Ind. Eng. Chem. Res. 1999, 38, 2641-2649
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MATERIALS AND INTERFACES High-Pressure Molding and Carbonation of Cementitious Materials F. Carl Knopf,† Amitava Roy,‡ Heather A. Samrow,‡ and Kerry M. Dooley*,‡ Departments of Chemical Engineering and Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
Bulk carbonation of cements can have several beneficial effects, including permeability and porosity reduction, increased compressive strength, and pH reduction. Using supercritical and near-critical CO2, we examined both in-situ molding processes and postsetting treatments of cement mixtures, including those with fly ash, cement slag, and reactive silica. Specimens were characterized by X-ray diffraction analysis, scanning electron microscopy, and thermogravimetric analysis (for carbonate content), pH by a contact method, and porosity measurements such as N2 adsorption and water absorption. Surface carbonation was almost instantaneous for cured cements using supercritical CO2, and rapid bulk carbonation of forms several millimeters thick could be effected using in-situ molding. Carbonation by supercritical CO2 formed a dense layer of interlocking CaCO3 crystals in minutes. The best way to rapidly carbonate large cement forms was to harden them in a mold under CO2 pressure; these materials cured at an accelerated rate, were densified, and showed enhanced formation of crystalline calcite. In some cases this was accomplished without significant loss of microporosity. The presence of different types of reinforcing fibers did not impede carbonation by this method. Introduction Supercritical Fluids for Cement Carbonation. Carbonated concretes have certain desirable properties which have prompted extensive studies on the carbonation reactions of cements. The improved durability and higher compressive strength of certain carbonated cements can allow the use of thinner blocks and thus less material for the same required strength,1,2 for example, in lightweight fire-resistant structural panels or cementbased roofing tiles. Of special interest is the use of lowcost reinforcing fibers and normal glass fibers in bulk carbonated cements; many such fibers are incompatible with the normal pH (∼13) of Portland cements. Between 3 and 4 billion lbs/yr of carpet fiber is being land-filled, but by incorporation of recycled carpet polymers in cement structures, the old carpets can become a useful resource rather than a waste. Carbonation and polymer reinforcement could also aid the petroleum, metallurgical, and chemical industries with strengthened and more chemical-resistant concrete, particularly with enhanced acid resistance.3,4 Finally, bulk-carbonated cements have essentially no die-swell or warpage, an advantage in the ceramics industry. We have found that a superior method to rapidly carbonate large cement forms is to shape and harden the cement in a mold under high CO2 pressure, at supercritical or near-critical conditions. Here uncured cement is exposed to supercritical or near-critical CO2 as part of a molding process. We show that this novel * To whom correspondence should be addressed. Fax: 225388-1476. Phone: 225-388-3063. E-mail:
[email protected]. † Department of Civil and Environmental Engineering. ‡ Department of Chemical Engineering.
carbonation method is more rapid than postmolding treatments with high-pressure CO2 or methods that admit relatively small amounts of CO2 to a mold at relatively low pressure and then compress the uncured mixture. We show that cements molded in this manner are also denser than otherwise comparable cements, either those not treated with CO2 or those treated with CO2 but after hardening. We further show that this method may be used with many cement compositions, including those made with waste products such as fly ash or cement slag. Rapid-set cements can be prepared by alkaline activation of these materials. Surface carbonation is almost instantaneous, and bulk carbonation of forms several millimeters thick is rapid. We show that by combining molding, curing, and carbonation into a single step carbonates are better distributed throughout the entire specimen or form, producing a more uniform cement product. Key steps in the interaction of a cement and supercritical or near-critical CO2 can be summarized. The CO2 increases the mobility of water present in the cement matrix. The pores initially contain CO2 at the pore entrance, an aqueous phase partly adsorbed on the pore walls, bound hydrates, and possibly free water at the CO2/water interface. The pore environment should eventually consist of a phase of water dissolved in CO2,5 with some water still adsorbed on the walls of the concrete pores. Supersaturation of water in a CO2-rich phase at high pressure is possible because phase separation in the pores of the concrete is expected to be slower than the rapid carbonation reaction.6,7 At high pressures the CO2 solubility in water also increases. For example, at 25 °C and 100 atm the solubility of CO2 in water is 2.5% and that of water in supercritical CO2 0.33%.8 Because there is now a measurable CO2 con-
10.1021/ie980705y CCC: $18.00 © 1999 American Chemical Society Published on Web 05/29/1999
2642 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999
centration present at the hydrated surface, reaction with, e.g., Ca(OH)2, can take place. That the surface remains hydrated is important, because a water layer is thought to be necessary to initiate carbonation reactions.9-11 Carbonation Reactions and Densification. All calcium-bearing phases are susceptible to carbonation.12 For Ca(OH)2 the reaction is
Ca(OH)2(s) + CO2(g) f CaCO3(s) + H2O(l)
Several papers and patents have recognized that treatment with CO2 would densify and harden various cement products, accelerating the rate of cure and increasing compressive strengths.1,2,24-30 However, in most cases these studies used low-density CO2 at ambient temperature. An exception was the work of Murray,25 who rapidly hardened concrete/polymer mixtures by jet imipingement with cold (∼-20 °C), and so dense, CO2.
