*3.5. Natural Carbonation*

The cumulative rainfall recorded in Madrid in the 6 months the specimens were exposed to outdoor conditions was 160 L/m2, whilst the mean temperature was on the order of 30 ◦C. The photographs in Table 5 depict the phenolphthalein staining in the specimens from which carbonation depth was deduced. Generally speaking, the shallowest depths were observed in the laboratory, intermediate penetration under outdoor sheltered conditions and the deepest in the specimens exposed to rainfall. That order of environmental aggressiveness is diametrically opposed to earlier reports. According to those data, penetration was deepest in specimens exposed to indoor environments or sheltered outdoor conditions, whilst carbonation was least intense in those exposed to rainfall, due to their higher or nearly optimal moisture content. The present findings were deemed accurate, however, for they were qualitatively identical in the 3-month and 6-month specimens. In this study, the specimens exposed for 6 months were also tested during the summer under high-temperature, low relative humidity conditions. In other seasons, with higher RH and more rain, the order may have differed. That is scantly relevant, however, for inasmuch as carbonation was intense in all the samples, the findings sufficed for the aim pursued, namely to compare the behaviour in the various cements.

Figure 11 plots the 3-month carbonation depths in the specimens bearing 10% bentonite against the respective references, and Figure 12 plots the same parameters in the 6-month samples. After 3 months, carbonation was less intense in a larger number of 10% bentonite than in reference specimens. The gap was smaller after 6 months, although in some cases carbonation was more intense in the blended cement than in the reference specimens.


**Table 5.** Phenolphthalein staining in the cements studied to determine carbonation depth.

**Figure 11.** Three-month carbonation depth in reference OPC vs. 10% bentonite-bearing Be10.

**Figure 12.** Six-month carbonation depth in reference OPC vs. 10% bentonite-bearing Be10.

As a rule, the flexural strengths (Figures 13–16) were fairly similar in the reference and blended samples. The compressive strength values were even closer in the two types of mortars. Inasmuch as the experiment was designed for purposes of comparison, the

inference drawn from these findings is that replacing 10% bentonite in the cement mix had no material effect on mortar behaviour.

**Figure 13.** Three-month flexural strength: OPC vs. Be10.

**Figure 14.** Three-month compressive strength: OPC vs. Be10.

**Figure 15.** Six-month flexural strength: OPC vs. Be10.

**Figure 16.** Six-month compressive strength: OPC vs. Be10.

#### **4. Discussion**

Rising to the challenge posed by the need to reduce the cement industry's carbon footprint may involve either large-scale technological change based on research into new manufacturing methods or adopting a more direct and technologically simple approach consisting in lowering the proportion of clinker in cements without altering their essential properties [1–3]. Replacing clinker with non-CO2-emitting materials is the most immediate alternative open to the industry.

By-products such as blast furnace slag, that are in themselves cementitious, have long been deployed to reduce clinker content [1], as have acid materials (pozzolans and more recently fly ash and silica fume) that react with the calcium hydroxide released in cement hydration. The use of natural pozzolans began to decline in the wake of their depletion in some natural reserves or because of the adverse impact of quarrying on the environment. In contrast, as an industrial by-product, fly ash was much less costly, for it did not have to be mined and the enormous stockpiles of coal industry waste had to be pared down. At around the same time, the nineteen seventies oil crisis raised the price of the fuel used to manufacture clinker. That combination of factors led to a general trend to add minerals to the clinker, which, depending on the commercial and legislative conditions prevailing in any given country, were either milled directly with the clinker or added at concrete plants.

As noted in the introduction, the evidence of climate change and the gradual reduction of the stock of such by-products have driven a return to former paradigms, including the use of additions other than slag or fly ash, such as precalcined clays [12]. Even with the investment involved in precalcination, clay has become competitive due to the rising cost of emissions [14]. Indisputably, however, the initial cost would be even more competitive if cement performance could be ensured with no need for precalcination.

Such precalcination entails, among other consequences, the loss of the bound water in the constituent minerals present in clay [8–12], which is recovered during hydration. Such thermal dehydration affects clay reactivity, i.e., cement hydration kinetics, but should not in principle impact component stability, for the hydrated compounds at issue are the same as they would be if the clay were used without precalcination. That is one of the many matters in connection with the use of non-pre-dehydrated natural clays, bentonite among them, in need of more thorough research.

In light of its vast diversity and fairly widespread geographic availability, clay is just one more local raw material [17,20] deployed in cement manufacture. The very same clays used in clinker kilns might, in certain proportions, constitute compatible additions to lower milling-related CO2 emissions (since milling clay is less energy-intensive than grinding clinker). That reasoning informed the initiative to undertake exploratory research along those lines, part of the results of which are described hereunder.

