Strength and Resistance to Sulfates, Carbonation and Chlorides Ingress by Substitution of Binder by Hydrotalcite in Several Cement Types

Abstract

Currently, the cement sector has become aware of the economic and environmental advantages of replacing clinker with other supplementary cementitious materials that have a lower carbon footprint in the design of eco-cements. In this study, hydrotalcite, a natural as well as synthetic clay, which can be fabricated at the cement plant site, was used as such an addition. The objective of this work was to evaluate the behavior of its physical–mechanical properties and durability in pastes and mortars, using a magnesium-type commercial hydrotalcite, Mg₆Al₂(OH)₁₆CO₃·4H₂O, as a substitute material for 10, 20 and 30% by weight of ordinary Portland cement (OPC). The mechanical strength was not affected by the substitution, the resistance to chlorides increased, as the hydrotalcite (HT) was able to bind chlorides, and the resistance to carbonation increased at 3 months but was almost the same as the reference specimen at 6 months, which indicates the need to have longer test durations.

1. Introduction

The evidence of climate change [1,2] promotes the study of reducing the energy costs associated with the concrete production process, paying special attention to decreasing the clinker content of cements to guarantee a reduction of the carbon footprint [3].

The durability of concrete and, therefore, its service life are compromised in environments with chlorides due to the corrosion of the reinforcement. On the other hand, the carbonation that occurs as a result of the penetration of carbon dioxide through the concrete pores is also a source of corrosion [4,5]. Over the years, different strategies have been used to prevent this premature corrosion, especially in environments with chlorides, including the use of inorganic additions, such as slag or fly ash [6,7], to retard the chloride penetration. The use of mineral additions in concrete has a very ancient origin, since Roman concrete was already a mixture of lime and natural pozzolan. In modern times in which cement is industrially produced, carbon dioxide released during clinker’s fabrication contributes to the greenhouse effect [8], because the production of one ton of clinker emits approximately 0.8–0.85 tons of CO₂ into the atmosphere when modern clinker kiln technology is considered [3]. The use of additions to clinker has increased over the years, not only in order to improve the properties of the material but also to reduce the proportion of clinker as an important objective within the Kyoto commitment to reduce gas emissions, it being a priority that this does not imply the alteration of its properties [9]. Specifically, these additions are advantageous in environments with chlorides because of the ability to combine their ions, thus delaying their arrival to the rebar; however, they can be detrimental in the case of carbonation.

In this study, mineral additions that can be synthesized, such as hydrotalcites (layered double hydroxides—LDHs), were used and, therefore, they can be produced at sites with scarce mineral resources of other clays. The objective was to explore the possibilities of their use as multifunctional mineral additions, that is, their provision of concrete with a protective capacity towards its reinforcements [10,11,12]; in addition, they may confer other properties not considered in this work.

Currently, there are few studies on the use of hydrotalcites as an addition to concrete [13,14], so tests were conducted that ranged from exploring the synthesis pathways of hydrotalcites to studying the durability properties of pastes and mortars prepared with the addition of these clays during the mixing [13,14]. Hydrotalcites are a large group of double-layered minerals that have the ability to capture ions and release them under certain conditions [15,16,17]. They can be represented with the general formula [M^II_1−xM^III_x(OH)₂]^x+[(Aⁿ⁻_x/n)]^x−·mH₂O, where M^II and M^III are di- and trivalent metals, and Aⁿ⁻ is an intercalated anion whose function is to neutralize the charge n. The value of x is in the range of 0.20–0.33. The cation layer can be Mg, Ca, Al, Ga or In and transition metals such as V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Y. The anions can be oxoanions, oxometalates, halides, polyoxometalates and organic anions [18,19]. Because of the ability to modulate the molecular structure in a unique way that these clays have, it has been thought that they have a great capacity to be artificially modified by means of the nanostructuring of the compounds that can be inserted between their layers [20].

