This paper details the main factors influencing the performance of limestone calcined clay cements (LC3). The kaolinite content plays a major role in the rheological properties as well as strength development. Even in the presence of secondary phases, kaolinite can be accurately quantified by thermogravimetric analysis. The performance of LC3 is slightly influenced by the calcination process of clay, but it can be optimized by using the correct calcination temperature and applying a specific mix design with adjusted sulfate and alkali content. The hydration reactions of LC3 are fully characterized. They vary slightly from plain cement. There is no significant change in terms of phase assemblage. The main properties of LC3 are also described. LC3 blends show a lower creep compliance and a delay in shrinkage strains compared with plain cement. Concerning durability, LC3 blends show outstanding performance with respect to resisting chloride ingress and expansion from the alkali–silica reaction.
|M kaolinite|| |
molecular weight of kaolinite
|M water|| |
molecular weight of water
|wt%calcined kaolinite|| |
calcined kaolinite content
water loss during kaolinite dehydroxylation
|wt%kaol-OH, calcined|| |
water loss during kaolinite dehydroxylation for incomplete calcination in calcined clay
Partial replacement of clinker by supplementary cementitious materials (SCMs) in blended cement or concrete is by far the most realistic strategy for lowering environmental impact. Unfortunately, supplies of the most widely used SCMs (i.e. granulated blast-furnace slag and coal fly ash) are limited to around 20% of cement production, and most suitable materials are already used in cement and concrete. Globally, kaolinitic clays are available in very much larger quantities and have excellent reactivity after calcination. Such materials can make a very substantial contribution to reducing the carbon dioxide (CO2) emission associated with the production of cementitious materials.1 The clays of interest are not necessarily those with high purity (i.e. high kaolinite content or metakaolin), and clays with a kaolinite content above around 40% also perform well.2
Limestone has found its position in the concrete industry as a cement replacement material due to its low price, high availability and low energy consumption during its grinding, enabling its use for adjustment of particle size distribution of cementitious components to enhance the workability and early-age strength of concrete. Furthermore, limestone promotes hydration of clinker by providing a suitable surface for nucleation of hydrates (filler effect) and contributes to hydration reactions in the presence of aluminates.3,4
This leads to the interest in cements with a coupled substitution of limestone with calcined clay, which are referred to as LC3: limestone calcined clay cement.5,6 The authors previously showed that a strong relationship exists between the strength development of LC3-50 blends (clinker content reduced to 50%) and the calcined kaolinite content of calcined clay.2 Moreover, using a clay with only 40% calcined kaolinite gives strength similar to that of plain cement (PC) after about 7 d. This paper presents several important aspects of LC3 cement and concrete, resulting from several years of research experience at the Laboratory of Construction Materials, École Polytechnique Fédérale de Lausanne, Switzerland. The paper starts by discussing the selection of suitable clays and the way of characterizing them. It continues with the most important processing aspects with the optimization of the calcined clay reactivity affected by calcination parameters. The paper then looks at hydration, mechanical properties and durability of LC3 systems.
The kaolinite content is a key criterion for the selection of suitable clay. The kaolinite content can be determined by thermogravimetric analysis (TGA) using the tangent method.7 The dehydroxylation of kaolinite AS2H2 occurs from about 400 to 650°C.8–10 This leads to the formation of the amorphous metakaolin phase AS2 according to the equation
The water loss during kaolinite dehydroxylation, wt%kaol-OH, allows the determination of the kaolinite content wt%kaolinite according to Equation 2. M kaolinite and M water refer to the molecular weights of kaolinite and water, respectively.
