Cement calorimeter

The cement calorimeter MC CAL - another very special calorimeter from C3 Prozess- und Analysentechnik - vas developed to study the hardening behavior of different cement blends with the help of the heat flow rate-time function. 

Several features of the early model (Fig. 6) have been modified in the present-day, high-temperature version of this calorimeter (Fig. 7) (37). Depending upon the temperature range envisaged, the block is made of refractory steel, alumina, or beryllium oxide and is machined to house the calorimeter itself. The thermoelectric pile (about 50 platinum to platinum-rhodium thermocouples) is affixed in the grooves of an alumina plate (A), which is permanently cemented to two cylindrical tubes of alumina (B). Cylindrical containers of platinum (C) ensure the uniformity of the temperature distribution within the calorimeter cells. 

With inert asbestos cement sheets in the measuring channel, the air-flow calorimeter was first equilibrated at an air temperature... 

The glass wall which surrounds the calorimeter block is naturally made as thin as possible, and it is desirable that there should be good thermal contact between them. Otherwise irregularities in the rate of change of temperature may easily be observed. The block was therefore cemented with Woods metal into the vacuum vessel. For this purpose some of the alloy was put into the vessel the block, electrically heated from the inside, was then introduced and pushed down a suitable depth into the Woods metal. 

The ARC calorimeter jacket and sample system are shown in Figure 11.49 (168). A spherical bomb is mounted inside a nickel-plated copper jacket with a swagelok fitting to a 0.0625 in. tee, on which is attached a pressure transducer and a sample thermocouple. The jacket is composed of three zones, top, side, and base, which are individually heated and controlled by the Nisil/Nicrosil type N thermocouples. The thermocouples are cemented on the inside surface of the jacket at a point one quarter the distance between the two cartridge heaters. The point is halfway between the hottesl and coldest spots of the jacket. The same type of thermocouple is clamped directly on the outside surface of the spherical sample bomb. All the thermocouples are referenced to the ice point that is designed to be stable to within 0.01°C. Adiabatic conditions are achieved by maintaining the bomb and jacket temperatures exactly equal. The sample holder has a capacity of 1-10 g of sample. Pressure in the system is monitored with a Serotec 0-2500 psi TJE pressure transducer pressure is limited in the vessel to 2500 psi. The maximum temperature of the system is 500°C. 

The group making use of the mixing method mainly involves Ordinary or Isoperibol calorimeters, but, curiously, not all of them. This is because the term mixing method eliminates the isoperibol experiments -which do not make use of a liquid medium to store the heat evolving from the sample, like cement hydration carried out in a Dewar vessel... 

Methods of testing cement. Physical tests. Test for heat of hydration Specification for calorimeter bombs Guide for determination of calorific values of solid, liquid and gaseous fuels (including definitions)... 

Abstract. Polyvinylalcohol (PVA) is a polymer soluble in hot water, it has the property of film formation and it can improve the concrete performance. The effects of PVA modified with nano clay on the cement hydration reaction have been investigated by means of semiadiabatic calorimeter, FTIR spectroscopy and SEM. FTIR spectroscopy was employed to monitor chemical transformation of cement. The morphology of the different samples was compared by means of SEM micrographs. With the semiadiabatic calorimeter the hydration kinetic was measured to compare the heat rate of the admixtures materials. Fixing the water-cement ratio, w/c, in 0,45, the ratio of polymer to cement (p/c) was 2 wt% and the ratio of clay to polymer was 4 wt% (0.8wt.% related to cement). The polymer and modified polymer admixtures produced a retardation effect on the kinetic of cement hydration, but the clay acts as nucleating agent. The increase of the temperature with time was measured and a new model with four parameters was employed and the kinetic parameters were determined for each sample. 

Measure technologies. The heat evolution of cement is measured using Sweden Thermalmat TAM Air 08 isothermal calorimeter. 

