NSF Carbon Sequestration. Concrete Masonry Block. 2 nd Trial. This report compares testing of carbon sequestration at low pressure CO2 (5psi) with a

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1 NSF Carbon Sequestration Concrete Masonry Block 2 nd Trial Purpose: This report compares testing of carbon sequestration at low pressure CO2 (5psi) with a fine water mist addition. CO2 CHAMBER, MATERIALS and TESTS The initial trial was comparing the standard production method using normal kilns and a baseline carbon dioxide, CO2 curing method. To enable acceleration of carbon sequestration, a special curing box was manufactured using local suppliers. The figure below is a schematic of this equipment: The CO2 curing chamber was designed with a vacuum system, thermocouples for monitoring temperature and humidity, ports for addition of CO2 gas and a fine mist 1

2 water vapor while holding pressure up to 20 psi. Research literature has noted the optimum carbon dioxide acceleration are relative humidity between 50% and 70%, CO2 of 100%, a higher temperature increases the rate of carbonation, proper moisture content, and a chamber pressure of 20psi. The concrete masonry mix was designed using a 70/30, 30% coarse to 70% fine aggregate ratio and a 10 to 1 aggregate to cement ratio. The batch was a 4000 pound mix, with 400 pounds of cement and mixed in three batches of 1200 lbs. The second test was performed with the same mix design as the first as noted above, except that vacuum was not used to empty the chamber and the batch was mixed in one sequence of 4000 lbs. The CO2 cured CMU s were immediately placed in the curing chamber, whereas the kiln cured CMU s were placed as side for a 2-hours pre-set time. These CMU s would not be placed in the kiln for curing, but the CMU s would be placed in the curing chamber without adding CO2, but adding a fine water mist at lower air psi, as compared to the CO2 pressure of 5psi. All of the other parameters of humidity, block weights and pressure will be monitored for the CO2 cured CMU s and the normal cured CMU s. The water temperature was heated to +100 degrees F for both types of curing. The cement used was a Type I, Portland cement, meeting the requirements of ASTM C150, Standard Specification for Portland Cement. Below is the specific analysis: Constituent Percentage C3S 59.0 C2S 11.4 C3A 8.0 C4AF 8.2 Freelime 1.1 Limestone 3.7 Gypsum, SO

3 Loss on Ignition 2.6 Total Alkali As Na2O 0.58 MgO 2.8 Insoluble 0.30 Curing Specimens that were to be CO2-cured were randomly selected from freshly molded batches and immediately moved into the curing chamber. The chamber was then pressurized with CO2 with a fine water mist (see photo) and low CO2 pressure applied. Fine Water Mist into Curing Chamber CMU s in Curing Chamber before CO2 curing 3

4 The blocks were left in the CO2 curing chamber for 2 hours, and temperatures and pressures were monitored using thermocouples. Compressive strength test specimens were prepared according to ASTM C140 and were conditioned for strength testing immediately after CO2 curing or after normal water curing in a mist atmosphere. Specimens were prepared for comparison testing between traditional steam curing and carbon dioxide curing using the variables of compressive strength, water absorption, and efflorescence. Study Background Early-age carbonation curing of concrete products result in improved strength, increased surface hardness, and reduced surface permeability to water, also well as the potential for efflorescence. Carbonation reaction between carbon dioxide and appropriate calcium compounds result in permanent fixture of the carbon dioxide in a thermodynamically stable calcium carbonate. The moisture content, relative humidity, of the hydrated system has considerable and important influence on the rate and ultimate extent of carbonation. The purpose of this study is to quantify carbon dioxide absorption levels in concrete masonry units based on mix design and curing methods. Low Pressure Trial Specimens were prepared for comparison testing between normal curing and carbon dioxide curing using the variables of compressive strength, water absorption, and efflorescence using phenolphthaein solution. 4

5 Temperature Temperature Continuous monitoring of temperature, relative humidity, and weight gain will be performed during the 2 hour period, note temperature profiles below: Temperature over Time CO2 cured CMU s Temperature Profile Temperature over Time Normal Cured CMU s Temperature Profile Note steady temperature from time CMU s are placed into chamber until completion of 2-hour CO2 addition with fine water mist. Maximum temperature reached was 30 degrees C (86 degrees F) as compared to the first trial from a starting temperature of 22 degrees C (71.6 degrees F) after 60 minutes and remained constant until end of the 2- hour CO2 test. This compares to a starting temperature of 19 degrees (66 degrees F) for the first test; the maximum is similar to the first test of 30 degrees C. The normal cured CMU s after the 2-hour preset started at a temperature point of 24.5 degrees C 5