On the other hand, it should be mentioned that quasi-hydrotalcites have been found in blast furnace slags added to the cement and also that the aluminate phases of the clinker have a double-layered structure. In the bibliography, it is possible to find References [21,22] in which the capacity of blast furnace slag to combine chlorides that penetrate through the network of concrete pores has been studied.

This work is part of an investigation to study how the substitution of up to 30% of cement by hydrotalcite affects the mechanical properties of paste and mortar, as well as how the depth of carbonation and the diffusion of chlorides and sulfates can be affected by the presence of 10% of this clay in three commercially used cements. For the conducted tests, simple Portland cement without additions or with standardized additions were used. The promising result regarding the behavior against carbonation is considered of interest, since the use of additions is usually accompanied by a worsening of the behavior against this aggressive agent.

2. Materials and Methods

2.1. Materials

The cements listed in

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

Cement	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2O	K2O	LOI	IR	Cl−
CEM I/42.5 R	20.24	3.99	2.92	62.88	1.41	3.47	0.08	0.86	2.78	0.10	0.02
CEM I 52.5R-SR 3	21.73	3.67	4.31	66.12	1.32	3.00	0.49	0.57	1.12	0.19	0.01
CEM II/A-P 42.5 R	28.17	6.20	3.13	52.41	1.32	3.11	0.38	0.67	2.34	-	0.05

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.

The hydrotalcites used for this work were of the magnesium type, which can be clays of natural origin or obtained with chemical synthesis. For the tests carried out on the paste specimens, hydrotalcites synthesized in the laboratory were used, but for the mortar specimens, higher quantities than those that could be synthetized in the laboratory were necessary because of the limitations of adequate means. Then, the tests of the paste and mortar specimens were conducted with commercial hydrotalcites, SORBACID 911, from the company CLARIANT (Moosburg, Germany), in which the main components were aluminum and magnesium, as well as those synthesized in the laboratory.

The process followed for the synthesis of the hydrotalcites in the laboratory is known as coprecipitation, with which we proceeded to the synthesis of magnesium and aluminum-based hydrotalcites and with an interlaminar OH⁻ anion, giving rise to the formation of MgAlOH⁻ (Figure 1).

2.2. Specimen Types

Different types of specimens were prepared depending on the test:

• For mechanical strength, sulfate attack and carbonation resistance testing, 10 × 10 × 60 mm cement paste specimens substituting 10%, 20% or 30% of cement by hydrotalcite were prepared at a water/cement ratio of 0.5. They were cured in a climatic chamber at 90% relative humidity, first in the molds for 24 h and after removal for 28 days prior to testing;
• For accelerated chloride diffusions, the 70 cubic mm cement mortar specimens used were prepared at a water/cement ratio of 0.5 and cement/sand ratio of 1/3. They were cured in a climatic chamber at 90% relative humidity, first in the molds for 24 h and after removal for 28 days prior to the application of an electric current to test for chloride diffusion.

2.3. Test Methods

2.3.1. X-ray Diffraction

The cement paste mineralogical composition was determined using a Bruker (Billerica, MA, USA) AXS DB Advance X-ray diffractor configured without a monochromator, fitted with a 3 kW (Cu Kα1.2) copper anode X-ray 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 with a 3 mm anti-scatter slit, a 2.50 rad secondary Soller slit and a 0.5% Ni-K beta filter.

2.3.2. Twenty-Eight Day Flexural and Compressive Strength

Testing for flexural strength consisted of bending the prismatic cement paste 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 [23,24].

2.3.3. Carbonation in Natural Environments

The 10 × 10 × 60 mm cement paste specimens having only the substitution of 10% of the cement by hydrotalcites 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 humidities and temperatures (Figure 2). The cements tested and their chemical compositions are provided in

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

Cement	Clinker	Pozzolan	Additional Components
CEM I/42.5 R	95	-	5
CEM I 52.5R-SR 3	95	-	5
CEM II/A-P 42.5 R	80	16	4

.

The carbonation depth was determined with phenolphthalein, an acid–base indicator [25,26,27,28,29], which was recorded for the 3-month and 6-month specimens, as depicted in the three environments shown in Figure 2.