The heat of hardening can be determined not only by a simplified ealculation, but by direct calorimetric measurements, or indirectly applying the Hess rule. According to this rule, the heat of reaction depends only on the initial and final state and this is the basis of the dissolution method. In this dissolution procedure, the heat of hardening is determined as a difference between the heat of neat cement dissolution and heat of hydration products (cement paste) dissolution. The mixture of nitric and fluoric acids is used for this purpose. Only the dissolution method is applied in the case of the longer hydration time. The most accurate, differential calorimetric method has been developed by Zielenkiewicz [164]. The completely hydrated mortar is placed in one container of calorimeter and in the second one— the fiesh mortar. 

In accordance with DIN 1164, Parts, the heat of hydration is determined with a heat-of-solution calorimeter. As stated there, "... this method is intended for the determination of the specific heat in J/g that is released when a cement undergoes hydration under isothermal conditions. The heat of solution of the unhydrated cement sample as well as that of the sample hydrated at 20 C (water-cement ratio w/c = 0.4) in a specified acid mixture is measured. The difference between the two heats of solution is the heat of hydration."... 

The test apparatus comprises a heat-of-solution calorimeter with accessories (Dewar flask, stirrer, funnel, etc.), an officially calibrated Beckmann thermometer and an appropriate acid mixture (nitric acid -l- hydrofluoric acid). The cement paste samples (their mix proportions, mixing procedure and temperature are specified) are stored in a water bath at 20° 0.5 C. The heats of solution of the unhydrated and of the hydrated cement are determined from the rise in temperature occurring when the samples dissolve (the test should be performed in constant-temperature surroundings) and from the determinations of the CaO content (or the losses on ignition, if applicable). Formulas for calculating the heat of solution from the test data are given in the Standard. It is an elaborate procedure. 

Development. The first conduction calorimeter was developed in 1923 [68] Subsequently, Carson applied this technique to the investigation of cement hydration, A systematic investigation of the effect of gypsum... 

Forrester, A., A Conduction Calorimeter for the Study of Cement Hydration,... 

The rate of hydration of cements can be determined through the heat development characteristics using a conduction calorimeter. In Fig. 13, the heat effect in the first few minutes is attributable to the heat of wetting and ettringite formation. Within a few hours, another strong exotherm appears due to the hydration of C3S. In some cases, depending on the composition of the cement, an additional peak is observed after the C3S peak. This is related to the reaction of C3A to form the low sulfoaluminate hydrate. 

Problem. In an adiabatic calorimeter the temperatnre 0c in a sample of hardening concrete is measrued. The cement content C of the concrete is 350 kg/m. The concrete density q is 2350 kg/m and the mass-specific heat capacity Cp of the concrete is calculated as 1.10 kJ/kgK. The measurement shows the following relation between hardening time t and the adiabatic concrete temperatnre 0c... 

The heat development of a concrete is investigated by measurement in an adiabatic calorimeter. The cement content of the tested concrete is C = 310kg/m and the concrete density is = 2320kg/m. A concrete specimen with the mass m = 12.5 kg is used, and the specific heat capacity of the concrete is Cp = 1.12 kJ/kg K. Bcised on this, determine 1) the heat capacity C of the specimen in (kJ/K) 2) the heat (kJ)... 

The heat development properties of cement is determined by calorimeter measurements. In practice, solution calorimetry, isothermal calorimetry and adiabatic calorimetry have all been used. The solution calorimeter is interesting in that it produces data that can be directly applied to thermo chemical calculations. 

Solution. For the calculations, the following notation is used PC denotes 1 g of unhydrated Portland cement nH20 denotes the amount of water added to 1 g of cement PC nH20 denotes hardened cement paste after 27 days A denotes the amount of acid added to the calorimeter SOL denotes the solution formed by cement and acid in the calorimeter. With this notation, this information can be written by the following thermochemical equations ... 

Figure 3.41. Simple solution calorimeter for measuring the reaction enthalpy AH solution calorimetry is, for example, used for determination of the hydration heat of cement.