Surprisingly, very few studies [16–26] on the subject were found in the literature other than reports of the widespread joint use of cement and bentonite in underground works [28,29], such as the nuclear waste storage [34–38], soil stabilisation or impermeable slurry wall construction [27].

To be compatible with cement and usable in concrete, additions must meet a series of short- and long-term requisites, summarised below.


This study addresses some but not all of those factors. The findings are deemed sufficiently promising to be made public, acknowledging, however, that the use of nonprecalcined clays will call for considerable research, in light of their enormous variety.

The following paragraphs discuss the more or less basic features of the use of nonprecalcined clays analysed here, i.e., the effect on mechanical strength, the nature of the hydration products forming and the impact of bentonites on resistance to sulfates, chloride ingress and carbonation.

Be it said from the outset in connection with flexural and compressive strength that bentonite thixotropy necessitates adjusting admixtures and a constant w/cm ratio to ensure suitable mix workability [31,32]. Such thixotropy, which has not been studied in depth, may be either a drawback or an advantage in terms of workability, depending on the

intended application of the concrete (such as precasting or additive manufacturing, also known as 3D printing) at issue [43]. In this study, carboxylate admixtures [31,32] were the simplest choice to avoid mixing problems.

One of the most prominent findings of this research was the rise in flexural strength with the proportion of bentonite. In the absence of supplementary testing, no reasons for such a rise can be ventured at this time. Although compressive strength was observed to decline as the replacement ratio rose, that development was readily attributable to the concomitantly lower clinker content.

The XRD findings for the hydrated pastes revealed that at the ages studied the cement barely reacted with bentonite. Further to reports on underground structures, for the nuclear industry in particular, high cement alkalinity induces the formation of a certain proportion of calcium silicate hydrates and calcium zeolites [35,36]. At ambient temperatures, however, that reaction is apparently slow enough to deem bentonite a nearly inert substance.

Although the results of the Koch–Steinegger sulfate resistance tests might be dismissed for their failure to represent actual conditions, they are nonetheless indicative of relatively short-term anomalous and expansive reactions. Longer-term tests using different solutions would be required to confirm the present initially promising results in this regard.

The lower diffusion coefficient measured for chloride ingress, in turn, was attributable to the timing differences between the tests conducted with the reference and with bentonite [46]. As the clay retards chloride penetration significantly [20], its use as an addition would be beneficial, although further research is called for to determine the reasons for this behaviour. One possibility might be the reduction of porosity (parameter not measured here), whereas any reaction between bentonite and chlorides would be all but ruled out in light of the negative charge in the clay's interlayers, which would accommodate cations but not anions.

The effect of bentonite on carbonation depth must be assessed in the realisation that its action supplemented the action of other additions present in the cement. In other words, at least two mineral additions were in place in the tests conducted here, accounting in some cases for a substantial fraction of the total. In the two blended CEM I cements used, carbonation was the same or even lower than when no bentonite was present. Of the other cements, the ones bearing natural pozzolans appeared to perform better than those carrying fly ash or slag. In neither case did the use of bentonite induce clearly poorer performance than already observed in those cements. However, such behaviour cannot be attributed to a reaction between the clay and carbon dioxide, for as noted above, bentonite cannot accommodate anions in its interlayers [47–49].

This feature, the reduction or at least non-alteration of carbonation depth, is deemed to be the most relevant finding of this study. For the opposite, lower carbonate resistance is one of the shortcomings identified in mineral additions in general. Bentonite could consequently be used to advantage instead of the 5% of inert matter or the up to 10% of limestone routinely added to clinker. It may improve one or several properties of the end product. Confirmation of the foregoing will nonetheless call for much more testing, in particular to detect possible adverse effects on shrinkage or creep.

In the context of the pursuit of a circular economy and climate change mitigation, the cement industry is undertaking new strategies to reach a net zero emissions target by 2050. One such strategy, the production and use of blended cements with a high pozzolanic material content, makes the need to find new additions the more pressing [5]. Bentonite is a well-known clay consisting mostly of montmorillonite, an aluminium phyllosilicate mineral whose microscopic (~1 μm in diameter) plate-shaped particles afford the clay a large surface area. Precalcined clay has been standardised (European standard EN 197-1:2011 [50]) under the category 'natural calcined pozzolana (Q)', defined as thermally treated clays, shales, sedimentary rocks or materials of volcanic origin. Inasmuch as bentonite is a clay that requires no thermal activation, it might well be classified under the designatory letter 'Q'.
