Ríos, J.A. Reduced Carbonation, Sulfate and Chloride Ingress Due to the Substitution of Cement by 10% Non-Precalcined Bentonite. *Materials* **2021**, *14*, 1300. https://doi.org/10.3390/ma14051300

Academic Editor: Alessandro P. Fantilli

Received: 13 February 2021 Accepted: 3 March 2021 Published: 8 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

gas emissions cloud the future of coal, however, for it is pivotal to the debate on energy and climate policy. A number of countries, committed to net-zero greenhouse gas (GHG) emission targets by 2050, have established an end date for coal power generation. In others, however, coal plays a key role in the supply of affordable energy [10]. That notwithstanding, coal's share in the global power mix is expected to decline by 10% by 2050, with a concomitant downturn in the stock of fly ash. Hence, it is imperative to seek replacements for clinker in nature to broaden the spectrum of alternative materials. The study described hereunder explored the use of non-precalcined bentonite, a widely available clay, as one such alternative.

As an anionic clay present in nature, bentonite can be obtained at low cost. It is also highly water absorbent and thixotropic (gel-like when vibrated). It was first discovered in 1988 in the United States and more specifically at Fort Benton, Wyoming [11], after which it is named. Its composition consists primarily of magnesium silicate, montmorillonite and aluminium hydrate, the third in the form of colloid-sized crystallites. Each individual montmorillonite crystal, in turn, comprises an octahedral layer of aluminium sandwiched between two tetrahedral layers of silicon. It carries a negative charge associated with isomorphic substitutions, such as Al3+ for Mg2+, in the crystallite network, which is offset by exchangeable alkaline metal cations [11].

Clay use as a mineral addition in cement is nothing new, although in most cases subject to precalcination [12]. A number of studies [13–15] have recently reported promising results around its application to replace more conventionally used additions such as fly ash or slag. Few studies on its non-precalcined use have been found in the literature, however, for that approach has consistently posed rheological problems [16–26]. That would explain why many building codes limit the presence of clay materials in aggregates and the much more common use of these materials in foundations and soil stabilisation than for structural applications [27–29]. Nonetheless, today's admixtures afford fresh concrete properties impossible to attain in the past, and modern laboratory techniques now in place can substantially shorten the time needed to design an optimal mix [30–33]. Non-precalcined bentonite has seldom been used to date in lieu of more conventional mineral additions [16–26]. This study was therefore designed to study the hydration mechanisms involved [19,24,26], fresh concrete properties such as flowability and bleeding [18,25], bentonite reactivity [19] and concrete water permeability [17,20,22] and compressive strength [16,25]. Durability was studied in terms of the replacement's effect on concrete resistance to freeze–thaw cycles and acid or sulfate attack [23]. Bentonite was reported to have a beneficial impact on preventing reinforcement corrosion [20] and carbonation resistance [21], although only one paper on each subject was located in the literature.

A review of the literature on the long-term performance of bentonite showed that it has exhibited excellent durability in underground works, where it has been used profusely as permanent formwork in concrete foundations often built long ago [27–29]. Additionally, whilst bentonite plays a non-calculated load-bearing role in such cases, none of the studies published report any long-term incompatibility between the two materials. It is likewise used in conjunction with concrete to generate impermeable slurry walls in highly radioactive waste storage facilities (designed to last for thousands of years) [34–36], where the caverns holding the radioactive waste are shotcreted and the encapsulated waste itself lies on a bed of bentonite. The use of such systems has given rise to research on how the alkaline nature of concrete may affect the long-term stability of bentonite clays. Such studies have verified the interaction between cement alkalinity and bentonite phases [34–37] or, equivalently, phase reactivity with clinker hydrated phases. From the standpoint of the role of clay as a clinker replacement, the findings have been initially promising thanks to the slow reactivity afforded by the high alkalinity of the pore solution, which dissolves the silicoaluminates in the bentonite. In terms of radioactive waste, such a result would be detrimental, however, if it affected the stability of the shotcrete/bentonite interface, given the many thousands of years they are intended to be in contact.

Standardised active additions such as fly ash, natural pozzolans, slag and silica fume, in turn, are known to effectively inhibit chloride and sulfate ingress [38,39], enhancing durability, although their presence in concrete impacts carbonation resistance adversely [13,16]. That is significant, for any decline in carbonate resistance is a primary long-term concern because it favours reinforcement corrosion [40–42] and the associated economic loss. The importance of seeking additions that either favour or at least are not detrimental to concrete durability cannot, therefore, be overstated.