TGA is also carried out on calcined clays to detect any water loss wt%kaol-OH, calcined corresponding to an incomplete calcination. The calcined kaolinite content is then obtained according to the equation
The influence of secondary phases on the determination of the kaolinite content was investigated to ensure that they do not significantly impact the kaolinite content quantification. Various phases lose water or carbon dioxide during calcination, but most of them do not interfere with kaolinite dehydroxylation. Among the thermally decomposing impurities most frequently identified in kaolinitic clays, goethite and gibbsite dehydroxylate at a lower temperature than kaolinite, at about 250–350°C.11–13 Calcite decarbonation occurs at a higher temperature, about 700–800°C. The presence of 2:1 clays with two tetrahedral silicate sheets and a single octahedral aluminate sheet (illite, muscovite, montmorillonite) was studied more deeply since they partially dehydroxylate over the temperature range of kaolinite dehydroxylation,14,15 as shown in Figure 1.
Since illite is the main phase interfering with kaolinite, mixes of these two clays were prepared; 25, 50 and 75% of kaolinitic clay were substituted by illitic clay. The TGA and differential thermal gravimetric (DTG) curves of these mixes are shown in Figure 2. The loss of water increases with the kaolinite fraction of the mix because kaolinite has a higher water mass fraction than illite.
The kaolinite content was then determined on these binary mixes using the tangent method. The comparison of the kaolinite content measured by TGA and the theoretical content is shown in Figure 3. The 1:1 line is also plotted to simulate a perfect fit. Very close results are obtained. The maximum deviation between the measured and the theoretical values is obtained for the mix containing 75% illitic clay, with 2·7% difference. Thus, even in the presence of a significant amount of illite, the kaolinite content can still be accurately determined.
Concerning the reproducibility that TGA provides, more than 50 different kaolinitic clays were tested in LC3. TGA was carried out three times for each clay. An average deviation of 1·0% was obtained for the kaolinite content. Therefore, it is concluded that TGA provides good reliability and reproducibility for the determination of the kaolinite content.
Another common way of quantifying the kaolinite content is the use of X-ray diffraction (XRD–Rietveld). However, the quantification of kaolinite by XRD is very challenging due to the structure of kaolinite, in which extensive layer disorder and preferred orientation are observed. XRD–Rietveld was carried out on 40 clays. The kaolinite content is compared with the values obtained by TGA in Figure 4. It shows that a general trend following the 1:1 line is obtained. However, some significant difference can also be observed. The maximal deviation obtained is 29·8%, and the average absolute difference is 6·1%. TGA is certainly preferable to XRD for such a purpose.
The dehydroxylation of kaolinite leads to the formation of metakaolin, and the highly disordered structure of metakaolin combined with the presence of penta-coordinated Al(V) are responsible for its reactivity. The calcination temperature of kaolinitic clays was investigated to optimize the reactivity of calcined clay. Batches of a clay with 50% kaolinite were calcined in an oven at five different calcination temperatures from 600 to 850°C. The TGA and DTG curves in Figure 5 show that kaolinite is partially dehydroxylated at 600°C and the dehydroxylation is complete from 700°C onward.
TGA indicates the efficiency of calcination but does not provide any information on the reactivity of calcined clay. The R3 pozzolanic test was developed as reactivity indicator of the calcined clay.2 This test also gives a good prediction of mortar strength, even at late ages. The R3 pozzolanic test consists of assessing the reactivity of calcined clays in model systems composed of portlandite, calcined clay and limestone with levels of sulfate and alkali adjusted to simulate the conditions in real blended cements. Two methods are possible to assess the reactivity. The monitoring of the heat release by isothermal calorimetry at 40°C is the most rapid way (24 h of testing), also giving the most relevant prediction of mortar strength. The determination of the bound water takes slightly more time but does not require any specific equipment other than an oven. Complete details of the test are given in the paper by Avet et al. 2
The R3 test was applied on the batches calcined at different temperatures. The cumulative heat release is shown in Figure 6. It shows that the highest heat after 24 h of testing is obtained for the clay calcined at 800°C, with a slightly lower reactivity for the clay calcined at 700 and 750°C. A similar ranking was obtained using the bound water alternative and mortar strength results, even for strength variations of only 10% between 700 and 850°C. The optimal pozzolanic reactivity is thus obtained 100°C higher than the completion of dehydroxylation. The reason for the lower reactivity at 600°C is the incomplete dehydroxylation of kaolinite. At 850°C, the decrease in reactivity is due to the physical coarsening and agglomeration of calcined clay particles.16
Calcined clay can be produced using different thermal processes. Among them, calcination in rotary kilns and flash calcination appear as the most promising alternatives at the industrial scale. According to the specific choice of calcination method, calcination parameters such as maximum temperature and residence time should be calibrated in order to ensure an optimized reactivity.
