CO.SUB.2 .-laden concrete precast products and the method of making the same

Abstract

The present invention relates to a process for producing precast products in an airtight enclosure, which comprises the steps of a carbonation of pre-dried concrete precast units by feeding CO.sub.2 gas into a closed airtight enclosure under near ambient atmospheric pressure (psig between 0 and 2) and/or low pressure (between 2 and 15 psig) conditions, wherein said pre-dried concrete units have lost between 25 to 60% of their initial mix water content.

Claims

1. A process for producing precast products in an airtight enclosure, which comprises the steps of: a) carbonation of pre-dried concrete precast units by feeding CO.sub.2 gas into a closed airtight enclosure under a pressure comprising between 0 and 15 psig, wherein said pre-dried concrete units have lost between 25 to 60% of their initial mix water content, and wherein a self-cleaning soaking step ensures that all the CO.sub.2 gas introduced into the enclosure during curing is consumed by the units, with minimal to zero residual CO.sub.2 present at the end of the curing cycle.

2. The process of claim 1, wherein said precast products are selected from the group consisting of masonry units, pavers, pipes, and hollow-core slabs.

3. The process of claim 1, wherein said airtight enclosure is a closed chamber.

4. The process of claim 1, which comprises step (i) to be performed before step (a): i) fan-assisted drying for accelerated water loss of wet newly formed precast concrete units to lose anywhere between 25 and 60% of a unit's initial mix water content.

5. The process of claim 1, wherein the process is a pseudo-dynamic process with regimented CO.sub.2 multi-injections.

6. The process of claim 1, wherein carbonation achieves a CO.sub.2 uptake equivalent to 15-25% mass of cement in a concrete mix.

7. The process of claim 1, further comprising the step of monitoring and recording at least one process variable pertaining to one chosen from injected gas flow rate, temperature, pressure, and CO.sub.2 concentration of interior of the enclosure.

8. The process of claim 1, further comprising, further comprising the step of controlling at least one process variable pertaining to gas flow rate, pressure, and CO.sub.2 concentration.

9. The process of claim 1, wherein carbonation curing is carried out at the low pressure conditions in an airtight pressurizable solid-walled enclosure.

10. The process of claim 1, wherein carbonation curing is carried out at the low pressure conditions in an airtight pressurizable solid-walled enclosure, preceded by a purging step to displace a volume of ambient air initially present in the enclosure.

11. The process of claim 1, wherein carbonation curing is carried out substantially at an ambient CO.sub.2 pressure in an airtight solid-walled enclosure, preceded by a purging step to displace in volume of ambient air initially present in the enclosure.

12. The process of claim 1, wherein carbonation curing is carried out substantially at an ambient CO.sub.2 pressure in an airtight flexible polymer enclosure, preceded by a vacuum step to exhaust 50 to 90% the volume of ambient air initially present in the enclosure.

13. The process of claim 1 wherein by-product-sourced CO.sub.2 gas with a purity ranging from 10 to 99% concentration can be used.

14. A precast concrete product prepared by the process claim 1, which has a high early-age compressive and flexural strength, and a high calcium-carbonate-reinforced CSH content, and is more resistant to freeze-thaw damage, sulfate attack, carbonation shrinkage, efflorescence, and chemical ion permeation, compared to a standard precast concrete product.

15. The precast concrete product prepared by the process claim 1, with a densified concrete paste matrix reinforced with property-enhancing nano-calcium-carbonate precipitates.

16. The precast concrete product prepared by the process of claim 1, with a cement content of 25 to 50% in said concrete mix.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present disclosure will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

(2) FIG. 1 illustrates the process flow diagram for Carboclave technology implemented for the making of concrete masonry blocks

(3) FIG. 2 illustrates a Carboclave masonry unit prepared as per the process flow diagram of FIG. 1;

(4) FIG. 3 illustrates the experimental pilot setup;

(5) FIG. 4 illustrates the schematic view of the chamber with Carboclave masonry units inside;

(6) FIG. 5 illustrates a graph that traces the interior gas pressure profile inside the experimental pilot chamber;

(7) FIG. 6 illustrates water loss profile for the monitored blocks and their resulting CO2 uptakes, expressed in weight % of initial cement content, for the first commercial-scale test carried out at an industrial autoclave;

(8) FIG. 7 illustrates pressure log within the industrial autoclave throughout the carbonation process, and cumulative CO2 level in the gas tanks displayed on the secondary vertical axis;

(9) FIG. 8 illustrates mass loss after 10 and 20 Freeze-Thaw cycles for differently cured masonry concrete slabs (Prior Art);

(10) FIG. 9 illustrates elongation of mortar bars under sulfate attack for differently cured specimens (Prior Art);

(11) FIG. 10 illustrates water loss profile for the monitored blocks and their resulting CO.sub.2 uptakes, expressed in weight % of initial cement content, for the second commercial-scale test carried out at an industrial autoclave;

(12) FIG. 11 illustrates Pressure log of autoclave throughout the carbonation process, and cumulative CO.sub.2 level in the gas tanks displayed on the secondary vertical axis.

(13) FIG. 12 is a schematic illustration of a polymer-based (geomembrane or polyurea) enclosure for concrete pavers capable of a vacuum pre-step prior to carbonation curing.

(14) FIG. 13 is another embodiment of a flexible polymer enclosure that can also undergo a vacuum pre-step prior to carbonation curing. Such an assembly can be suitable for various precast products, especially pipes.

(15) FIG. 14 is a simple illustration of an HMI display of the control system for an autoclave assembly.

(16) FIG. 15 concrete block forming of units to undergo commercial-scale carbonation as per Carboclave technology in an industrial autoclave assembly.

(17) FIG. 16 closely-monitored presetting of concrete blocks in drying tunnel before being subject to carbonation curing.

