# Hydration and Durability of Ternary Binders
Based on Metakaolin and Limestone Filler

E. Rozière · A. Loukili

Institut de Recherche en Génie Civil et Mécanique (GeM), Centrale Nantes,
Nantes, France e-mail: emmanuel.roziere@ec-nantes.fr

G. Medjigbodo

UMR Ecologie Des Forêts de Guyane (EcoFoG), Université de Guyane, Kourou,
France

L. Izoret

Association Technique de L'Industrie Des Liants Hydrauliques (ATILH), Paris,
France

**Abstract** The combination of Portland cement and mineral additions allows reducing the carbon footprint of cement-based materials and improving their resistance to several environmental actions. The development of binary binders is limited by the reactivity of pozzolanic materials at high substitution levels and the availability of industrial by-products such as slags and fly ash. Thus, it is necessary to develop new cements from natural raw materials such as clay and limestone and to combine them to design ternary binders with higher hydraulic activity. The study is focused on mortar based on binary and ternary binders (Portland cement, metakaolin, limestone filler) with a maximum substitution level of 45%. Two sets of mortar mixtures with different water-to-cement ratios were designed. The experimental program includes the determination of strength, porosity, hydration heat, portlandite content, shrinkage and natural carbonation. The analysis of data aimed at correlating the evolution of mechanical properties with hydration degree and reactivity of calcined clays. The results actually showed that the performances of ternary binders closely depend on the properties of the three studied metakaolins, especially their production process and physical properties. For a given substitution level, the studied ternary binders clearly showed better performances than other mineral additions. The reduction of water-to-cement ratio resulted in an acceleration of pozzolanic reaction. This allowed an improvement of short-term strength as well as potential durability.

**Keywords** Hydration · Shrinkage · Carbonation
# 1 Introduction

The use of calcined clay and limestone filler now appears as a promising alternative to plain Portland cement (PC) and industrial by-products. The raw materials are relatively abundant, and the calcination process mainly consists in de-hydroxylation, which results in lower CO₂ emission than clinker production.

Metakaolin-based binders have been widely studied. The reactive phase of metakaolin (AS₂) consists of aluminates Al₂O₃ (A) and silicates SiO₂ (S). They allow the production of C(A)SH [1], which improves the strength and durability of obtained cement-based materials. The optimum metakaolin (MK) proportion depends on its properties and mix-design parameters but generally remains lower than 30% [2].

Higher replacement thus lower carbon footprint can be reached by combining MK or calcined clay with limestone filler [3]. Besides its pozzolanic activity, MK actually reacts with limestone filler (LF) and portlandite to produce carboaluminates and stabilize ettringite. However, the CO₂ emissions should only be compared at equivalent durability, and the performances of ternary binders depend on several parameters, especially the physicochemical properties of raw materials.

The studies on the durability of ternary systems are still relatively scarce. For a given water-to-binder ratio, a high replacement of Portland cement affects early-age strength and durability. Reducing the water-to-binder ratio improves both properties [4] and influences shrinkage [5], but there are still very few references on calcined clay [6]. The experimental study presented in this paper investigates binary and ternary binders based on Portland cement (PC), metakaolin, limestone and siliceous fillers (SFs). The results are analyzed in order to understand the behavior of PC-MK-LF binders and to identify the relevant parameters allowing to improve their strength and durability.

# 2 Materials and Procedures

The results presented here are a part of a comprehensive study [11] described in Table 1. A Portland cement CEM I 52.5 N (PC) was combined with three different metakaolins (MKi), two limestone fillers (LF) and two siliceous fillers, at different PC replacement levels at water-to-cement ratios (W/C₀).