Flash calcination exposes the material to much higher temperature gradients (103–105°C/s) over short periods of time (usually 0·2–1 s),17 leading to a higher specific surface area compared to calcination in a rotary kiln due to the rapid release of water vapor.18 Thus, flash calcination has been found to produce calcined clay with slightly higher reactivity compared to static or rotary calcination.19 The reactivity of a clay with 50% kaolinite calcined in an industrial flash calcination unit is compared with the same clay batch calcined at 800°C for 60 min in a static oven in Figure 7. In both cases, the reactivity was assessed by monitoring the heat release with the R3 test. The specific surface area was measured using nitrogen adsorption. Specific surface area values of 51·8 and 62·6 m2/g were obtained for the static- and flash-calcined clay, respectively. The effect of the specific surface area is visible during the first 24 h of the R3 test. Afterwards, the reactivity of both calcined clays is roughly the same and remains dominated by the calcined kaolinite content of the calcined clay.
Ensuring a proper grinding and therefore high fineness of clinker, limestone and calcined clays is crucial to achieve good reactivity and mechanical performance.20 In laboratory conditions, LC3 constituents are normally ground separately in an open-circuit grinding configuration. On the other hand, the most common grinding process in cement plants is based on intergrinding of cement constituents in closed-circuit units. The main difference between separate grinding and intergrinding is that during intergrinding the components interact with one another. These interactions are mostly due to their differences in grindability.21 In the case of LC3, calcined clay and limestone have higher grindabilities (softer particles) compared to clinker (harder particles). Thus, upon intergrinding, clinker tends to remain concentrated in the coarse fraction (reducing its reactivity), while calcined clay and limestone become much finer, which may have a detrimental effect on workability. It was observed that increasing the limestone amount in the clay/limestone fraction helps to overcome this issue partially.22 However, the highest reactivity is achieved when clinker is ground separately from limestone and calcined clay.
Clays are mixtures of clay minerals (such as kaolinite, illite and montmorillonite) and other impurities, such as quartz, iron oxide and other rock-forming minerals. Due to this inherent heterogeneity of the material, the grinding of clays results in a characteristic bimodal particle size distribution. This characteristic distribution is observed when grinding both raw and calcined clays and also at industrial-scale grinding setups. Mineralogical analysis of the different particle populations shows that kaolinite is concentrated in the finer fraction due to its higher grindability compared to quartz, which remains mainly in the coarse fraction of the distribution. This opens the possibility of applying particle classification processes to increase the kaolinite content of a given calcined clay.
The sulfate content in cement is adjusted based on the aluminate content from the clinker. In LC3 systems, part of clinker is substituted by calcined clay containing an aluminate-rich metakaolin phase. Thus, the sulfate content used for a properly sulfated PC cannot be simply diluted when using LC3. A new sulfate optimization is required. An example is shown in Figure 8, where different gypsum additions (in addition to the amount in the cement) from 0 to 3% are used for an LC3-50 containing a calcined clay with 50% calcined kaolinite. A 2:1 calcined clay-to-limestone mass ratio is used in this study (30 parts of calcined clay for 15 parts of limestone). It shows that without adjustment, silicate and aluminate peaks occur close to each other, leading to a lower total heat release and reactivity, whereas with a proper gypsum addition of 1%, it is possible to differentiate the silicate from the aluminate peak. This ensures the optimal properties of the blended cement.23 In this case, the total sulfur trioxide (SO3) content of the LC3 system is actually lower than that of PC (1·9% for LC3-50 against 2·6% for reference PC).