(18) FIG. 17 loading of preset concrete blocks into autoclave.

(19) FIG. 18 Tank carrying liquefied by-product-sourced high purity CO2 gas and vaporizer assembly.

(20) FIG. 19 Pressure gauge displaying interior autoclave pressure.

(21) FIG. 20 Example of a CO2 and 02 concentration reading for the interior of the autoclave.

(22) FIG. 21 Freeze/thaw cycling of cut sections from concrete blocks that underwent carbonation curing and conventional hydration curing. Graphs display the mass loss experienced after every fifth cycle.

(23) FIG. 22 Subsequent internal rehydration due to the encapsulation effect arising from the CaCO3 densified outer periphery of the concrete component. This promotes pH rebound, high subsequent strength gain, and protection of steel reinforcement. The graph reveals the strength of a Carboclave CMU after 1 day, and after 28 days.

(24) FIG. 23 Microstructural model for the pore structure of a cement paste slurry prior and post carbonation curing.

(25) FIG. 24 A microstructural illustration of the cement paste (a) before, and (b) after, carbonation activation.

(26) It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(27) The concrete units (referred herein as Carboclave) are manufactured from a binder blend composed of Portland cement and a supplementary cementitious material (SCM) replacing between 0-50% cement content, and activated by carbon dioxide for consolidation strength and enhanced durability. Carboclave products present a more sustainable alternative to commercial precast benchmarks in that their production associates a lower carbon footprint, and additionally converts CO.sub.2 gas into embedded, property-enhancing nanoscale calcium carbonate crystals (CaCO.sub.3). The nano-CaCO.sub.3 precipitates effectively reinforce the hardened cement paste, lending the final concrete product better mechanical performance and improved durability. Standardized test results show Carboclave concrete masonry units (CMU) as more resistant to common deleterious mechanisms (freeze/thaw cycling, sulfate attack, foreign ion ingress, etc. . . . ) in comparison to commercial blocks. Currently, standard CMU's are most commonly produced using steam-curing. Carboclave units on the other hand are produced through a carefully regimented carbonation process. The process entails a presetting (or drying) step prior to carbonation, whereby partial loss of mix water is achieved to facilitate CO.sub.2 diffusion within the concrete. Carbonation is conducted in low pressure (<15 psi) conditions preferably in an airtight solid or flexible enclosure. It is prolonged until the calculated amount of CO2 gas fed into the chamber is entirely consumed by the blocks during processing. This feature ensures that minimum to no residual CO.sub.2 gas is released to the atmosphere at the end of the processing cycle, an approach we coined self-cleaning. One full production cycle (presetting and carbonation) would last between 24-30 hours.

(28) The resulting Carboclave CMU's are characterized for being high in strength, with the capability of permanently storing an average of 0.3 kg (0.7 lb) of CO.sub.2 gas per block. This is equivalent to embedding 680 g of nano-CaCO.sub.3 crystals within the block, specifically within the resulting hardened paste (the binding matrix). The precipitation of these carbonates associates a densification effect that reduces porosity and pore-connectivity, thereby limiting ingress and the permeation of deleterious ions in and out of the concrete's structure. These blocks also displayed low water absorption, an important property for improved service-life performance.

(29) The high strength achieved by the Carboclave concrete articles allows for reduction of cement content. This is an important environmental measure since cement is the most expensive and ecologically-taxing component of concrete. To this effect, Carboclave blocks have been demonstrated to replace 25% of the cement content with waste-derived SCM (secondary cementitious materials) like Lafarge Newcem or Newcem-plus. The high-volume use of these additives is equivalent to diverting additional CO.sub.2 from the atmosphere in terms of carbon footprint per block. This along with the physical fixation of CO.sub.2 gas during processing, makes Carboclave blocks arguably the most sustainable and resilient CMU product in the market.

(30) The proposed processing method applies to all precast concrete products (reinforced and non-reinforced) that employ Portland cement as binder, as well as other binder systems that comprise CO2-reactive minerals. It also works for all air-tight curing assemblies that can and cannot be withstand elevated pressures (between 2 and 15 psig). An near-ambient pressure (between 0 and 2 psig) curing system is also presented that either displaces ambient chamber atmosphere via a purging step (solid wall chambers) or a vacuum step (flexible polymer wall) prior to carbonation curing.

(31) Market

(32) Annually, approximately 4.3 billion CMUs are produced between Canada and the USA [1], with CMU's presenting only a small segment of precast products. Moreover, regulations for alleviating global carbon footprint will mandate companies in the near future to reduce and even capture their CO.sub.2 emissions. In such case, there will be plenty of pure, industry-recovered CO.sub.2 for utilization. The monetizing of CO.sub.2 on a per ton basis is the ultimate aim of emerging carbon-trading/taxing green economies. Sequestration of CO.sub.2 may therefore present a source of revenue in such a framework.

(33) Carboclave Production Process

(34) FIG. 1 illustrates the process flow diagram for the processing of the Carboclave blocks. Table 1 is an example of an adopted preferred mix design for CMU.

(35) Mix Design ():

(36) An example of a preferred Carboclave mix design is summarized in Table 1. The proportions were devised for the block to be the most sustainable, with a 25% replacement of cement by SCM. Considering that the production of 1 ton of ordinary Portland cement (OPC) generates around 0.85 tons of CO.sub.2 [4], a 25% replacement in a block translates to a CO.sub.2 footprint reduction from 1.42 kg to 1.06 kg CO.sub.2 per block.