2.3.4. Sulfate Resistance: Koch–Steinegger Method

Cement paste resistance to sulfate ions was tested on 10 × 10 × 60 mm cement paste specimens with 10% substitution of cement by hydrotalcites further to the Koch–Steinegger method, based on comparing the flexural strength in such specimens soaked for 56 days 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 days (Figure 3). All specimens were cured in a humidity chamber for 28 days prior to testing [30].

2.3.5. Accelerated Chloride Ingress

The accelerated chloride diffusion test, described in Spanish standard UNE 83992-2 EX [31] (Figure 4a), was conducted on steel bars embedded in 70 cubic mm mortar specimens with 10% hydrotalcite substitution, comparing their performance to that of reference CEM I 42.5SR specimens of the same dimensions [32,33].

The test consisted in connecting specimens made with different types of mortar to an electrical current that accelerated diffusion (i.e., migration) across the matrix. The corrosion potential and corrosion rate were monitored (Figure 4b) in the specimens until corrosion was electrochemically detected in the embedded reinforcement bars (see the setup in Figure 4b).

3. Results

3.1. Flexural and Compressive Strength

As Figure 5 shows, the substitution of 10%, 20% and 30% of cement CEM I 52.5R–SR 3 by hydrotalcite increased the 28-day flexural strength relative to the reference. However, while unaffected by hydrotalcite at a replacement ratio of 10% (Figure 6), the compressive strength decreased at ratios of 20% or 30%. These findings informed the decision to use only the 10% hydrotalcite in all of the subsequent tests as a conservative proportion.

3.2. X-ray Diffraction-Based Characterization

The possible reactivity and stability of hydrotalcite-substituted cement paste were also explored. The diffractograms for 28-day cured pastes bearing 10%, 20% and 30% hydrotalcite are reproduced in Figure 7, while the relative contents (counts, in percent) of the various phases are graphed in Figure 8.

The reflections attributable to hydrotalcite (002, 004, 009) increased in intensity between 10% and 20% replacement, although no such rise was visible between 20% and 30%. The portlandite content was similar to the reference in the former two mixes but declined significantly at 30% replacement. Again, these findings recommended the use of only 10% in this exploratory work.

3.3. Sulfate Attack

The results for this test type are deemed acceptable when the strength of the substituted 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% hydrotalcite, as graphed in Figure 9 and Figure 10, the strength was higher in both the reference and in the specimen bearing the hydrotalcite presence when soaked in the sulfate solution than when soaked in water. The increase in the resistance was mainly attributed to the increase in the hydration degree because of the longer age after the 56 days of testing with respect to the 28 days of curing. Comparing the reference with the hydrotalcite substituted specimens, the resistances were very similar. This fact enabled the deduction that, in this preliminary study, the hydrotalcite did not modify the performance of the cement in the absence of the substitution with regard to the sulfate attack.

3.4. Chloride Resistance Test

This test aimed to determine the effects of the hydrotalcite addition on chloride transport in the mortar matrix and the chloride ion threshold at which the reinforcing steel began to corrode. Figure 11 shows the trends of the corrosion potential and of the corrosion rate during the experiment until a shift of both was detected and the depassivation occurred. In the present results, the corrosion rate shifted up after around 70 days of testing both in the reference and in the substituted specimens, but the values were above 0.1 µA/cm² for the reference while the substituted by hydrotalcites only moved to values > 0.1 µA/cm² at 130 days (

Table 3. Time to depassivation, penetration depth and chloride non-steady-state diffusion coefficient (D_ns) in reference ordinary Portland cement (OPC) and with substitution of 10% hydrotalcite-bearing specimens (HtMg10).

Sample	Test Time (h)	Maximum Penetration (mm)		Mean Penetration (mm)	Dns Diffusion Coefficient (cm2/s)
OPC—1	70	34.04	33.03	33.54	17 × 10−12
OPC—2	70	33.94	33.14	33.54	15 × 10−12
HtMg10—1	130	34.22	34.29	34.22	2.25 × 10−12
HtMg10—2	130	33.98	34.28	34.13	2.05 × 10−12

) just when the potential also shifted towards more negative values.