Alkali content is known to affect hydration reactions, mechanical properties and durability properties of concrete significantly. It is accepted that alkalis accelerate the early hydration of cement,24 while resulting in reduced hydration and strength at later ages.25 A certain level of alkalinity is also necessary to favor pozzolanic reaction.26,27 Figure 9 shows the heat flow for an LC3-50 system with a clay containing 50% kaolinite and varying alkali contents from 0·3 to 0·6% sodium oxide equivalent (Na2Oeq). It is clearly seen that the increase in alkali content accelerates the reaction of silicate and aluminate phases. The optimum for LC3 consists of providing enough alkalinity to enhance the pozzolanic reaction without significantly impairing the properties at late ages. The optimum in this case is 0·48% Na2Oeq.
The rheological properties of blended cement-based materials depend strongly on mixture proportions and the characteristics of the components.28 It has been shown that the most relevant factors governing the rheological behavior of a cement paste are the water-to-cement ratio and the specific surface of the constituents.29 In the case of calcined clay, the high surface area of the clay minerals may have a negative impact on workability.30
The use of water-reducing admixtures such as superplasticizers is a strategy that can contribute to overcome the workability reduction observed when using calcined clay. Figure 10 shows the superplasticizer (polycarboxylate ether based) demand of LC3-50 by unit flow as a function of calcined kaolinite content. As observed, the calcined kaolinite content dominates the rheological properties, with a global increase in the amount of superplasticizer with the calcined kaolinite content to reach the same flow value. Thus, compared with pure metakaolin, the use of suitable clays without about 40% calcined kaolinite permits significantly reducing the amount of superplasticizer needed.
During hydration of LC3, additional reactions occur compared with PC. Metakaolin in calcined clay reacts as pozzolanic material, consuming portlandite and forming mainly calcium aluminum silicate hydrate (C-A-S-H).31 Limestone also reacts with the C3A from clinker to form carboaluminate hydrates.32 In LC3, the formation of carboaluminate hydrates is also enhanced thanks to the reaction of aluminate from metakaolin.23 In order to characterize the phase assemblage evolution in LC3, clinker hydration degree can be easily quantified using XRD, as well as the amount of reacted limestone. The determination of the reaction degree of metakaolin is more challenging. A comparative study was carried out to determine the best method.33 For the various clays tested, the mass balance approach is the most suitable method. It consists of calculating the phase assemblage forming based on the consumption of anhydrous phases. The amount of reacted metakaolin is unknown and considered as the free variable of the system. It is varied from 0 to 100%, and the actual amount is determined from the best agreement with experimental data. The results obtained for the amount of reacted metakaolin for LC3-50 blends is shown in Figure 11 for various calcined kaolinitic clays. The metakaolin content of each calcined clay is indicated for each system. It shows that the amount of reacted metakaolin increases with time and globally increases at late ages with the grade of the calcined clay. However, this increase becomes less important with the metakaolin content of calcined clay.
From these data, the whole phase assemblage of LC3-50 can be characterized, as shown in Figure 12 at 3 and 28 d of hydration. Globally, the phase assemblage does not show significant differences from PC. Similar hydration products form. Carboaluminate hydrates are the main AFm phases in LC3; this is similar to most modern PCs, which contain small amounts of limestone (<5%). Monosulfoaluminate is only formed in PCs with no limestone addition.34 The highest amount of C-A-S-H is found in PC, due to the higher initial clinker content. The pH of the pore solution is very similar to that of PC, from 13·2 for LC3-50 (95·0%) with fairly pure metakaolin to 13·8 for LC3-50 (0%) containing quartz.