(37) TABLE-US-00001 TABLE 1 Mix proportions of Carboclave blocks Carboclave (CMU) Mix Design Material Mass (%) Aggregates 87.00 Cement 7.20-7.50 SCM* 2.50-2.80 Water 3.38 water-to-cement (w/c) 0.35 SCM: Supplementary Cementitious Material (Lafarge Newcem or Newcem-Plus)

(38) Presetting Stage () :

(39) Presetting is an important conditioning step to dry the blocks in order to create space and facilitate the diffusion of CO.sub.2 within the block. This is done to achieve optimum carbonation degrees. From an extensive parametric study, a mass loss in the range of 35 to 40% of the total water in the block yields optimal results in terms of reaction. The residual water content in the block is somewhat of a critical parameter. Too much water hinders CO.sub.2 diffusion; too little water results in water starvation. In both cases the carbonation reaction is limited. Therefore, there is an optimum water content that needs to be respected within the blocks before their carbonation. Water is integral as it is the medium for the multi-step carbonation reaction and where both CO.sub.2 gas solvates and calcium-silicates dissolve. However, it does not only serve as a medium, but is also a reagent, where it is consumed to form CSH. Both CSH and CaCO3 precipitates form in sites previously occupied by the water medium in the pore structure.

(40) For example, to calculate the mass of water that needs to be lost by a block, it is important to consider the aggregates' water absorption degree. The target mass loss per block, say 35%, can be calculated as such:
WL.sub.35%=[(M.sub.agg.A.sub.agg.)+(M.sub.block% Water)]35%(3) WL.sub.35%: Mass of 35% target water loss M.sub.Agg.: Mass of aggregates in block A.sub.Agg.: Absorption of aggregates M.sub.Block: Mass of block

(41) Carbonation and Self-Cleaning Concept () :

(42) The self-cleaning concept was developed to make sure that the CO.sub.2 gas introduced into the chamber is fully consumed by the blocks, avoiding the release of gas to the atmosphere when opening the chamber for retrieval of the samples at the end of the carbonation cycle.

(43) For this reason, the amount of CO.sub.2 introduced into the chamber has to be carefully regulated and based on the optimum amount that can be absorbed by the processed blocks. We optimize this regimen by means of mass balancing the CO.sub.2 feed and CO.sub.2 uptake achievable by the blocks. Since we are confined by the volume of a pressure chamber and the operating pressure under which carbonation is carried at, feeding of CO.sub.2 will need to be done in sequential increments until blocks reach their optimal storing capacity (15-20% cement mass). The chamber will be intermittently replenished with CO.sub.2 in response to pressure drops resulting from the reaction. Feeding is stopped once the entire mass of CO.sub.2 that can be consumed by the blocks is supplied.

(44) The number of times a chamber needs to be fully replenished with CO.sub.2 depends on the volume of the chamber, total volume of the loaded blocks, CO.sub.2 sequestration capacity of the blocks, and density of CO.sub.2 gas at the given operating pressure. The number of chamber refills is assigned the symbol , and presented in Equation 4 below:

(45) $\begin{array}{cc}=\frac{\mathrm{Mass}\mathrm{CO}2\mathrm{absorbed}\mathrm{by}\mathrm{blocks}}{\mathrm{Mass}\mathrm{CO}2\mathrm{occupied}\mathrm{by}\mathrm{chamber}\mathrm{freespace}}=\frac{\left(\%\mathrm{Cement}\mathrm{in}\mathrm{mix}{M}_{\mathrm{Block}}\right)U_{\mathrm{CO}2}Q}{\left[V_{\mathrm{chamber}}-\left(V_{\mathrm{Block}}Q\right)\right]K}& \left(4\right)\end{array}$

(46) Where, n: Number of chamber fillings M Block: Mass of block (17-18 kg) U.sub.CO2: % CO2 uptake per cement mass (between 15-20%) Q: Number of blocks loaded in chamber V.sub.chamber: Volume of chamber V.sub.Block: Volume of block (7.8 L) K: Mass-volume constant (Table 2)

(47) TABLE-US-00002 TABLE 2 Experimental calibration by filling a chamber with CO.sub.2 gas till a specific pressure is reached and recording the associated weight gain. Experimental Calibration, Chamber Volume = 5.65 L Pressure (psig) CO.sub.2 (g) K (g/L) 0 0 0 5 3.5 0.611 10 6.9 1.221 14.5 9.4 1.664 29.0 19.9 3.522 43.5 30.6 5.416 58.0 40.8 7.221 72.5 51.9 9.186

(48) Values agree with, and were verified against, the ideal gas law (PV=nRT).

(49) Limitation of this approach is that it does not account for CO.sub.2 absorbed during the primary filling of the container, as the carbonation reaction is quite rapid at the initial stages of exposure. To address this, a CO.sub.2 flowmeter can be employed to monitor the exact amount of gas injected into the chamber.

(50) The CO.sub.2 uptake achieved by the blocks is calculated by the equation below. During the course of carbonation, the reactions taking place are exothermic in nature (Eq. 1 and 2), and associate the release of heat. This is also met with the evaporation of residual water in the blocks. In order to properly determine the mass of CO.sub.2 taken up by a block, the vaporized and condensed water within the chamber need to be collected and accounted for as shown below.
CO.sub.2Uptake.sub.per block=(M.sub.Block(final)M.sub.Block(initial))+
M.sub.Evaporated Water(5)

(51) TABLE-US-00003 TABLE 3 Illustrative comparison between standard CMU and Carboclave units Comparison of Concrete Masonry Units (CMU) Standard CMU Carboclave Unit Dimensions = 8 8 16 inch Dimensions = 8 8 16 inch Volume = 7.8 L Volume = 7.8 L Average Block wt. = 17.0 kg Average Block wt. = 17.0 kg Mix Proportions: Mix Proportions: Aggregate = 87% Aggregate = 87% Portland cement = 9.8% Portland cement = 7.35% Water = 3.38% Newcem-plus = 2.45% Water = 3.38% Portland cement use = 1.67 kg Portland cement use = 1.25 kg (1 kg cement generates 0.85 kg (1 kg cement generates 0.85 kg CO.sub.2) CO.sub.2) CO.sub.2 footprint = 1.06 kg CO.sub.2 footprint = 1.42 kg Average CO.sub.2 uptake per block 300 g (offset) Absolute CO.sub.2 footprint = 1.06 0.30 = 0.76 kg Equivalent reinforcing nano- CaCO.sub.3 = 681.2 g Distinguishing Properties: Lower porosity, pore diameter, and pore connectivity Improved freeze/thaw resistance Improved sulfate-attack resistance Improved ingress resistance Improved resistance to carbonation shrinkage

(52) The present invention will be more readily understood by referring to the following examples which are given to illustrate the invention rather than to limit its scope.

