Heat Evolution in Cement Hydration
Knowledge of the thermal response of cement is important in the design and construction of high early strength concrete mixtures as well as mass concrete structures. This is because such structures present a high potential for cracking as a result of stresses induced by temperature gradients within the concrete. Heat production results from the cement hydration reactions and both the quantity of heat as well as the rate at which this heat is evolved depends on the mineralogical and morphological characteristics of the cement clinker. This page explains a technique for assessment of heat of hydration. Cements were prepared in the laboratory by grinding clinkers from various cement plants with gypsum from a single source and tested in an adiabatic calorimeter to determine the heat evolution characteristics of each of the clinker samples. X-ray fluorescence (XRF) analysis and optical microscopy were used to determine the chemical composition and to characterise the crystal size, morphology, and distribution of clinker phases.
Concrete structures such as mass concrete elements or those designed to achieve high early strength are subject to potential cracking as a result of the development of temperature gradients across the structure. This occurs because of the exothermic reactions involved in the hydration of cement together with the uneven heat loss from different sections of the structure. In the case of mass concrete structures, the problem occurs because of the high thermal inertia of concrete, while a high cement content is the principal cause of the problem in high-early strength concretes. The progress and kinetics of hydration of Portland cements are influenced by various internal and external factors1-4 such as:
Portland cement clinker is typically composed6 of 50-70% alite (C3S), 15-30% belite (C2S), 5-10% calcium-aluminate (C3A), and 5-15% calcium-alumino-ferrite (C4AF) as the dominant crystallographic phases. The morphology and composition of these phases can vary significantly depending on the manufacturing process and raw materials used and these variations can have a significant influence on the amount and rate of heat evolution during hydration. Furthermore, the presence of impurities can alter the structure and composition of the different phases and thereby alter their reactivity.
Various techniques have been developed to measure the heat evolution
pattern of cement, which includes isothermal calorimetry, conduction calorimetry
and adiabatic calorimetry. Details regarding these techniques have been
critically discussed by Lawrence7. Of these techniques, the adiabatic
method is considered to be relatively easy to perform while giving fairly
accurate and representative results. Gibbon et al.8 developed a low cost
adiabatic calorimeter to study the heat evolution in concrete, which has
the advantage that the test can be conducted on the actual concrete mixture
proportions used in the structure. Ballim and Graham9 proposed a form
of expressing the adiabatic heat rate function in maturity form to obtain
a normalised function that accounts for variations in the time-temperature
conditions under which the test was conducted. Ballim10 also showed how
this form of the adiabatic heat rate curve can be used in a model for
predicting the time-temperature profiles in large concrete structures.
Cement clinker samples (IN1, IN2 and IN3) were obtained from three leading cement manufacturing plants in South India. The gypsum used for producing cement from these clinkers was obtained from one of the cement plants and this gypsum was added to the clinkers so that the resultant cement had an SO3 content of 2.3%. Two South African cement clinkers were also selected, one considered to be a low heat clinker (SL) and the other considered as a medium heat clinker (SM). Although different gypsum was used to prepare the cements with the South African clinkers, the SO3 content of these cements was also kept at 2.3%.
The oxide composition of the clinkers and the gypsums, shown in Table 1, was determined using X-ray fluorescence analysis. These results were also used to determine the amount of gypsum required for producing the cements. The compound composition of the clinkers was estimated using quantitative X ray diffraction analysis (Rietveld refinement) and the results of this analysis are shown in Table 2.
The clinkers with the appropriate amount of gypsum were ground
in a laboratory ball mill. The mill was periodically stopped and the specific
area of the sample was determined using the Blaine test. Grinding was
stopped when the measured specific surface area of the sample was 3200
± 50 cm2/g. Clinker IN1 (hardest of the group) took 37.5 hours
of grinding to reach the target specific surface area of 3200 cm2/g. Clinkers
IN2 and IN3 were considerably easier to grind and only required about
6 hours of grinding time. Each of these laboratory-manufactured cements
was then used to prepare a concrete for adiabatic calorimeter testing
to determine the amount of heat and the rate at which heat is evolved
Table 1. XRF Analysis of Clinkers and Gypsums used in the study
Table 2. Compound composition (%) of clinkers determined from quantitative XRD
Note: M – Monoclinic, T – Triclinic, C - Cubic
An adiabatic calorimeter similar to that described by Gibbon et al.8 was used to determine the amount and rate of heat evolved by each of the laboratory cement samples. Fig.1 shows a schematic arrangement of this adiabatic calorimeter. Briefly, this test uses a one-litre concrete sample, placed inside a plastic container and separated from the water in a temperature-controlled water bath by a 40 mm thick air space. The air space is necessary to dampen any harmonic responses between the sample and the water temperature as a result of inherent measurement errors of the thermal probes. A temperature probe is inserted into the centre of the freshly cast sample up to the middle of the container. Another temperature probe is immersed in the water bath to monitor the bath temperature. The concrete sample temperature is then monitored over time by a personal computer fitted with an analogue-to-digital input/output card. A heater element in the water bath is turned on or off in response to the concrete temperature, to maintain the water bath at the same temperature as that of the concrete.