(1) Experimental Section
The CaCO3 can crystallize in one of several forms, but calcite is the most stable. Ca(OH)2 is assumed to first dissolve in water, and following reaction, CaCO3 precipitates.11 Atmospheric CO2 (0.04%) will not react with completely dry concrete. Also, if the concrete pores are filled with water, carbonation essentially stops at low pressures, because the solubility and diffusivity of CO2 in water is low under such conditions.9-11,13 Bulk carbonation at atmospheric pressure and ambient temperatures results from years of exposure to atmospheric CO2. The pH of cement in fully carbonated zones is lowered from a basic ∼13 to ∼8. The nonhydrated phases as well as the normal calcium silicate hydrate (C-S-H, ideally, 3CaO‚2SiO2‚ 3H2O)14 phase of cements can be carbonated. When standard shorthand is adopted for specifying cement component compositions (C ) CaO, S ) SiO2, A ) Al2O3, h ) CO2), these reactions can be M ) MgO, H ) H2O, C written as
h f C3S2C h 3H3 C3S2H3 + 3C
(2)
C3S + 3C h + xH f SHx + 3CC h
(3)
C2S + 2C h + xH f SHx + 2CC h
(4)
C3AH6 + C h + 5H f C3AC h H11
(5)
The last reaction apparently takes place in fly ash and other high aluminate cements.15 The evidence for reactions (2)-(4) has been reviewed in ref 9. There is strong experimental evidence [X-ray diffraction (XRD), scanning electron microscopy (SEM), and elemental analysis of sections] for the occurrence of the direct carbonation reactions (3) and (4) of the unhydrated silicates.2,16-18 However, reactions (3) and (4) are slow at lower pressures, and therefore at these conditions mostly the calcium silicate and calcium aluminate hydrates are formed, with a little carbonate.2,9,19-20 Other constituents of cements containing Mg and K can also be carbonated. Carbonation can reduce the permeability of cement by 3-6 orders of magnitude.21 This permeability reduction is attributed to precipitation of carbonates in the cement macro- and micropore structures. For example, in cement grout carbonation shifts a bimodal pore distribution (2-10 and 10-900 nm) to a more unimodal distribution of 2-10 nm.9,21 Smaller pore diameters also slow rates of diffusion in carbonated cements; Cl- and I- diffusion coefficients were 2-3 orders of magnitude smaller in carbonated compared to noncarbonated paste.22 Similar results were obtained for carbon-14 migration in carbonated cement-based materials.23 This provides the opportunity to produce an impermeable surface on the order of a few millimeters, which will can protect concretes from strong acids or corrosive marine environments.
