Calcium aluminate cement is frequently preferred in refractory applications due to its performance and resistance to high temperatures. In the first article of our series, we examined in detail what refractory is — you can read that post here. The second article, which continued the series, focused on how calcium aluminate cement is used as a refractory material — you can access it here.
In this final article of the series, we will explore the experimental studies conducted on the use of calcium aluminate cement as a refractory and present the results.
Materials and Experimental Works
Calcium aluminate cement with an aluminate content of 40%, which has been compliant with TS 6271, has been used in this study. The properties of the cement used have been presented in Chart 1, while the characteristics of the fireclay incorporated into the mortars have been detailed in Chart 2.
Chart 1. Chemical, Physical and Mechanical Properties of Calcium Aluminate Cement
Chart 2. Properties of Fireclay
Preparation of Mortar Mixtures and Samples
Castable refractory mortar has been produced through mechanical mixing method.
- Mixtures with a Fireclay / Calcium aluminate cement ratio of 3 and Water/Cement ratio of 1.27 have been used.
- Castable refractory mortar has been mixed for 3 minutes in its dry form before the addition of water in order to obtain a homogeneous mixture.
- The calculated required amount of water has been added slowly while paying attention to the material in order to avoid clumps on the surface.
- Following the addition of water, mixtures have been prepared by mixing at low speed for 3 minutes and mixing at high speed for 2 minutes.
- Prepared mixtures have been placed into the 7x7x7 cm casts with three cubic sections by vibrating them on the vibratory table unit for 3 minutes as well as using 25 hammer blows to compress them.
- The mixer has been placed close to casting equipment and drinking water has been used in the mixture. The setting time is 24 hours.
- The casts have been kept for 24 hours in the maintenance room with 95% relative humidity at 20°C temperature and the samples have been taken out of the casts at the end of 24 hours.
Experiments Conducted After Sample Preparation
The thermal processes have been carried out on the prepared samples following the steps outlined below.
- The samples have been kept for 24 hours in an environment with 65% relative humidity at 20°C; at the end of this period the the uncombined water has been removed by placing the samples in a furnace at a constant temperature of 110°C for 24 hours and furnace was used again to cool it down to the room temperature.
- The samples then have been reheated to the desired degree and they have been kept in the furnace for them to cool down to room temperature. In this way, 12 different test sets have been prepared by heating them again, starting at 200°C and going up to 1300°C with 100-degree increments.
- The total heating duration has been 4 hours: 1 hour to reach the target temperature and 3 hours of holding at that temperature.
Following thermal process, the Archimedes experiment has been conducted on the cube samples to find their volume changes and these have been recorded. In addition, 3 of each (7x7x7) cm cube samples among the prepared test sets have been subjected to pressure tests. Parameters regarding wearing have been examined on 3 of each (7x7x7) cm cube samples.
Test Results
The results of the experiment have shown significant changes in compressive strength, ultrasonic speed transition and wear losses on the samples made using calcium aluminate cement when subjected to high heat.
According to the volume change results, the samples subjected to 200°C and 300°C have exhibited very low volume loss, approximately at the level of 1%. Figure 1 presents the volume loss chart after the thermal process. The volume of samples before heating has been taken as 1 in order to better observe the volume change after the thermal process.
Figure 1. Volume Losses After Thermal Process
According to the results of pressure test on the mortar samples made using CAC, all samples which have received thermal treatment have shown better results except 200 and 300°C samples. Figure 2 presents compressive strength chart before and after the thermal process.
Figure 2. Temperature-Compressive Strength Chart for Before and After Thermal Process
In terms of compressive strength, the increases have started to become more prominent after 400°C compared to reference samples. The compressive strengths of the samples subjected to the thermal process at 900°C have been observed to increase by up to 100% compared to the reference samples. After 900°C, compressive strength has been observed to decline, but it has not fallen below the levels of the reference samples at any point.
Moreover, the test results have shown that the samples made with calcium aluminate cement have experienced a reduction in firing time. This reduction has demonstrated that the expansion issues observed in other refractories do not occur in this case. It has been observed that the structure of the samples becomes porous at higher temperatures due to the elimination of crystallized water. Therefore, it can be concluded that the wear rate increases with rising temperature.
According to the results of the experiment;
It has been observed that, as an alternative to the fireclay and calcium aluminate cement mixture used in this study, mortars incorporating refractory sands with different properties can be produced. In addition, samples using aluminate cement in different ratios can be produced for refractory concrete production following mortar tests.
How and Why to Use Calcium Aluminate Cement as Refractory?
In the previous articles of the series, we have examined what refractory is and how to use calcium aluminate cement as refractory. The experimental studies have been conducted on the subject of calcium aluminate cement use as refractory to understand the usage of calcium aluminate cement (CAC) which a special type of cement is produced by sintering or melting bauxite and limestone instead of clay as refractory. For more information about calcium aluminate cement, you can check out our article here. Mechanical behaviors of the cubic samples prepared with the mortars produced using CAC and fireclay at room temperature have been identified through experiments conducted at temperatures between 100°C and 1200°C with incremental increases of 100°C. Volume and compressive strength values have been measured at every temperature range the samples have been subjected. The changes in measured values against temperatures have been determined. The results of the reference samples kept at the laboratory and the results of the samples prepared during the experiments have been compared. In light of the findings obtained in scope of these tests; the thermal strengths of concretes made using calcium aluminate cement have been analyzed and it has been concluded that these can be used as refractory.
References:
- Chotard T., Gimet-Breart N., Smith A., Fargeot D., Bonnet J.P., Gault C. ‘‘Application of ultrasonic testing to describe the hydration of calcium aluminate cement at the early age’’ Cement and Concrete Research, Vol.30, pp.405, 412, 2001
- Yeprem H.A., ‘‘Yerli Boksitlerin Dökülebilir Refrakterlerde Dolgu Malzemesi Olarak Kullanılması’’ Metalurji Dergisi, Year.17, Issue 135, pp. 49-54, 2003.