The mixtures CX_L include L% (in mass) of material X, with X = {MK1; MK2; MK3; LF; μLF, SF1; SF2}. The mixtures CX_M Y_N include M% of X and N% of Y, with W = {LF; μLF, SF1; SF2}. The workability was assessed using a 150 mm high mini-cone. The workability of mixtures including MK2 and MK3 was adjusted by adding superplasticizer TEMPO11 to reach a slump of 50 mm in order to make the casting of mortar samples easier. Limestone fillers had a beneficial effect on the workability of MK-based mixtures due to its lower water demand.
<table><caption>**Table 1** Experimental program</caption><thead><tr><th>Parameter</th><th>Mineral addition</th><th>Metakaolin properties</th><th>Water-to-binder ratio</th></tr></thead><tbody><tr><td>Binary binders</td><td>CMK130<br>CFC30<br>CμFC30<br>CFS130<br>CFS230</td><td>CMK130<br>CMK230</td><td>CMK315<br>CMK330</td></tr><tr><td>Ternary binders</td><td>CMK130LF15<br>CMK130μLF15<br>CMK130SF115<br>CMK130SF215</td><td>CMK130FC15<br>CMK230FC15</td><td>CMK330FC15</td></tr><tr><td>Water-to-binder ratio</td><td>0.50</td><td>0.50</td><td>0.42 and 0.50<br>for each binder</td></tr><tr><td>Characterization</td><td>Compressive strength<br>Hydration degree<br>Portlandite content</td><td></td><td>Porosity<br>Shrinkage<br>Carbonation</td></tr></tbody></table>

## 2.1 Materials and Mixtures

The physical and chemical properties of studied materials are given in Table 2. Metakaolin 1 results from the flash calcination [1] of clay extracted in Fumel, France. It showed relatively high quartz content, and its reactive material content (Al₂O₃(SiO₂)₂ or AS₂) of 53% was deduced from X-ray diffraction. Metakaolins

<table><caption>**Table 2** Chemical and physical properties of constituents</caption><thead><tr><th rowspan="2"></th><th rowspan="2">PC</th><th colspan="3">Metakaolin</th><th colspan="2">Limestone filler</th><th colspan="2">Siliceous filler</th></tr><tr><th>MK1</th><th>MK2</th><th>MK3</th><th>LF</th><th>μLF</th><th>SF1</th><th>SF2</th></tr></thead><tbody><tr><td colspan="9"><em>Chemical composition</em></td></tr><tr><td>SiO<sub>2</sub></td><td>20.6</td><td>68</td><td>55</td><td>55</td><td>0.0005</td><td>0.3</td><td>99.1</td><td>99</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>5.3</td><td>25</td><td>40</td><td>39</td><td>-</td><td>-</td><td>0.0048</td><td>0.0052</td></tr><tr><td>CaO</td><td>66.3</td><td>0.4</td><td>0.3</td><td>0.6</td><td>98.8</td><td>99.6</td><td>0.0003</td><td>0.0003</td></tr><tr><td colspan="9"><em>Physical properties</em></td></tr><tr><td>Density (g/cm<sup>3</sup>)</td><td>3.12</td><td>2.5</td><td>2.4</td><td>2.2</td><td>2.71</td><td>2.7</td><td>2.65</td><td>2.65</td></tr><tr><td>Specific surface BET (m<sup>2</sup>/g)</td><td>0.34</td><td>16</td><td>17</td><td>19</td><td>0.88</td><td>2.2</td><td>0.29</td><td>0.47</td></tr><tr><td>Water demand</td><td>0.34</td><td>0.56</td><td>0.90</td><td>1.65</td><td>0.31</td><td>0.10</td><td>0.29</td><td>0.52</td></tr><tr><td>Pozzolanic index (mg Ca(OH)<sub>2</sub>/g)</td><td>-</td><td>754</td><td>1000</td><td>1400</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>d<sub>50</sub> (μm)</td><td>12</td><td>31</td><td>15</td><td>1.5</td><td>8.5</td><td>3.7</td><td>34</td><td>17</td></tr></tbody></table>
MK2 and MK3 with AS₂ of 68% were obtained from the calcination (in rotary and flash kiln respectively) of kaolinite-rich clay from the Charentes Basin, France [6]. They only contain traces or original kaolinite. Limestone filler LF comes from Orgon, France, and microfiller µFC from Arboç, Spain. The mortar mixtures were designed keeping the volume of 0–2 mm crushed quartzite sand constant (47%). The studied mixtures were derived from the initial PC composition with constant water-to-binder ratio.

## 2.2 Experimental Procedures

**Compressive strength.** Compression tests were performed on three 40 × 40 × 80 mm³ specimens after 1, 2, 7 and 28 days of water curing. The activity index *i* is defined as the ratio of the strength *f*<sub>p</sub> of the mortar with the replacement of PC p% (in mass) to the strength of the plain PC mortar specimens *f*<sub>0</sub> (Eq. 1).

$$i = \frac{f_p}{f_o} \qquad (1)$$

**Hydration.** The studied materials are likely to influence the hydration kinetics as well as the type of hydration products. This was investigated using isothermal calorimetry and thermogravimetric analysis (TGA).