Preparation of Carbonated and Molded Samples. For initial carbonation experiments using cured cements, an existing supercritical fluid continuous treatment system was used. In it, liquid CO2 was compressed by a positive displacement diaphragm compressor (American Lewa model ELM-1) to 10.44 MPa. The compressed CO2 was stored in surge tanks to dampen any pressure fluctuations. The pressure was controlled by a Tescom regulator (model 44-1124) to (0.03 MPa and monitored by a Heise digital pressure gauge (model 710A). The specimen (10 × 10 × 40 mm) was contained in a tube immersed in a constant-temperature bath held at 25 °C. A CO2 flow rate of ∼0.8 g/s was used, and the run time was 1 h. A prototype device was also constructed to evaluate methods of “one-step” molding, curing, and supercriticalCO2 treatment. Specimens were prepared using the apparatus of Figure 1, a cylindrical mold operated by a piston, which is sealed on its outer surface by O-rings. CO2 gas (at ∼6.3 MPa) is introduced below the piston. The pressure above the piston was rapidly increased using water as the driver fluid (13.9 MPa) to initiate molding. The specimens, 39 mm diameter and 13 mm height, were typically molded for 2 h, although examination of some samples after 40 min suggested that setting times in the presence of high-pressure CO2 were much shorter. The maximum amount of CO2 added to the cement matrix could be controlled by adjusting the initial height of the piston (using spacers) above the cement. The normal amount was 8 g of CO2/25 g of cement. Characterization of Chemical and Physical Properties. Surface areas were estimated using the onepoint N2 adsorption method at 30% relative saturation, using a Micromeritics 2700 pulse chemisorption apparatus. Water was first removed under vacuum at 1 torr for 24 h at 100 °C and then under flowing N2/He for at least 2 h. The surface areas of selected samples were checked by the full multipoint Brunauer-Emmett-Teller (BET) N2 adsorption method using an Omnitherm (model Omnisorp 360) adsorption apparatus. The pore volume was determined in water by displacement (Archimedes’ principle). All specimens used in density and void fraction measurements were dried under vacuum at 1 Torr at ambient temperature prior to measurement. A Scintag PAD-V XRD using Cu KR radiation was used to identify crystalline phases in the specimens. Samples were step-scanned from 3 to 60° 2θ, at 0.02° step size, for 3 s/step. A Perkin-Elmer TGA operating at atmospheric pressure was used to quantify the weight losses due to water evolution (from hydrates), hydroxide (e.g., CH) to oxide conversions, and carbonate (e.g., CC h) to oxide conversions. The carrier gas was He, and the temperature program was 200-700 °C, 5 °C/min, and then hold at 700 °C.
Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2643
Figure 1. Prototype system for in-situ molding and carbonation. On the left, the mold is filled with CO2 at ∼6.3 MPa. On the right, molding and curing take place at ∼13.6 MPa. Table 1. XRD Phase Characterization of Carbonated Specimens, Continuous Flow ratio XRD heights, Portlandite/calcite additives lime calcite plasticizer (WRDA 19) fiber: E-glass fiber: Kevlar 49
control 1
test 1
2.9
0.029
control 2 3.9 0.021 0.007
Cement pH was measured by the contact method using bromothymol blue indicator (1%) in ethanol and 30 mL of solution/25 g cement, as a molded disk specimen. While these tests normally use phenolphthalein indicator solution,31 for the purposes of this work we wanted an indicator which changes color over a lower pH range (7.0-7.6). Results and Discussion Specimens Prepared in a Continuous-Flow Apparatus. These carbonations used near-critical CO2 (10.44 MPa, 25 °C). However, the CO2 density at these conditions is 0.83 g/cm3, well above the density (0.46 g/cm3) at the critical pressure and temperature. Table 1 summarizes XRD results for five different cement mortar (type III) mixes. For each mix both a control (no carbonation) and test (carbonated sample) are reported. The weights of the additives are normalized to the initial weight of the cement mortar. For all samples, a water/ cement (w/c) weight ratio of 0.60 was used. The five mixes represent a typical fast-set cement mortar, and such a mortar with typical additives includes glass and Kevlar fibers, calcite, lime, and a plasticizer. The key result reported in Table 1 is the ratio of the portlandite (CH) to calcite (CC h ) peak heights, from XRD of the samples. The portlandite peak is at 18.1° 2θ and the calcite peak at 29.5°. This ratio is not strictly
test 2 500 °C) (11) It was impossible in most instances to resolve the small peaks of some of the more stable hydrates and the MH and MC h peaks from the CH peak. Also, dehydration reactions of surface hydroxyls in, e.g., silica take place at temperatures which overlap CC h decomposition. For fly ash, more hydrogarnet (C3AH6) than CH is dehydrated in the 350-450 °C region,36 but because the calcium aluminum oxides can also be carbonated, the analogy to CH in PC is valid. Therefore, the TGA results should be viewed as estimates of the actual amounts of (i) CH and other hydroxides of
2646 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 3. TGA Results for Cement and Cement Slag Samples
Figure 4. DTGA of ground physical mixtures of Portland cement, set in air (2/3) with the additives shown (1/3).