Example 1

Field Test Calculations

(53) The following scenario was experimentally tested.

(54) # of blocks (Q)=10 as shown in FIG. 4.

(55) Volume of Chamber (V.sub.chamber)=287 L

(56) Mass of block (M.sub.Block)=17 kg

(57) Avg. CO.sub.2 uptake by cement mass (U.sub.CO2)=18%

(58) Volume of one block (V.sub.Block)=7.8 L

(59) Pressure of chamber (P.sub.chamber)=15 psi

(60) Absorption by aggregates (A.sub.agg.)=3%

(61) Mix design: Aggregates: 87.00% Cement: 7.35% SCM: 2.45% Water: 3.38%

(62) Presetting water loss, according to Equation 3:
WL35%=[[(17 kg0.87)0.03]+[17 kg0.0338]]0.35=0.356 kg

(63) A demolded block of 17 kg needs to lose a target 0.356 kg of water prior to undergoing carbonation.

(64) Number of chamber fillings for the given scenario, according to Equation 4:

(65) $K@15\mathrm{psi}\left(\mathrm{from}\mathrm{Table}3\right)=1.664g\text{/}L\left(i.e.9.4g\text{/}5.65L\right)$$=\frac{\left(0.09717,000g\right)0.1810}{\left[287L-\left(7.8L10\right)\right]1.664g\text{/}L}=\frac{2968.2g}{347.7g}=8.5$

(66) The 10 blocks can absorb a total of 2968.2 g of CO.sub.2. Respecting the maximum operating chamber pressure of 15 psi, complete refilling of the chamber will need to be carried out 8.5 times. (Filling the freespace of the chamber to 15 psi amounts to a total CO.sub.2 mass of 347.7 g).

(67) FIG. 5 illustrates a graph that traces the pressure profile inside the chamber. After approximately 18 hours, 94% of the CO.sub.2 that can be consumed by the blocks was achieved.

(68) CO2 uptake per block, according to Equation 5:

(69) Total evaporated water collected=986 g

(70) Water evaporated per block=986/10=98.6 g

(71) Average CO2 Uptake per block=30435.6 g CO2

(72) Average 1-day Compressive Strength of Carboclave blocks=22.61.4 MPa

(73) Average 1-day Compressive Strength of Hydrated reference blocks=16.61.1 MPa

Example 2

(74) Full Scale Pilot

(75) This site pilot was a step closer towards the practical realization of carbonation curing at Boehmers (by Hargest Block). CMU's were the precast products for this commercial pilot. The data shared in this report details the three major stages for the prescribed manufacturing process: 1. the pre-carbonation drying step; 2. low-pressure carbonation; and 3. the self-cleaning soak. Two full scale trials were conducted over the four day testing period, differentiated by the varied concrete mix-design batches. The first trial was conducted on normal-weight concrete, and referred to for short as the Day 1 batch. The Day 2 batch consisted of light-weight blocks. A 2-day period was allocated for each trial in order to accommodate the time-consuming steps of drying and carbonation. A summary of the results are tabulated below.

(76) For the Day 1 trial, drying was unassisted and lasted 16 hours. Carbonation prolonged for 24 hours and an average uptake of 0.435 kg (0.96 lbs) CO.sub.2 was achieved per normal-weight block. An initial purge was implemented for this trial to help flush out the residing air in the autoclave. An open release valve resulted in some reading discrepancies since it contributed to partial depressurization of the autoclave. For the Day 2 trial, all release-valves were capped, and initial purging avoided. The lightweight blocks achieved an average CO.sub.2 uptake of 0.356 kg (0.78 lbs) per block. Their full sequestration potential could not be reached as a result of high moisture content, beyond optimum levels for effective carbonation. A higher degree of drying needed to be achieved.

(77) For future considerations, a minimalist purging approach can be regimented by aid of a CO.sub.2 sensor affixed to the furthest release valve, where purging is halted as soon as a slightly elevated concentration of CO.sub.2 is detected. Drying can be expedited by fan/heat assistance, to reduce processing time. The target water loss for the normal-weight concrete should be between 35 and 40% of initial water, and a minimum of 40% for the lightweight blocks.

(78) TABLE-US-00004 TESTING SUMMARY DAY 1 Normal Weight (7 racks) DAY 2 Normal Weight - High-Strength (1 rack) Light-Weight (8 racks) Normal Weight - 25% Newcem-plus SCM (1 rack) Light-Weight - High-Strength (1 rack) Casting: @ 2 pm (Dec. 15) @ 12 pm (Dec. 16) Drying: 2pm-5:30am (Dec. 16), 29.2-38.6% 12pm-6am (Dec. 17), 25.4-34.9% 15.5 hrs water loss 18 hrs water loss Carbonation: 6:30am-6am (Dec. 17), 0.9-1.0 lb 9am-10am (Dec. 18), 0.7-0.8 lb 24 hrs CO.sub.2/block 25 hrs CO.sub.2/block Strength: (4 readings) 26.8-33.9 MPa (2 readings) 17.0-20.5 MPa

(79) Day 1: Normal Weight Concrete Blocks

(80) The autoclave can take up to 9 racks of concrete blocks. For this trial, 1 rack was reserved for a High-Strength blocks, and another rack for blocks using 25% Newcem-plus as an SCM. The remaining racks were normal weight concrete. Based on previous findings, the total CO.sub.2 uptake that could potentially be consumed in this trial was worked out to be 1,264 kg (2780 lbs). This breakdown is shown below.