For the adiabatic tests, each of the cements obtained by grinding
clinkers IN1, IN2, IN3, SL and SM was used to make a concrete sample with
the mixture composition as shown in Table 3. All the mixture components,
including the water, were stored in the same temperature controlled room
as that of the calorimeter at least 24 hours before mixing. This enabled
the temperature of the materials to be in equilibrium with the room temperature,
which was controlled at 19 ± 1 oC. Concrete was prepared by manual
mixing in a steel bowl and the adiabatic test was started within 15 minutes
after the water had been added to the concrete mixture. Temperature measurement
in the calorimeter was continued until there was no appreciable increase
in temperature of concrete (approximately four days). The silica sand
used in the concretes was obtained in three size fractions and these were
recombined to ensure uniform sand grading for concrete mixture proportioning.
Figure 1: Schematic layout of the adiabatic calorimeter
Table 3. Mixture composition (kg/m3) of the concrete used for adiabatic calorimetry
The microscopical examination of cement clinkers provides a visual appreciation of crystal size, morphology, and distribution of clinker phases. A geological optical microscope with reflected light capability was used to examine the structure and morphology of clinkers. Each clinker sample was lightly crushed and the fragments passing through 2.36 mm sieve and retained on 1.18 mm sieve were used to prepare the polished sections. The fragments were embedded in a low viscosity epoxy resin under vacuum, polished initially with 600 grit carborundum paper, followed by progressively finer abrasion systems until a final polish with 0.25 µm diamond paste on a lapping disk with a non-aqueous lubricant.
The polished samples were then etched with potassium hydroxide
(KOH) followed by nital using a procedure described by Ballim and Graham9.
While a wide variety of etching techniques have been developed to highlight
the different phases of clinkers11, the KOH-nital etch was found to give
a suitable general image of the clinkers. Digital photographs were obtained
of the etched surfaces and an image analysis system was used to measure
crystal sizes and to provide a qualitative description of the clinker
RESULTS AND DISCUSSION
Figs. 2 – 5 show the etched sections of clinkers viewed under reflected light in an optical microscope. These figures show the important phases of alite (C3S, generally angular), belite (C2S, generally rounded), interstitial material (C3A and C4AF, light colour regions) and epoxy resin in clinker pores.
Figure 2: Clinker IN1 showing (Left) typical C3S (alite) structure with relatively high proportion of aluminate phases (white areas) and (Right) a zone of high C2S (belite) content; width of field 400 µm
Figure 3: Clinker IN2 showing (Left) high C2S content surrounded by C3S crystals and (Right) fine grained, nested C2S zone with much porosity in clinker; width of field 400 µm
Figure 4: Clinker IN3 showing (Left and Right) relatively large proportion of C2S and very high porosity; width of field 400 µm. The blue colour of the crystals is an indication of the longer time of exposure to etch that was required to reveal crystal structures.
Figure 5: South African clinkers SL (Left) and SM (Right) showing relatively large C2S crystals with fairly low porosity; width of fields 200 µm
It is important to note that the photo-micrographs shown in Figs. 2 to 5 are selective views of the clinkers and do not depict all the features noted with respect to each of the clinkers and interpretation of kiln conditions. Such interpretation was drawn from the results of several techniques of examination and study of several clinkers particles in the sample. Various factors that constitute burning conditions are related to some aspect of the formed microstructure. The principal kiln conditions such as heating rate, cooling rate, kiln atmosphere, maximum temperature, and other factors will determine the size, morphology, and abundance of various compounds11.