Isothermal calorimetry was performed on mortar samples at 20 °C according to the method described by Lenormand et al. [10]. The technique allows determining the heat flow *q*(*t*) and by integration the cumulated heat release *Q*(*t*) (in Joule per gram of cement). The hydration degree is assumed proportional to this value; thus, the time evolution of hydration degree *α*(*t*) can be estimated through Eq. (2).

$$\alpha(t) \approx \frac{Q(t)}{Q_{\text{tot}}} = \frac{\int_{0}^{t} q(\tau) d\tau}{Q_{\text{tot}}} \qquad (2)$$

$Q_{\text{tot}}$ is the long-term value of heat release estimated by the extrapolation of experimental data using the $1/\sqrt{t}$ function. The evolution is almost linear for the lowest values of $1/\sqrt{t}$, and the intersection of this linear function with the y-axis gives an estimation of $Q_{\text{tot}}$.

**Durability.** The actual durability of cement-based materials depends on many factors. Three properties have been investigated: water porosity, drying shrinkage and carbonation. The water porosity has been assessed according to French standard NF P18-459. Cylindrical mortar samples were saturated with water under vacuum then dried under constant mass. Total shrinkage and mass loss were measured on 20 × 20 × 160 mm³ mortar prisms stored at 20 °C and 50% relative humidity from the age of $t_0 = 1$ day. The shrinkage data $\varepsilon(t)$ were analyzed using the hyperbolic function (3) in order to deduce the long-term shrinkage magnitude $\varepsilon_{\infty}$ and the shrinkage halftime
$N_s$, 40 × 40 × 160 mm³ mortar prisms were exposed to natural carbonation in the laboratory conditions at 0.04% CO₂, 20 °C and 50% relative humidity from the age of 1 day. Carbonated depth was estimated by spraying phenolphthalein indicator at 28 days.

# 3 Results and Discussion

## 3.1 Activity Index and Hydration of Binary and Ternary Binders

The activity indices are plotted as a function of PC replacement (in mass) in Fig. 1. The binary binders clearly show the difference between metakaolin and fillers. The four fillers lead to the same values of activity index. The drop in 28-day strength is actually due to the clinker dilution effect [10]. These materials, especially limestone filler, accelerate hydration at early age through nucleation effect [11], but this effect vanishes at 28 days as the degree of hydration of Portland cement becomes closer to its ultimate value. The higher activity index of MK-based binders indicates its chemical reactivity.

The relatively high pH and high calcium content provided by Portland cement foster the hydration of MK, which produces C(A)SH and hydrated aluminate phases contributing to strength. This corresponds to pozzolanic reactivity resulting from the progressive activation of the aluminates and silicates from the amorphous fraction of MK [1]. However, the quantitative results differ from the values reported by Antoni et al. [2]. Their 30% MK mortars reached 120–130% of 28-day strength of reference

<figure><img src="image_1.png" /><figcaption>Fig. 1 Activity indices at 28 days (W/C₀ = 0.50)</figcaption></figure>
mortar, whereas they did not exceed 74% here. This discrepancy could mainly come from the properties of cements and metakaolins. Antoni et al. [2] used a CEM I 42.5 cement and a metakaolin with a significantly higher content of reactive phase. Avet et al. [3] actually showed a linear increase of compressive strength with calcined kaolinite content. From their results, the strength decreases by 30% when this content is reduced by 50%. As a consequence, the optimum replacement of Portland cement strongly depends on metakaolin properties [13].

Figure 1 also allows highlighting the synergistic effect of metakaolin and limestone filler. The CMK1<sub>20</sub>LF<sub>10</sub> mixture reached the highest activity index of 0.83 at 30% replacement. The activity index of PC-MK-LF binders did not exceed 0.65 at 45% replacement, but it was significantly higher than PC-MK-SF binders, which confirm the interaction between the three constituents and the beneficial effect of formed phases on compressive strength. TGA results (not shown here) actually showed a higher portlandite consumption in PC-MK-LF binders. XRD analyses performed by Antoni et al. [3] showed the formation of hemi- and monocarboaluminate. Carbonate phases are more stable than corresponding sulfates; thus, the addition of limestone filler favors the stability of ettringite, whereas it would transform into monosulfoaluminate without calcium carbonate.