(81) Projected CO.sub.2 Uptake:

(82) TABLE-US-00005 9 Racks total per kiln 468 Blocks per rack 4212 Blocks total 1 Batch = 122 blocks Rack = 4 batches For Day 1: 7 Racks Normal weight 7 468 = 3276 blocks (~300 g CO.sub.2/block) blocks: 1 Rack Normal weight, 1 468 = 468 blocks (~200 g CO.sub.2/block) 25% Newcem-plus: 1 Rack Normal weight, 1 468 = 468 blocks (~400 g CO.sub.2/block) High-Strength: TOTAL = 4212 blocks (~1264 kg CO.sub.2 Stored)

(83) TABLE-US-00006 MIX DESIGN B NORMAL WEIGHT, HIGH-STRENGTH, 17.9 kg/unit Aggregates 14.62 kg 81.70% PC 2.38 kg 13.30% Water 0.90 kg 5.04% w/c 0.38 MIX DESIGN A NORMAL WEIGHT, 16.8 kg/unit Aggregates 14.62 kg 87.00% PC/25% NewCem+ 1.63 kg 9.70% Water 0.56 kg 3.33% w/c 0.35 Water absorption by aggregates: Absorption by Sand ~4% Absorption by Agg. ~2% AVG. Absorption assumed ~3%

(84) TABLE-US-00007 TABLE 4 example calculation for target water loss in a normal-weight block WATER LOSS CALCULATIONS (e.g. Normal weight block) Initial Block wt.: .sub.16,800.sub.g .sup.{circle around (1)} Mix Water Water in Agg. (.sub.3.sub.% abs .sup.{circle around (3)}) 3.33% = .sub.560.sub.g .sup.{circle around (2)} 87% = .sub.438.sub.g .sup.{circle around (4)} Total Initial Water: + = .sub.998.sub.g .sup.{circle around (5)} Target Water Loss: 30.0% g Target Water Loss: 30.0% = _299.sub.g.sup.{circle around (6)} Target Block Weight: = .sub.16501.sub.g

(85) For the Day 1 trial, a total of 4 blocks were retrieved from the production line during casting to serve as the representative control specimens for profiling the water loss during the drying step, and quantifying the CO.sub.2 uptake from the weight differential after carbonation. The freshly cast blocks were collected during the preparation of different racks. The preparation of an entire charge normally takes 3 hours. Loading and unloading lasts 1 hour each.

(86) Table 5 summarizes the results associated with each monitored block for the Day 1 trial. All blocks achieved their minimum required water loss except for Block 1-3. This block represented the high-strength concrete batch, which was expected to take longer since this mix design entailed a higher total water content in the initial block, and also contained more cement than the original normal weight blocks.

(87) TABLE-US-00008 TABLE 5 Tabulated results for the monitored blocks of the Day 1 trial DAY 1 TRIAL RESULTS Block ID: {circle around (1)}-{circle around (1)} {circle around (1)}-{circle around (2)} {circle around (1)}-{circle around (3)} {circle around (1)}-{circle around (4)} Obtained from casting 3 2 1 9 of rack: Description: Normal-weight Normal-weight Normal-weight Normal-weight 25% Newcem SCM High-Strength Mix Design: A A B A Casting wt. (g): 17,767 18,032 18,312 18,294 Target water loss (%): 30 30 30 30 Actual water loss (%): 38.8 36.8 29.2 38.6 Pre-carbonation wt. (g): 17,244 17,278 17,794 17,636 Post-carbonation wt. (g): 17,570 17,588 18,154 17,980 *Adjusted final wt. (g): 17,670 17,688 18,254 18,080 wt. CO.sub.2 (g): 426 410 460 444 wt. CO.sub.2 (lb): 0.9 0.9 1.0 1.0 Strength (MPa): 33.9 Strength of arbitrarily 28.3 (Rack 4, top of rack) chosen blocks (MPa): 29.3 (Rack 5, mid rack) 26.8 (Rack 6, bottom of rack) *Adjusted final weight - accounts for water lost by blocks during carbonation, which from previous trials was found to equal around ~100 g per block.

(88) Interestingly, the monitored blocks achieved higher CO.sub.2 uptakes than in the previous miniature site tests. This could be due to a more precise and regimented drying process. The adjusted final weight values correct for water loss arising from the carbonation of the blocks. On average, each block experiences 100 g weight drop, a value that was repeatedly and carefully measured during previous miniature site tests. FIG. 6 shows the water loss profile for the monitored blocks and expresses the respective CO.sub.2 uptake values in terms of weight fraction of initial cement content. As shown, the block with the highest water loss achieved the highest carbonation degree.

(89) FIG. 7 displays the pressure log of the autoclave recorded throughout the carbonation process. There was no pressure build-up for the first 1.5 hours since an initial purge was executed in order to flush the autoclave's residing air. Purging was stopped as soon as a high CO.sub.2 concentration was detected atop the exterior stack, after which the back valve was closed and pressurization initiated. The primary fill of the autoclave to 10 psi took 55 min. A significant amount of the carbonation reaction was expected to have occurred during the purge and initial filling of the chamber. This extent of the reaction could not be accounted for through monitoring the autoclave's pressure drop and/or recording the decrease of CO.sub.2 levels in the tanks, since these methods could not differentiate the fraction of CO.sub.2 reacted with the blocks and the fraction emitted through the exhaust stack. More specific methods are more accurate, such as monitoring the individual block's weight differential, or conducting thermal decomposition analysis as this technique is the most effective in determining the absolute CO.sub.2 content within a block.