A benchmark test on compressive strength was carried out on standard mortar, using LC3-50. More than 50 clays were tested, with their calcined kaolinite content ranging from 0 up to 95% at ages of 1, 3, 7, 28 and 90 d.35 Results are presented in Figure 13, showing the dependency of compressive strength on calcined kaolinite content. At 1 d, mechanical strength is only slightly affected by the clay grade. This is expected, as the pozzolanic reaction of metakaolin in calcined clay is just starting. Between 3 and 28 d of hydration, the positive effect of clay grade on the compressive strength is clearly visible, as strength is almost linearly correlated with the calcined kaolinite content. This is the key finding of this benchmark test, as it demonstrates that – for the LC3-50 design – strength differences are a first-order function of the calcined kaolinite content, independent of the secondary clay phases. Those phases can be anything from metastable clay minerals, such as metaillite or inert phases, such as quartz or hematite. In addition, the dashed horizontal lines represent the PC strength at different ages – from 3 to 90 d. Interestingly, LC3 using very high-grade clays can catch up with PC after only 3 d and clays with a calcined kaolinite content as low as 40% are also able to reach PC strength at 28 d. The blends used in this test were produced by simple mixing of the calcined clay and limestone with PC. It is highly likely the early-age properties could be further improved by optimizing the grinding and blending process.
It is also worth noting that clays with a calcined kaolinite content from about 40 to 75% exhibit the highest strength gain between 7 and 28 d. This is also valid from 28 to 90 d, as the highest-grade clays (>80%) already reached their maximal strength after 1 month and the lowest-grade clays (<30%) have probably depleted their metakaolin. Thus, there is no major benefit of using clays with more than 50–60% calcined kaolinite since the strength gain is not significant at late ages.
Some clays were also selected for a testing campaign on flexural strength, on standard mortar as well. The trends in flexural strength evolution follow those of compressive strength; see Figure 14. The ratio of compressive strength to flexural strength is very similar between PC and LC3-50 blends.
The evolution of Young’s modulus against compressive strength was also monitored on mortar samples, through non-destructive compressive measurements; these are shown in Figure 15. A similar trend is observed for PC and LC3-50 blends.
Uniaxial compressive creep was assessed on mature paste samples in sealed conditions, for LC3-50 binders and a PC reference. Samples were loaded after 28 d (t 0) of sealed curing at a constant stress corresponding to 15% of their strength, for a duration of 28 d. This level of loading ensures being in the linear regime between strain and applied stress.36 The results are reported in Figure 16, shown as the basic creep compliance over loading time. Creep compliance is obtained by subtracting the elastic strain at loading and the autogenous shrinkage strain (see the following section for details) from the total measured strain and then dividing this strain by the applied load. The most striking feature is the sharp reduction in the creep compliance of LC3 binders compared to that of the PC reference. This is consistent with previous results from the study of Brooks and Johari,37 although that study focused on concrete and clinker substitution by metakaolin only. This decrease seems independent of the clay purity – at least within the studied range – and could be attributed to both a refined pore structure and a lower amount of the viscous C-A-S-H phase in LC3.
The autogenous shrinkage of paste specimens was measured according to the ASTM C 1698-0938 standard in an oil bath kept at 20 ± 0·1°C. LC3-50 mixes containing calcined clays with a metakaolin content ranging from 25 to 60% were compared to a PC reference. Results in Figure 17 show the autogenous shrinkage over the first month of hydration. Curves are zeroed after the initial expansion to display only the pure shrinkage part. Note that this initial expansion was more important for the PC reference than the LC3 samples. All ternary blends show a delay of the shrinkage onset, postponing it to around 4 d for the studied systems. This is a valuable feature as shrinkage strains seem to be limited at young age, when the strength of the material is still low and therefore subject to cracking. After 28 d of hydration, the total autogenous shrinkage amplitude is similar between PC and LC3 mixes.