Ternary binders are likely to influence the hydration kinetics as well as the hydration products and their contribution to strength. In order to distinguish both effects, the activity index has been plotted against hydration degree in Fig. 2. The difference between the ternary binders with limestone and siliceous fillers increases between 1 and 7 days. The increase of specific surface area provided by microfiller did not result in significant accelerating of hydration, but the activity index was higher at early age. This can be attributed to higher initial packing density of cement paste due to particle size distribution of µLF. This allows optimizing short-term strength this durability (see Sect. 3.2).

<figure><img src="image_2.png" /><figcaption>Fig. 2 Activity index and hydration degree of ternary binders ($W/C_0$ = 0.50)</figcaption></figure>
<table><caption>**Table 3** Shrinkage magnitude, hydrous compressibility factor (K) and water porosity</caption><thead><tr><th></th><th></th><th>CEM I</th><th>CMK3<sub>15</sub></th><th>CMK3<sub>30</sub></th><th>CMK3<sub>30</sub>LF<sub>315</sub></th></tr></thead><tbody><tr><td rowspan="5">W/C<sub>0</sub></td><td>Ultimate shrinkage (μm/m)</td><td>978</td><td>833</td><td>558</td><td>890</td></tr><tr><td>K</td><td>270</td><td>237</td><td>140</td><td>189</td></tr><tr><td>Porosity (%)</td><td>22</td><td>24.5</td><td>25.2</td><td>26.6</td></tr><tr><td>1-day strength (MPa)</td><td>17.4</td><td>14.4</td><td>8</td><td>6.2</td></tr><tr><td>28-day carbonated depth (mm)</td><td>2.5</td><td>3.0</td><td>4.1</td><td>5.8</td></tr><tr><td rowspan="5">0.42</td><td>Ultimate shrinkage (μm/m)</td><td>773</td><td>572</td><td>522</td><td>488</td></tr><tr><td>K</td><td>276</td><td>199</td><td>—</td><td>132</td></tr><tr><td>Porosity (%)</td><td>19.9</td><td>23.0</td><td>23.3</td><td>25.0</td></tr><tr><td>1-day strength (MPa)</td><td>23.5</td><td>19.5</td><td>13.2</td><td>9.5</td></tr><tr><td>28-day carbonated depth (mm)</td><td>1.7</td><td>1.7</td><td>2.7</td><td>3.4</td></tr></tbody></table>

## 3.2 Durability-Related Properties

The parameters deduced from water porosity and shrinkage tests are given in Table 3. For a given replacement, the shrinkage magnitude increased with W/C₀ ratio. This trend has already been reported in previous studies. Higher W/C₀ actually results in coarser pores, thus lower capillary tensions according to Kelvin–Laplace equation, but lower elastic modulus decreases and higher creep strain, thus shrinkage finally increases.

At both W/C₀ ratios, the replacement of cement by the studied materials led to lower ultimate shrinkage. This could be due to higher capillary tension induced by the finer pores of plain Portland cement paste. In order to confirm this assumption, the shrinkage versus mass-loss curves was analyzed. The second stage of this evolution shows a linear increase that can be characterized by a coefficient K. The PC mortar actually had higher K values; in other words, a given mass loss induces higher shrinkage. The hydration of plain Portland cement is actually faster than binary or ternary binders; thus, the porosity of these mortars was finer when exposed to drying at 1 day.

The porosity increased with W/C₀ ratio, which was attributed by Bredy et al. [14] to unreacted MK particles. Ambroise et al. [2] reported a decrease of porosity, but at W/C₀ ratios lower than 0.3. This confirms the strong influence of MK properties and mix-design parameters on the behavior of these materials.

For a given water-to-binder ratio, the carbonated depth increased due to coarser porosity and lower portlandite content. However, reducing the W/C₀ resulted in an acceleration of hydration [7] at early age, which was beneficial for strength and carbonation [8].
## 4 Conclusion

The study aimed at understanding the behavior of ternary binders based on Portland cement, metakaolin and limestone filler. The experimental results were promising in terms of strength and potential durability. Advanced characterization based on isothermal calorimetry and thermogravimetric analysis allowed correlating the development of strength, hydration and durability parameters such as porosity, shrinkage and carbonation.