There are many different types of calorimeters that can be used in the cement field, and some of them have many uses. The most common technique is isothermal (heat conduction) calorimetry in which the heat production rate (thermal power) from small samples of paste or mortar is directly measured. This technique is the focus of this chapter as it is the most versatile calorimetric technique in the cement field. Typical commercial instruments of this type used in the cement field are TAM Air (TA Instruments, U.S.), TCal (Calmetrix, U.S.), MC CAL (C3 Prozess- und Analysentechnik, Germany), ToniCAL III and ToniCAL TRIO (Toni Technik, Germany) and C80 (Setaram, France). 

Solution calorimeters are used for determination of heat of hydration typically after 7 days as specified, e.g. in European Standard EN 196-8 (2010) or in American Standard ASTM C186 (2013). Heat of hydration is calculated from the measured heats of dissolution of a cement paste sample that has hydrated for 7 days and of the dry cement powder. Solution calorimetry has significantly decreased in use in the last decades as the acids needed to dissolve the samples pose severe occupational hazards. 

Isothermal and semiadiabatic calorimeters both quantify cement hydration kinetics, but they do so in different ways. This is illustrated in Figure 2.1. In isothermal (heat conduction) calorimetry the heat production rate ( ) in a small sample (S) is measured by a heat flow sensor as heat is conducted to a heat sink that is placed in a thermostated environment. It is also necessary to have a reference sample (R) with the same properties (especially the same heat capacity) as the sample but without any heat production. This arrangement significantly reduces the noise in the measurements. The output from the calorimeter is the difference between the sample signal and the reference signal. 

Figure 2.2 Typical results from 2 days of measurements of (a) thermal power and (b) heat of hydration with an isothermal calorimeter for a rapid-hardening cement (CEM I) and a slow-hardening cement (CEM IV/B).

To summarise, isothermal, semiadiabatic and adiabatic calorimeters are all used to study the cement hydration process, but they follow different time-temperature trajectories. This is illustrated in Figure 2.4. 

A stable laboratory temperature can improve the precision of calorimetric measurements, but it is not needed for qualitative and many quantitative applications since the calorimeter has its own temperature control, which for most calorimeters is far more precise than normal laboratory air-conditioning. However, several standards place demands on laboratory temperature for example, ASTM C1679 (2014) demands the requirements of ASTM C511 (2013) for cement mixing rooms (23.0 4.0°C and a relative humidity of not less than 50%) to reduce the disturbance when charging samples. 

The third calibration parameter, which is needed when rapidly changing processes are studied, is the time constant. This is a measure of the thermal inertia of the sample that blurs details in rapid events. The time constant is used in the Tian equation - named after a pioneer in isothermal calorimetry - to correct for this. Typical time constants in isothermal calorimeters are 100-1000 s. As the main hydration has timescales much longer than this, the Tian equation is not needed in cement calorimetry when the main hydration is studied, but it is needed when early reactions are studied. Further information on the Tian equation is given by Wadso (2005), and other similar methods are discussed by Evju (2003). 

It is common that commercial calorimeters have internal, automatic calibration. Although this makes a calorimeter user friendly, it is problematic if the user does not know whether the calibrations are accurate. One way to check whether the instrument is working properly and whether the user is performing the measurement in a correct way is to run a validation procedure, i.e. an experiment with a known outcome (proficiency test). A number of such chemical calibration systems are described by Wadso and Goldberg (2001) however, none is similar to cement hydration measurements. It is therefore of interest to establish reference cements - or other similar systems, for example, based on calcium hemihydrate - that can be used to validate the quality of calorimetric cement measurements in a laboratory. 

Isothermal calorimeters have two vial positions one for the sample vial and one for a reference vial. The latter should be charged with a substance with a similar heat capacity as the sample but with no heat production. Convenient materials are water and quartz sand and a method for selecting a proper reference is discussed by Wadso (2010). Do not use old hydrated cement paste samples as such samples may still produce heat or produce heat when the temperature changes. 

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