The penetration depth of the colorimetric front is depicted in Figure 12 confirming the arrival of chlorides to the rebar surface. The chloride diffusion coefficients calculated from the time taken to depassivation are listed in Table 3. Those of the mortar with the 10% substitution of hydrotalcite were much smaller. In other words, it took much longer to reach the penetration shown in the figures in the hydrotalcite-bearing than in the reference specimens.

The values of the chloride content are given in

Table 4. Concentration of chlorides in the surface of the specimen at the end of the experiment and in the surface of the steel bar.

Concentration of Chlorides	Bar in Reference	Bar in HtMg10
Surface chloride concentration (% mass mortar)	1.10	1.41
Chloride threshold (% mass mortar)	0.35	0.24

. The surface concentration is higher in the case of the presence of hydrotalcite, but opposite, the chloride threshold is lower with hydrotalcite.

3.5. Natural Carbonation

The photographs in

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

Cement	3 Months						6 Months
	Indoors		Outdoors Unsheltered		Outdoors Sheltered		Indoors		Outdoors Unsheltered		Outdoors Sheltered
	Reference	HtMg10	Reference	HtMg10	Reference	HtMg10	Reference	HtMg10	Reference	HtMg10	Reference	HtMg10
CEM I 42,5R
CEM I 52,5R-SR3
CEMII/A-P(16) 42,5R

depict the phenolphthalein staining in the specimens from which the 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 opposed to earlier recordings and general experience [3]. According to present data and opposite the expectation, the penetration was lowest in specimens exposed to indoor environments or sheltered outdoor conditions, while carbonation deeper in those exposed to rainfall, assumed to be due to their higher or nearly optimal moisture content. The results seem correct because they were qualitatively similar in the exposures for 3 months and 6 months. In this study, the specimens exposed for 6 months were tested during the summer under high temperature and low relative humidity (RH) conditions. In other seasons, with higher RH and more rain, the order may have differed. This is hardly relevant, however, for inasmuch as the carbonation was intense in all of the samples, the findings sufficed for the aim pursued, namely, to compare the behavior in the three types of cements with and without hydrotalcite substitution.

Figure 13a plots the 3-month carbonation depths in the specimens bearing 10% hydrotalcite against the respective references and Figure 13b the same parameters in the 6-month samples. As can be deduced from Table 5, the carbonation penetration was not symmetrical in all of the faces, which made the determination of the average value for the faces not straightforward. As a general appreciation, after 3 months the carbonation was less intense in a larger number of the 10% hydrotalcite than reference specimens. The gap was smaller after 6 months, although the carbonation was greater in several of the substituted cements than in the reference specimens.

With respect to the mechanical performance of the carbonated specimens, the flexural strengths were fairly similar in the reference and substituted samples (Figure 14). The compressive strength values were even closer in the two types of mortars. Inasmuch as the experiment was designed for the purposes of comparison, the inference drawn from these findings is that substituting 10% hydrotalcite into the cement mix had no consistent negative effect on the mortar’s behavior when carbonated (Figure 15).

4. Discussion

At present, the need to reduce the carbon footprint poses a great challenge to the cement industry because large-scale technological changes are necessary, as well as the investigation of the production of clinker from alternative decarbonated fuels and biomass and electric energy with zero carbon dioxide emissions for the manufacture of cement to reduce the clinker factor without altering their essential properties.

The already evident climate change and the limitation In the stock of said industrial by-products has led to the need to resort to other types of additions, such as precalcined clays, even with the added cost that implies precalcination. The use of clays [34] has become a solid alternative on the path to reducing greenhouse gas emissions. If, in addition, these clays could be used without the need for precalcination, costs would be reduced and it would be much more competitive, as long as the performance of the cement can be guaranteed. Hydrotalcite is present in nature, and it is also possible to chemically synthesize it with a simple process, so it is easy to use it in the cement manufacturing process.