Crystal size of alite depends on the rate of temperature rise
in the sintering zone and fineness of raw feed. A quick-heating rate and
fine raw feed will produce smaller alite crystals11. Alite is quick to
react; and its properties (abundance, size, reactivity) affect early age
strength. Long burning times produce larger belite crystals, whereas a
shorter burning time will produce smaller belite crystals. Belite hydration
primarily has an effect on later age compressive strength. Table 4 presents
a summary of the observations made during the microscopic assessment of
Table 4. Summary of observations made during the microscopic examination of clinkers
In measuring the heat evolution of the laboratory cements, the adiabatic test was terminated when the temperature of the concrete tended towards a constant value. This is the point at which the rate of hydration is so slow that the heat being evolved is not sufficient to measurably increase the temperature of the concrete sample. For all the cements tested, this point was reached after approximately 5 days of measurement.
The measured temperature change of the concrete sample was used to calculate the amount of heat evolved at any time after the start of the test using Equation 1. The specific heat of the concrete was taken as the mass weighted average of the mixture components and was calculated to be 1115.8 J/kg.K. The total heat function, as obtained from Equation 1, was then numerically differentiated to obtain the heat rate function.
where qt is the total heat evolved up to time t; Cp is the specific heat capacity of the concrete; Tt is the concrete temperature at time t; To is the concrete temperature at the start of the test; ms is the mass of the sample and mc is the mass of cement in the concrete sample.
Fig. 6 shows the results of the total heat liberated (per unit mass) by the cements over the first 100 hours of testing. The total heat liberated was found to be approximately 270 kJ/kg for cements IN1 and IN2, compared to approximately 300 kJ/kg for IN3 and the South African cements. Given that the Indian clinkers showed lower total alite and belite contents compared to the South African clinkers, the higher total heat evolved by IN3 is unexpected. This may well be the effect of the higher belite content (highest of the group) combined with the smaller belite crystal size resulting in a more rapid and more extensive hydration of this phase than would normally be expected.
Figure 6: Total heat liberated by the cements over the first 100 hours of testing
Figure 7: Maturity heat rate curves of cements
Fig. 7 shows the maturity heat rates for these cements with respect to the maturity time of hydration (t20 hours is the time taken for concrete at any temperature to attain the same maturity as that of concrete cured at 20 oC8,9,13). The figure shows the heat rates over the first 60 t20 hours. Hereafter, the curves cluster close to each other, slowly tending towards the zero heat rate line. The important feature of Fig. 7 is the peak heat rate that occurs between 8 and 20 t20 hours. The maximum measured heat rates for the cements lie in the range 1.16 W/kg for cement IN2, to 3.2 W/kg for cement IN3. Based on the classification used by Ballim and Graham13, cements IN1 and IN3 fall in the category of medium heat cements while cement IN2 falls in the low heat category.
It has been generally accepted that clinkers with relatively well developed crystals with distinct grain boundaries lead to higher strengths than those with indistinct crystallisation, indistinct grain boundaries and islands or fragments of ill formed minerals7. This generalised rule appears to apply in the case of clinkers IN1, IN2 and the South African clinkers. However, although clinker IN3 showed a poorly developed and sometimes indistinct crystal grain structure, it nevertheless gave the highest heat rate of the group tested and showed a total heat evolution similar to that of the South African cements. This may be explained by the higher C3A content (explaining the high early heat rate) and the more reactive belite structure (explaining the relatively high total heat evolved).
This project has shown that there are significant physico-chemical differences between the three Indian clinkers assessed. Furthermore, differences in kiln operation in manufacturing these clinkers were illustrated by the fact that clinker IN1 took significantly longer than clinkers IN2 and IN3 to achieve the target degree of fineness. This points to differences of grindability of the clinkers which depends on both raw material chemistry as well as the conditions of burning.
The counter-intuitive heat performance of clinker IN3 indicates that further and more detailed analysis of clinker chemistry and crystal morphology is required to relate such information to the heat evolution characteristics. Nevertheless, it is clear that the morphology of the clinker minerals can indicate the quality of the clinkers and gives a reasonable idea of the likely heat evolution patterns. In addition, it is understood that morphological and chemical characterisation techniques, on their own, may not account for possible differences in the hydration characteristics of otherwise nominally similar cement clinker minerals.
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