The compressive strength of studied mortars confirmed the synergistic effect of metakaolin and limestone filler used as a substitution of Portland cement. These ternary binders actually induced higher portlandite consumption during hydration. The activity index closely depends on metakaolin properties.

Mortars with binary and ternary binders had lower ultimate shrinkage but lower shrinkage halftime. The analysis of shrinkage versus mass-loss curves showed a coarser porosity at short-term resulting from the coupling between hydration and drying, especially for the water-to-binder ratio of 0.50.

For the given water-to-binder ratio, the porosity and carbonated depth increased. However, for a given replacement, reducing the water-to-binder ratio allowed mitigating carbonation as well as increasing compressive strength.

**Acknowledgements** The authors would like to acknowledge ATILH and Ecole Centrale de Nantes for their financial support and all the providers of raw materials.

## References

1. Ambroise, J., Maximilien, S., Pera, J.: Properties of metakaolin blended cements. Adv. Cem. Based Mater. **1**(4), 161–168 (1994)

2. Antoni, M., Rossen, J., Martirena, F., Scrivener, K.: Cement substitution by a combination of metakaolin and limestone. Cem. Concr. Res. **42**(12), 1579–1589 (2012)

3. Avet, F., Snellings, R., Alujas, D.A., Ben, H.M., Scrivener, K.: Development of a new rapid, relevant and reliable (R3) test method to evaluate the pozzolanic reactivity of calcined kaolinitic clays. Cem. Concr. Res. **85**, 1–11 (2016)

4. Darquennes, A., Roziere, E., Khokhar, M.I.A., et al.: Long-term deformations and cracking risk of concrete with high content of mineral additions. Mater. Struct. **45**(11), 1705–1716 (2012)

5. Bredy, P., Chabannet, M., Pera, J.: Microstructure and porosity of metakaolin blended cements. MRS Proc. **136**, 275 (1988)

6. Cassagnabère, F., Diederich, P., Mouret, M., Escadeillas, G., Lachemi, M.: Impact of metakaolin characteristics on the rheological properties of mortar in the fresh state. Cem. Concr. Compos. **37**, 95–107 (2013)

7. Ferreiro, S., Herfort, D., Damtoft, J.S.: Effect of raw clay type, fineness, water-to-cement ratio and fly ash addition on workability and strength performance of calcined clay–Limestone Portland cements. Cem. Concr. Res. **101**, 1–12 (2017)

8. Khokhar, M.I.A., Roziere, E., Turcry, P., Grondin, F., Loukili, A.: Mix design of concrete with high content of mineral additions: optimisation to improve early age strength. Cem. Concr. Compos. **32**(5), 377–385 (2010)
9. Lawrence, P., Cyr, M., Ringot, E.: Mineral admixtures in mortars effect of type, amount and fineness of fine constituents on compressive strength. Cem. Concr. Res. **35**(6), 1092–1105 (2005)

10. Lenormand, T., Rozière, E., Loukili, A., Staquet, S.: Incorporation of treated municipal solid waste incineration electrostatic precipitator fly ash as partial replacement of Portland cement: Effect on early age behaviour and mechanical properties. Constr. Build. Mater. **96**, 256–269 (2015)

11. Medjigbodo, G., Rozière, E., Charrier, K., Izoret, L., Loukili, A.: Hydration, shrinkage, and durability of ternary binders containing Portland cement, limestone filler and metakaolin. Constr. Build. Mater. **183**, 114–126 (2018)

12. Poon, C.S., Kou, S.C., Lam, L.: Compressive strength, chloride diffusivity and pore structure of high performance metakaolin and silica fume concrete. Constr. Build. Mater. **20**(10), 858–865 (2006)

13. San Nicolas, R., Cyr, M., Escadeillas, G.: Characteristics and applications of flash metakaolins. Appl. Clay Sci. **83**, 253–262 (2013)

14. Ye, G., Liu, X., De Schutter, G., Poppe, M., Taerwe, L.: Influence of limestone powder used as filler in SCC on hydration and microstructure of cement pastes. Cem. Concr. Compos. **29**(2), 94–102 (2007)