The additions used in concrete to be compatible with cement must meet a series of requirements both in the short and long term, among which are:

• They must be inert or, at least, they must not induce expansive or degenerative reactions;
• They must improve or, at least, not alter its mechanical resistance;
• They should improve or, at least, not affect the durability of the concrete.

Regarding the resistance to flexion and the compression studied, it should be stressed, although it has been previously commented, that the use of admixtures and the modification of the w/c ratio have been necessary to guarantee a correct workability of the mix due to the hydrotalcite thixotropy.

One of the most interesting findings of this study, although for reasons not understood yet, is the increase in the flexural strength with the addition of hydrotalcites. Regarding the compressive strength, a decrease was observed as the substitution percentage increased, which is attributed to the lower cement content. Perhaps an optimization of the fineness of the different components can improve the behavior under compression, for which reason proportions greater than 10% of hydrotalcite have not been used.

In terms of the XRD results for the hydrated cement pastes, it was observed that at the age studied (28 days) the cement hardly reacted with the hydrotalcites, which indicates that it is possibly inert from a hydration point of view, even if it does not have the ability to retain aggressive exteriors.

According to the results of the Koch–Steinegger sulfate resistance test, the absence of anomalous or expansive reactions in the short term was evident. To confirm these initially promising results, it would be necessary to carry out longer-term tests [34,35,36,37,38].

Regarding the diffusion of chloride ions, it was observed, as research by other authors [17,18] has indicated, that hydrotalcites significantly delay the penetration of these ions by blocking their transport from being absorbed between the double layer; therefore, their use as an addition would be beneficial, although it is necessary to carry out more studies in the longer term to observe their stability over time in terms of retention, especially if carbonation occurs.

Finally, regarding the depth of carbonation, it was studied taking into account the action of other additions present in the cements under study. In the two CEM I mortars prepared, a carbonation depth equal to or even less than when hydrotalcites were not present was observed. For cements with natural pozzolans, the results obtained seem to indicate a better behavior against carbonation than those with fly ash and slag [18,39,40,41,42]. As one of the deficiencies identified when using mineral additions, in general, is the lower resistance to carbonation, the use of hydrotalcite could be an advantage, with a 10% substitution instead of limestone that is usually added to clinker, if economically it would also be profitable [43,44,45].

These results are attributed, as in the case of chlorides, to the fact that hydrotalcites have positive charges in their interlayers and, therefore, capture the carbonate ion for which they have a great affinity. However, as the carbonates provide thermodynamically more stable hydrotalcite phases than those of the rest of the anions, this indicates the need in the future to check if carbonation displaces and releases previously captured chlorides [46,47,48].

5. Conclusions

This article describes exploratory experimentation on some of the properties of pastes and mortars made with different proportions of hydrotalcite. The conclusions that may be drawn from the findings include the following.

Regarding the compatibility of hydrotalcites, it was observed that their presence results in cementitious materials that require the use of admixtures to improve the workability after which their mixing characteristics are adequate. In addition, they do not modify the crystalline phases of the cement, remaining inert with respect to hydration for at least 6 months of this test.

Substituting 10% of the cement does not impair the compressive strength of the tested mortars, but proportions greater than 10% do lower the resistance. On the other hand, the flexotraction resistance increases significantly with the three proportions tested.

In reference to the attack by sulfates, materials were obtained that were equally resistant to the reference cement, which was resistant in the Koch–Steinegger test.

As regard the chloride ions, as other authors have found, with the addition of hydrotalcites it was possible to significantly reduce their diffusion coefficient, although it is necessary to explore whether the bound chlorides are not released when a subsequent carbonation occurs.

Finally, in relation to the behavior against carbonation, the substitution of 10% of the cement by hydrotalcite did not significantly worsen the behavior, which is positive since the use of other mineral additions usually implies a significantly lower resistance towards this type of attack.