(90) Over a carbonation period of 24 hours around 6 fills were injected, which by conversion to mass equivalents from the calibration curve cumulatively amount to a total sequestration of 1395 Kg (3069 lbs) CO2, or an average of 0.76 lbs/block. This may not be very accurate since this approach fails to account for the blocks' carbonation engagement during the purging step and subsequent fillings. Also, during this trial one of the autoclave's release valves was open, thereby partially contributing to the depressurization of the autoclave. This made deductions solely from the pressure log slightly unreliable. An alternate approximation was through monitoring tank level drops, which indicate that a total of 5023 lbs (2283 kg) CO.sub.2 were emptied from the tanks for the Day 1 trial. Again, not the entire amount is expected to have been absorbed by the blocks since a considerable portion of the gas was ejected out of the autoclave during purging and the valve leak during carbonation. The more representative approximation was that obtained from the weight gain experienced by the monitored blocks (Table 5), which averaged an uptake of 0.435 kg (0.96 lb) CO.sub.2 per block.

(91) Nonetheless, the most accurate determination for the absolute CO.sub.2 content of a block can be attained from thermal analysis, where weight loss between 650-850 C. is attributed to the release of CO.sub.2 from the decomposition of CaCO.sub.3, the primary product of carbonation. This analysis will be performed shortly on representative specimens obtained from each block.

(92) Freeze-thaw and Sulfate-attack Performance:

(93) The following table details standardized laboratory testing conducted to evaluate the performance of carbonated concrete subject to freeze-thaw cycling and sulfate-attack.

(94) TABLE-US-00009 TABLE 6 Freeze-Thaw and Sulfate-Attack testing as carried out by referenced study herein .sup.1 Testing Deterioration Protocol quantification Experimental conditions Results Freeze-Thaw Mass-loss due 3% NaCl solution Results graphically depicted in FIG. 1 CSA A231.2 to deleterious Concrete slabs: 40 76 127 mm Slabs with 9% CO.sub.2 uptake by (1995) internal Mix Design: cement mass yielded better expansion of Cement: 286 kg/m.sup.3 resistance to F/T deterioration in water Agg.: 730 kg/m.sup.3 this comparative study crystals Sand: 1050 kg/m.sup.3 Mass loss was the lowest for these Water: 100 kg/m.sup.3 slab specimens w/c = 0.35 The carbonation-modified surface 18 hrs freezing @ 15 C. lowers permeability, thereby 6 hrs thawing @ 21 C. reducing water ingress and 20 cycles therefore frost damage and scaling. Mass loss measured every 10 cycles Sulfate Dimensional 5% Na.sub.2SO.sub.4 Results graphically depicted in FIG. 2 Attack elongation of mortar bars: 25 25 285 mm Carbonated bars displayed better ASTM C1012 specimens cement/sand: 1/2.75 resistance to sulfate attack immersed in w/c = 0.36 These bars measured the least sulfate length of bars monitored weekly longitudinal expansion solution Improved performance possibly owed to the reduced gypsum and ettringite formation as a result of carbonation's consumption of hydration product Ca(OH).sub.2. .sup.1 Rostami, V.; Shao, Y.; Boyd, A. J. Carbonation Curing versus Steam Curing for Precast Concrete production. Journal of Materials in Civil Engineering, 2012, 24(9), 1221-1229.

(95) FIG. 8 shows that the carbonated masonry slabs generally performed better than the steam-cured batch. The best performance was displayed by the batch subject to carbonation and followed by subsequent hydration. Subsequent hydration was achieved by replenishing the slabs via intermittent water spraying (this could have also been achieved by placing the slabs in a fog room, i.e. 100% relative humidity). This batch appeared intact post testing and only amounted to an overall mass loss of 8.6% after 20 freeze-thaw cycles, compared to the heavily fragmented steam-cured slabs, which experienced almost 68% mass loss under the same exposure conditions.

(96) FIG. 9 summarizes the sulfate-induced deterioration for differently cured mortar bars. Again, the carbonated specimens demonstrated the most dynamic stability, as these bars experienced the least longitudinal expansion. The deterioration mechanism is usually facilitated by the presence of Ca(OH).sub.2, an abundant by-product in concrete originating from the hydration of cement. Carbonated specimens display considerably lower Ca(OH).sub.2 content since these crystals are normally consumed by the carbonation reaction to form the much less-soluble CaCO3 precipitates. This, in effect, hinders the formation of gypsum and ettringite, which are key ingredients for deleterious dimensional instability and loss of strength.

Example 3

(97) Day 2: Light-Weight Concrete Blocks

(98) For the Day 2 trial, the autoclave was charged with lightweight concrete blocks. One of the racks was reserved for high-strength lightweight blocks. Compared to the previous full-scale trial, a few modifications were made. 1. All release-valves were plugged to make sure depressurization of the autoclave was solely attributed to the carbonation of the blocks, and not from leakage. 2. No purging step was implemented, i.e. a closed system from beginning to end. 3. Carbonation pressure was raised to 14 psi rather than 10 psi. This will help reduce the number of autoclave refills.

(99) Lightweight blocks should be able to achieve higher CO.sub.2 uptakes than normal weight blocks as their mix design includes a higher cement content. However, these blocks require more intense drying since they contain 1.5 times the initial water content of normal concrete. The expanded-slag aggregates used in these blocks exhibit high water absorption behavior. For this reason, the drying of the full charge of blocks was assisted by fanning the tunnel from both ends.