Eluding the extra cost and additional handling involved in precalcining clays at temperatures of up to 1000 ◦C would carry obvious advantages. The present study consequently aims to explore the physical-mechanical properties and durability of concrete prepared with a non-precalcined bentonite as a substitute for clinker at different (wt/wt) replacement ratios.

This study sought to determine how replacing up to 30% clinker with non-precalcined bentonite may affect mortar mechanical properties and how carbonation depth and chloride and sulfate diffusion may be impacted by the presence of 10% of the clay with different types of binders bearing mineral additions. The findings are highly promising, particularly as regards carbonation, the weak point observed in other mineral additions. In the tests conducted, carbonation resistance either remained essentially unchanged or improved in the mortar prepared with the blended cement relative to the reference material. Plain Portland cement or cement bearing standardised additions was used throughout [43].

### **2. Materials and Methods**

*2.1. Materials*

All the cements listed in Table 1 were used in the carbonation tests, whereas cement CEM I 52.5 R-SR 3 served as the basis for the mechanical strength and sulfate and chloride diffusion trials.


**Table 1.** Chemical composition of the cements used in the tests.

A commercial bentonite (Mapeproof Seal), distributed by Mapei for purposes other than those studied here, was used to ensure consistent composition and particle size distribution throughout. According to the vendor's specifications sheet, the material contained over 95 wt % montmorillonite.

The particle size distribution and volume density curves were found by analysing bentonite powder on a Mastersizer 3000 laser diffractometer diffractor (Malvern Panalytical, Madrid, Spain) (Figure 1). Ninety per cent of the particles were <86 μm, whilst most lay within the 10 to 15 μm range (see the volume density curve). Such greater fineness than observed for the cement was initially deemed suitable, although optimisable. Addition fineness plays a significant role in the strength and rheology of composite cements, for the distribution curves for those materials complement the curve for the cement itself. This parameter must consequently be analyzed in depth in future research [43,44].

**Figure 1.** Particle size distribution and volume density plots for bentonite.

Another factor of particular interest in bentonite materials, which lies outside the scope of the present study, is water demand, affected not only by fineness but also by its sodium content [8].

In all the pastes and mortars prepared, bentonite replaced the corresponding binder content, and the water cement ratio was calculated as cement plus bentonite: w/cm.

#### *2.2. Specimen Types*

Different types of specimens were prepared, depending on the test.


#### *2.3. Test Methods*

#### 2.3.1. X-ray Diffraction

The mineralogical composition of the cement paste ground and sieved to 45 μm [40] was determined on a Bruker AXS DB Advance X-ray diffractor (Bruker, Madrid, Spain) configured without a monochromator, fitted with a 3 kW (Cu Kα1.2) copper anode Xray source and a wolfram cathode. A 30 mA current was applied to the X-ray tube at a voltage of 40 kV. A 0.5 mm fixed divergence slit was used. The instrument was also fitted with a 2.5 rad primary Soller slit and a Lynx-eye X-ray super-speed detector diffractor (Hamamatsu, Hamamatsu, Spain) with a 3 mm anti-scatter slit, a 2.50 rad secondary Soller slit and a 0.5% Ni-K beta filter. The specific reflection peak used was 2θ = 35◦.

#### 2.3.2. Twenty-Eight Day Flexural and Compressive Strength

Testing for flexural strength [42] consisted in bending the prismatic specimens by applying a force perpendicular to their longitudinal axis, on a Netsch test frame specifically designed for small specimens.

The test was deemed valid only when the specimen failed across the middle.

The two halves of the specimens resulting from the flexural test were subsequently used for compression testing.

Compressive strength was found by exposing the specimens to two axial forces with equal modulus and orientation but coursing in opposite and convergent directions, on an Ibertest Autotest 200/10-SW test frame [45].

#### 2.3.3. Carbonation in Natural Environments

The cements tested and their chemical compositions are given in Table 2. The 10 × 10 × 60 mm specimens were exposed to natural carbonation at the atmospheric CO2 pressure prevailing in the city of Madrid, in an indoor laboratory environment and two outdoor environments, one sheltered and the other unsheltered from rainfall, i.e., environments with varying relative humidity and temperatures (Figure 2).


**Table 2.** Composition (%) of the cements used in the tests.

**Figure 2.** Exposure to natural carbonation: environments: (**a**) indoor (laboratory) environment, (**b**) outdoor unsheltered environment and (**c**) outdoor sheltered environment.