6 Humidity (% Humidity (% (76 degrees F due to thermal temperature from the CO2 cured test). Note the trend of lower temperatures for 60 minutes into the test until the heat of hydration from the cement and the hot water mist spray started the hydration sequence. The maximum of 40 degrees C (104 degrees F) was not achieved (as noted in previous report). Additional, note the relative humidity curve below reached a maximum at start of CO2 addition of 89%RH and continued at a steady upward trend of 97% RH with the hot water mist. Whereas the normal cured CMU s started at a higher 97% RH and reached a maximum of 99% RH. This again was due to the humidity level within the curing chamber from the previous CO2 test. Relative Humidy CO2 cured CMU s Relative Humidity Relative Humidy Normal Cured CMU s Relative Humidity 6

7 Weigh Weigh Note the consistent gain of CMU weight for the CO2 cured CMU s as compared to the normal cured CMU s. The carbon sequestration technique using CO2 curing, gain of weight is critical reflecting the acceleration of reaction, whereas the normal cured CMU s are not as dependent on gain of weight. Weight over Time (lb) CO2 Cured CMU s Weight Gain Weight over Time (lb) Normal Cured CMU s Weight Gain The total weight gain for the CO2 cured CMU s amounted to 12 pounds of water for 18 CMU s in the curing chamber or a gain of 0.67 lbs per CMU. A dramatic difference as compared to the first trial of a gain of only 0.11lbs per CMU, thus the fine hot water mist with the CO2 does create a more optimum environment for reaction. Analysis of Compressive Strengths Compressive strengths were compared at 2-hour, 24-hour, and 7-day interval for the CO2 accelerated blocks and the normal cured CMU s. 7

8 PSI NSF CO2 Curing Hr 1-Day 7-Day 28-Day Time Normal Cure CO2 Cure 2nd Normal Cure 2nd CO2 Cure CO2 cured blocks developed better strength gain during the first 2 hours of cure as compared to the normal cured CMU s. CMU s Strengths 2-hr 1-day 7-day CO2 curing CO2 Curing Avg 310 psi 1090 psi 1160 psi Normal Curing Normal Curing Avg 160 psi 1280 psi 1830 psi After 1-day of curing the CO2 cured CMU s and the normal cured CMU s were close in strength gain. At 7-days, normal cured CMU s out performed the CO2 accelerated CMU s. CMU s compressive strength variance, at early age, 2-hrs is within 50 psi for CO2 cured CMU s as compared to 20 psi for normal cured CMU s. At 1-day the variance increases for CO2 cured CMU s to 140 psi as compared to 280 psi for normal cured CMU s and 8

9 7-day variance for the CO2 cured CMU s increases to 510 psi as compared to the normal cured CMU s of 940 psi for a set of 3 blocks per age. Thus compressive strength psi variance increases with age of CMU. Thus the amount of compaction and vibration influences the optimum average psi of these CMU s. An optimum moisture condition is necessary to promote the carbon sequestration reaction but the proper amount of CO2 is necessary for the pore structure to absorbed the CO2 into the matrix of the CMU. The second trial was necessary in order to seek the proper mist spray, but the low CO2 pressure was not enough to develop proper strength gain. Absorption Results Comparing water absorption results between the CO2 cured versus the normal cured CMU s as per ASTM C140 at 14-day of age: CO2 cured CMU s Normal Cured CMU s Variable 2-hrs 24-hrs 7-days 2-hrs 24-hrs 7-days Compression 310 psi 1090 psi 1160 psi 160 psi 1280 psi 1830 psi Absorption 8.76 pcf 8.76 pcf 8.76 pcf 8.59 pcf 8.59 pcf 8.59 pcf Moisture 42.13% 42.13% 42.13% 58.39% 58.39% 58.38% Oven-dry Density pcf pcf pcf pcf pcf pcf The maximum ASTM C140 CMU s water absorption permitted for an average of 3 units is 13 lbs/ft3. Whereas the minimum Dry-Oven Density of concrete permitted are 125 lbs/ft3 Conclusions and Recommendations Again the CO2 accelerated CMU s were outperformed the normal cured CMU s during the initial 2-hours of testing. The amount of moisture, relative humidity, pressure and concentration of CO2 gas is critical to the optimum performance of carbon sequestration. 9

10 The minimum pressure of CO2 is greater than 5psi, but less than 20 psi needed for reaction to occur. It has been reported that carbon sequestration will react 0.5kg (1.1 lbs) of CO2 for every 1.0kg of cement, if all the cement is reacted during the curing regime. The maximum CO2 uptake by the cement can be estimated based upon the chemical composition where the amount of CaO (63.2%), using the Steinour equation: CO2 (wt%) = (CaO 0.7SO2) MgO Na2O K2O The calculated carbonation capacities of the cement use for this trial are about 50%, thus the maximum theoretical weight gain for the concrete masonry unit is 0.55lbs. From the amount of moisture in the CO2 curing chamber (0.67 lbs), there was more than sufficient moisture available, but not the optimum amount of CO2. In conclusion, the third test should be with a CO2 pressure more than 5psi, but less than 20 psi, probably 10 psi would work and keeping the fine hot water mist spray. Additionally 28-day strength will be out on July 5 th and the test for efflorescence needs To be performed on either the 7-day or the 28-day CMU s. 10