(100) The total CO.sub.2 uptake that could potentially be consumed in this trial was worked out to be 1,395 kg (3069 lbs), according to the following breakdown:

(101) Projected CO.sub.2 Uptake by Kiln:

(102) TABLE-US-00010 9 Racks total per kiln 468 Blocks per rack 4212 Blocks total Batch = 122 blocks 1 Rack = 4 batches For Day 2: 8 Racks lightweight 8 468 = 3744 blocks (~315 g CO.sub.2/block) blocks: 1 Rack lightweight, 1 468 = 468 blocks (~460g CO.sub.2/block) High-Strength: TOTAL = 4212 blocks (~1395 kg CO.sub.2 Stored)

(103) TABLE-US-00011 MIX DESIGN C MIX DESIGN D LIGHT-WEIGHT, LIGHT-WEIGHT, HIGH-STRENGTH, 14.2 kg/unit 15.1 kg/unit Sand 1.25 kg 8.80% Stone 0.51 kg 3.35% Exp. Slag 10.42 kg 73.40% Exp. Slag 10.78 kg 71.40% PC 1.87 kg 13.20% PC 2.76 kg 18.30% Water 0.67 kg 4.62% Water 1.05 kg 6.95% w/c 0.35 w/c 0.38 Water loss: Absorption by Sand ~4% Absorption by Agg. = 8% Assume absorption only by expanded slag, with an average of = 7.5%

(104) TABLE-US-00012 TABLE 7 example calculation for target water loss in a light-weight block WATER LOSS CALCULATIONS (e.g. lightweight block) Initial Block wt.: .sub.14,200.sub.g .sup.{circle around (1)} Mix Water Water in Agg. (.sub.7.5.sub.% abs .sup.{circle around (3)} ) 4.62% = .sub.656.sub.g .sup.{circle around (2)} 82.2% = .sub.875.sub.g .sup.{circle around (4)} Total Initial Water: + = .sub.1531.sub.g .sup.{circle around (5)} Target Water Loss: 30.0% g Target Water Loss: 30.0% = _460.sub.g.sup.{circle around (6)} Target Block Weight: = .sub.13,740.sub.g

(105) Again, 4 blocks were retrieved from the production line during casting to serve as the representative control specimens for profiling the water loss during the drying step, and quantifying the CO.sub.2 uptake from the weight differential after carbonation.

(106) TABLE-US-00013 TABLE 8 Tabulated results for the monitored blocks of the Day 2 trial DAY 2 TRIAL RESULTS Block ID: {circle around (2)}-{circle around (1)} {circle around (2)}-{circle around (2)} {circle around (2)}-{circle around (3)} {circle around (2)}-{circle around (4)} Obtained from casting 3 6 1 9 of rack: Description: Lightweight Lightweight Lightweight Lightweight High-Strength Mix Design: C C D C Casting wt. (g): 15,000 15,049 15,562 14,834 Target water loss (%): 30 30 30 30 Actual water loss (%): 34.0 34.9 25.4 25.6 Pre-carbonation wt. (g): 14,398 14,432 14,975 14,288 Post-carbonation wt. (g): 14,668 14,698 15,254 14,496 *Adjusted final wt. (g): 14,768 14,798 15,354 14,596 wt. CO.sub.2 (g): 370 366 379 308 wt. CO.sub.2 (lb): 0.8 0.8 0.8 0.7 Strength (MPa): 17.0 Strength of arbitrarily 20.5 (Rack 8) chosen blocks (MPa): *Adjusted final weight - accounts for water lost by blocks during carbonation, which from previous trials was found to equal around ~100 g per block.

(107) Results for this trial's monitored blocks are summarized in Table 8. The water loss profile for the blocks are graphed in FIG. 10. Both blocks 2-3 and 2-4 could not reach their target water loss. Consequently, they displayed the least carbonation reactivity in terms of cement engagement. The blocks of this trial have a much higher sequestration potential than the normal-weight blocks of the Day 1 trial, but were unable to reach their optimum CO.sub.2 reactivity. Closer visual and numerical observations seemed to suggest that the blocks were slightly more water saturated than required, an effect that hinders the diffusion of CO.sub.2 within the blocks and, therefore, overall reactivity. The expanded slag aggregates used in these blocks have a high absorption (8%), which may be inflicting a saturation effect as it replenishes the cement paste with water during carbonation. A minimum loss of 40% of total initial water may prove more appropriate for these blocks. It is highly recommended that drying of these blocks be thoroughly assisted. The log for the internal pressure of the autoclave is graphically depicted in FIG. 11, along with the cumulative CO.sub.2 levels in the tanks. The initial filling of the autoclave to 14 psi lasted 30 min, considerably faster than the previous full-scale. This is largely owed to capping all release valves (no leaks) and also adjusting for a higher flow-rate. The autoclave was manually regulated at 14 psi by replenishing the CO.sub.2 gas after each considerable pressure drop. At the 13 hour mark, all the remaining CO.sub.2 in the tanks was pushed into the autoclave, which resulted in a surge in pressure to 16 psi. The inlet was then closed. At the 24 hour mark, the internal pressure was 8 psi, which meant that not all the injected CO.sub.2 was absorbed by the blocks, and the residual gas was released into the exhaust stack. Self-cleaning could not be attained. This is primarily due to the high moisture content of the blocks.