Carbonation depth as found with phenolphthalein, an acid-base indicator, was recorded for the 3-month and 6-month specimens, depicted in the three environments in Figure 2.

#### 2.3.4. Sulfate Resistance: The Koch–Steinegger Method

Cement paste resistance to sulfate ions was tested on 10 × 10 × 60 mm specimens further to the Koch–Steinegger method, based on comparing flexural strength in such specimens soaked for 56 d in an aggressive solution (here, sodium sulfate at a concentration of 4.4 g/L) to the strength of analogous specimens soaked in water, likewise for 56 d (Figure 3). All the specimens had been cured in a humidity chamber for 28 d prior to testing.

**Figure 3.** Koch–Steinegger exposure to sulfate attack.

#### 2.3.5. Accelerated Chloride Ingress

The accelerated chloride diffusion test described in Spanish standard UNE 83992-2 EX [36] was conducted on 70 cubic mm mortar specimens, each bearing an embedded steel bar. Performance by the samples with 10% bentonite was compared to the results observed for reference CEM I 42.5SR specimens of the same dimensions.

The test consisted in connecting specimens made with different types of mortar to an electrical current that accelerated chloride ion diffusion (migration) across the matrix toward the bar (see setup in Figure 4). The steel was assumed to begin to corrode when surface contact with the chlorides was electrochemically detected. That initial corrosion time and the amount of chloride on the bar surface were the parameters used to calculate the diffusion coefficient.

**Figure 4.** Accelerated corrosion test setup.

#### **3. Results**

#### *3.1. Flexural and Comprenssive Strength*

Additions should not alter, except to improve, mix mechanical performance. As Figure 5a shows, replacing 10% or 20% of cement CEM I 52.5R–SR 3 with bentonite raised 28 d flexural strength relative to the reference cement except at a replacement ratio of 30%. Adding 30% bentonite yielded lower compressive strength than in the reference and in the materials with 10% or 20% replacement.

While unaffected by bentonite at a replacement ratio of 10% (Figure 5b), compressive strength declined at ratios of 20% or 30%. Those findings informed the decision to use only the 10% bentonite in all the subsequent tests as the most conservative option.

**Figure 5.** (**a**) Flexural and (**b**) compressive strength in cement pastes prepared with the reference and blended cements at replacement ratios of 10%, 20% or 30%.

#### *3.2. X-ray Diffraction-Based Characterisation*

The possible reactivity and stability of bentonite-bearing cement paste were also explored. The diffractograms for 28 d pastes bearing 10%, 20% and 30% bentonite are reproduced in Figure 6, whilst the relative content (counts, in per cent) of the various phases is graphed in Figure 7.

**Figure 6.** XRD patterns for unadditioned CEM I 52.5R–SR 3 and the same cement with 10%, 20% or 30% bentonite.

**Figure 7.** Crystalline phases identified on the XRD patterns for the reference and bentonite-bearing mixes: counts (%).

The reflections attributable to bentonite (montmorillonite and quartz) rose in intensity between 10% and 20% replacement, although no such rise was visible between 20% and 30% [44].

Portlandite content was similar to the reference in the former two mixes, but declined significantly at 30% replacement. According to [34], that decline might be explained by the formation of a calcium zeolite in the interaction between bentonite and the pore solution in the hydrated cement; confirmation of such a premise lies outside the scope of the present study.

Inasmuch as the carbonate phases would have been generated by carbonation occurring during the test, for the time being the rise in intensity with bentonite content need not be attributed to that higher proportion of the clay.

As ettringite content, in turn, followed neither an upward nor a downward pattern, its presence would be due to the cement and therefore may be deemed unaffected by the addition.

#### *3.3. Sulfate Attack*

The results for this test are deemed acceptable when the strength of the blended cement is greater than 70% of the value recorded for the control soaked in distilled water. Further to the flexural and compressive strengths of the reference specimen soaked in water and the specimens bearing 10% bentonite graphed in Figure 8, strength was higher in both the reference and in the specimen bearing the bentonite presence when soaked in the sulfate solution than when soaked in water. That rise in strength was attributed to the higher degree of hydration resulting from the difference in specimen ages: 56 d rather than 28 d. The values observed for the reference paste and the paste prepared with the blended cement were very similar. On the grounds of those data, the presence of bentonite may be deemed to have had no effect on cement resistance to sulfate attack.

**Figure 8.** (**a**) Flexural and (**b**) compressive strength in reference specimens and specimens bearing 10% bentonite soaked in distilled water (W) or in sulfate (S).