Corrosion of steel reinforcement in concrete due to chemical attack of chloride ions (from seawater or deicing salts) is the most important durability concern of reinforced-concrete structures worldwide. Diffusion of chloride ions through concrete is mainly governed by the porosity characteristics of concrete,39 as well as the phase assemblage of the binder.40,41 Figure 18 shows the profile of chloride ions in a one-directional ponding experiment in 3 wt.% sodium chloride (NaCl) solution over 2 years for PC and three LC3 blends, all prepared with a same clay of 50% kaolinite content. In LC3-65, the substitution level is lower with a clinker content of 65%. The calcined clay-to-limestone ratio was also changed from 2:1 to 1:1. The chloride profile in LC3-50 blends indicates significant improvement with respect to chloride ion diffusion compared to the systems with higher clinker content. A higher calcined clay-to-limestone ratio tends to reduce further the penetration depth of chloride. Furthermore, the authors’ recent analysis has shown that the performance of the LC3-50 blend is better than or similar to that of ternary blends prepared with fly ash or slag (instead of calcined clay) at the same clinker replacement levels. Thus, significant improvement in the service life of LC3-50 concrete structures in saline environments is expected.
The main reason for such excellent chloride resistance is explained in Figure 19 by the significant refinement of pore connectivity. Even for a calcined kaolinite content of 38·9%, the critical pore entry radius is three times smaller than that for PC at 28 d of hydration.
The carbonation rate of concrete with blended cements is generally known to be higher than that of PC, and this is also the case for LC3 systems.42 The rate of carbonation is mainly controlled by the calcium oxide (CaO) content of concrete, which is the main component binding carbon dioxide from the atmosphere in concrete. Figure 20 shows how the initial calcium oxide content of concrete determines the rate of carbonation. Portland pozzolanic cement blends were composed of binary PC–calcined clay, PC–fly ash or PC–slag systems. Calcined clays with higher purity have slightly better carbonation resistance than low-grade clays and comparable to other conventional SCMs, such as fly ash and slag.
The carbonation resistance of LC3, and other blended systems, can be improved by good curing of concrete prior to exposure, as seen in Figure 21. Also, it should be noted that carbonation is highly related to the percentage of relative humidity (RH%) of the environment. At very low or very high humidity, the carbonation rate is slow. As such, at locations where LC3 has the highest potential of use, such as tropical countries, or in marine structures where the RH is generally high, carbonation will not be a concern.
For the study of the alkali–silica reaction (ASR), the reactive aggregate (Jobe) was used. The clay tested in LC3-65 and LC3-50 contains 50% calcined kaolinite. The mortar bars after curing in fog condition for 28 d were soaked in 0·32 M sodium hydroxide (NaOH) solutions at 38°C. Generally, the use of SCMs has an effective preventive effect against ASR in concrete43,44 due to the lower alkalinity and the presence of aluminum (Al) in pore solution.45 Figure 22 shows that LC3 is extremely promising for mitigating ASR. As previously observed for chloride resistance, even better results are obtained with increasing the clinker substitution level.
The following briefly highlights the most important aspects of LC3 cement and concrete.
The selection of a suitable clay is made possible by the quantification of the kaolinite content by TGA and by the assessment of reactivity using the R3 test. A minimum of kaolinite of 40% of clay is necessary in LC3-50 to reach strength similar to that of PC from about 7 d onward. There is no real advantage of using clays with more than 60% kaolinite.
Clays with 40–50% kaolinite are also better than purer clays in terms of workability. The optimization of limestone particle size distribution further improves the rheological properties of LC3.
Clays with 40–50% kaolinite have excellent chloride resistance for LC3 paste and mortar. Chloride transport is significantly better than in PC and blends with other conventional SCMs. This is mainly attributed to porosity refinement in LC3 systems. ASR is also mitigated using LC3. LC3-50 performs even better than LC3-65.
Carbonation of the LC3 system is faster than that of PC. Clays have a similar effect to other SCMs in terms of lowering the calcium oxide content of concrete, which causes a higher rate of carbonation. Prolonged curing before exposure can mitigate carbonation.
The authors would like to acknowledge financial support by Swiss Agency of Development and Cooperation grant 81026665. WH would like to thank SCG Cement-Building materials Co., Ltd. The Swiss Federal Commission for Scholarships for Foreign Students is acknowledged for supporting FZ’s studies through scholarship 2016.0719.