(108) TABLE-US-00014 TABLE 9 CO.sub.2 uptake approximations as determined by different approaches DAY-1: CO.sub.2 Uptake (4209 blocks) Remarks Recording CO.sub.2 tank level drop Approach 1: Total CO.sub.2 CO.sub.2/block (kg) CO.sub.2/block (lbs) Amount not fully absorbed by blocks 1322 kg 0.314 kg/ block 0.69 lbs/block as there was residual gas at the end of carbonation that had to be flushed out Mass conversion of autoclave's internal pressure log Approach 2: Total CO.sub.2 CO.sub.2/block CO.sub.2/block Does not account for reaction 797 kg 0.189 kg/block 0.42 lbs/block occurring during filling steps. Average weight differential of monitored blocks Approach 3: Total CO.sub.2 CO.sub.2/block (kg) CO.sub.2/block (lbs) More accurate than preceding two 1498 Kg 0.356 kg/block 0.78 lbs/block approaches However, results only based on the 4 monitored blocks Thermal decomposition of CaCO.sub.3 between 650-850 C. Approach 4: Total CO.sub.2 CO.sub.2/block (kg) CO.sub.2/block (lbs) Most accurate determination of the ? ? ? absolute CO.sub.2 content

(109) For this Day 2 trial, no purging was implemented and all valves were tightly capped. This meant that the depressurization of the autoclave was solely owed to the blocks' reaction with CO.sub.2. Table 9 lists the different approaches used to approximate the CO.sub.2 uptake. The individual bulk approaches of monitoring tank levels and autoclave logs may not be accurately reflective since not all the gas injected was fully consumed, and reactions occurring during fillings could not be accounted for by these approaches. From the weight gain of monitored blocks, the average CO.sub.2 uptake measured was 0.356 kg (0.78 lbs) CO.sub.2 per block.

(110) FIGS. 12 through 20 are ones taken for the abovementioned examples pertaining to the commercial pilot trials.

(111) Table 10 below displays how Carboclave blocks compare to Beohmer's own premium autoclave products. While heavier and denser due to carbon loading, Carboclave blocks also associate higher physical resilience, as clearly demonstrated by strength values.

(112) TABLE-US-00015 TABLE 10 Average values for 20 cm masonry blocks prepared via conventional autoclaving and via Carboclave technology. Boehmers Normal Weight 20 cm Masonry Blocks Physical Property Autoclave Carboclave Oven Dry Mass (kg) 16.511 16.965 Density (kg/m.sup.3) 2129 2213 Absorption (%) 5.924 4.990 Suction (%) 0.670 0.174 Dry Shrinkage (%) 0.0129 0.0196 Splitting-tensile Strength, 1 day (MPa) 1.83 2.04 Compressive Strength, 1 day (MPa) 23.6 35.6 Compressive Strength, 28 day (MPa) 27.2 52.5

(113) Increased Product Resilience:

(114) Concrete mansonry units prepared via the presented Carboclave methodologies exhibit tangibly improvements in resilience durability. FIG. 21 summarizes results obtained from a standardized freeze/thaw cycling test. The results pictographically compare concrete specimens retrieved from a Carboclave masonry unit and an identical unit that had undergone conventional hydration. The adjacent graphs reveal plot the relatively intact, only losing 4.8% of its initial weight after 20 cycles, while the hydrated specimen lost 29.4% of its mass.

(115) FIG. 22 reveals a phenolphthalein-sprayed cross section of a concrete slab prepared via Carboclave technology and left to hydrate for a subsequent 28 days. The cross-section reveal pH gradient, with a highly alkaline core and a less alkaline periphery, which experiences the heaviest degree of carbonation. This densified outer layer also functions as a form of encapsulation to promote further internal hydration of the unreacted cement portion within the concrete. The very high compressive strength achieved by Carboclave blocks after 28 days is a reflection of this feature. Moreover, this internal hydration also incurs a pH rebound effect, bringing the pH back up to alkaline ranges typical of normal concrete and re-promoting the passivation protection of steel-reinforcement where applicable.

(116) Other embodiments for Carboclave technology implementable in enclosure assemblies enabling near-ambient pressure conditions are presented in FIGS. 12 and 13.

(117) Theoretical Discussion:

(118) To better illustrate the evolution of the pore structure of the cement paste as a result of carbonation, FIG. 23 presents a simplified microstructural schematic. In the initial paste (cement+water), cement grains are densely packed such that the small voids separating them constitute the pore structure. These voids are filled with water initially. After drying presetting, the voids become partially depleted of water, promoting enhanced gas permeation within the paste. After carbonation, the cement grains are almost entirely consumed to form reaction products CSH and CaCO.sub.3, which form an enveloping composite matrix that is expansive due to a lower specific density. The ensuing pore structure comprises void pores, capillary pores, and gel pores (nano-pores within the CSH structure) as respectively indicated by the arrows in the figure.

(119) FIG. 24 presents another schematic illustration of the formation of the composite paste matrix. Initially, water and cement grains only constitute the paste slurry. After carbonation, CSH and CaCO.sub.3 are generated within the interstitial spaces previously occupied by water, where CSH forms the binding matrix, and the randomly oriented carbonate precipitates act as a sort of granular reinforcement to the matrix, very much like aggregates reinforce concrete.

(120) While preferred embodiments have been described above and illustrated in the accompanying drawings, it will be evident to those skilled in the art that modifications may be made without departing from this disclosure. Such modifications are considered as possible variants comprised in the scope of the disclosure.

REFERENCES

(121) 1. El-Hassan, H.; Shao, Y.; Ghouleh, Z. 2013. Effect of Initial Curing on Carbonation of Lightweight Concrete Masonry Units. ACI Materials Journal 110(4), 441-450. 2. Young, J. F.; Berger, R. L.; Breese, J. 1974. Accelerated Curing of Compacted Calcium silicate Mortars on Exposure to CO2. Journal of the American Ceramic Society 57(9), 394-397 3. Bukowski, J. M.; Berger, R. L. 1979. Reactivity and Strength Development of CO2 Activated Non-Hydraulic Calcium Silicates. Cement and Concrete Research 9(1), 57-68. 4. Puertas, F.; Garcia-Diaz, I.; Barba, A.; Gazulla, M. F.; Palacios, M.; Gomez, M. P.; Martinez-Ramirez, S. 2008. Ceramic Wastes as Alternative Raw Materials for Portland Cement Clinker Production. Cement and Concrete Composites 30(9), 798-805.