sample
% loss
% hydroxide as Ca(OH)2
% carbonate as CaCO3
3Ama 3Bm 20Bm (foamed) 20Bt (foamed) 22Am 22At 22Bm 22Bt 24Bm 24Bt 26Bm 26Bt 45Bm 45Bt 4Am (BFS) 4Bm 6Am (foamed) 6Bm (foamed) 9Bm 12Bm (postset) 19At 19Am 19Bt 19Bm
1.5 1.3 1.6 1.9 2.2 2.0 2.4 2.8 1.1 1.1 1.8 1.7 1.8 1.9 1.8 0.86 0.45 0.56 1.1 1.2 0.81 1.3 0.76 0.87
16 13 16 12 11 7.5 4.1 5.8 5.7 8.0 13 11 11.7 9.9 14.7 18.5 14.4 12.6 16.7 14.3 6.1 7.8 3.4 4.2
5.1 7.0 2.5 7.3 1.5 5.5 11 18 6.7 8.0 1.9 2.6 1.9 5.4 3.8 7.7 1.1 2.3 5.8 2.8 0.64 0.55 2.9 1.8
color change, s 2 10 7 3 >300 >300 n.d.b n.d. 2 3 >300 3 5 n.d. 4 38 >300
a A ) noncarbonated; B ) carbonated; m ) from middle of disk; t ) from top of disk. b Not done.
Table 4. TGA Results for Fly Ash Samples
Figure 5. DTGA of molded fly ash specimens. The labels in the legend refer to sample designators in Table 2; A ) noncarbonated, B ) carbonated, m ) from middle of disk, and t ) from top of disk.
calcium and (ii) CC h and other carbonates. The results are useful only in relative comparisons of carbonated vs noncarbonated (but otherwise identical) materials, as done here. The standard samples shown in Figure 4 were used to define appropriate temperature ranges of the dehydration and decarbonation reactions in our TGA analysis. Each standard was a homogeneous physical mixture, 2/3 from mold specimen 3A (Portland cement, set in air) and 1/3 additive. These were ground to powder using a mortar and pestle. In sample 1 the additive is CC h , and the high-temperature reaction is evident. In sample 2 the additive is CH, and there are multiple dehydrations from ∼350 to 450 °C; the peak multiplicity arises from such reactions taking place in several calcareous hydroxide phases in the cement as well as in pure CH. In sample 3 the additive is aluminum hydroxide, and it is seen that its bulk dehydration takes place at low temperatures, in the hydrate-loss region. In sample 4, the additive is sodium metasilicate, and the extra peak at ∼570-640 °C is evidence that the final reaction to evolve water from silicate surfaces will affect quantitation of the carbonate peak, a problem for FAC and BFS as well because of their higher silica contents. However, the relatively small size of the peak, when such a large quantity of silicate was added, suggests that composition estimates of CC h and related carbonates by TGA are still possible. Examples of carbonated (B) vs noncarbonated (A) DTGAs are shown in Figure 5 for the fly ash samples. The designations “m” and “t” refer to samples removed from the middle (at 6.5 mm depth) and top (surface, the one away from the CO2 inlet) of the disk, respectively.
sample 2Aa 2B 11Bt (postset) 11Bm (postset) 5Bm 5Bt 16Bm (foamed)
color % loss, % hydroxide % carbonate as CaCO3 change, s hydrates as Ca(OH)2 2.8 1.6 2.3 2.7 0.89 0.74 0.55
9.6 3.3 6.0 5.2 3.8 4.0 3.7
6.5 14 5.9 6.0 11 11 15
6 >300 210 n.d.b n.d. n.d.
a A ) noncarbonated; B ) carbonated; m ) from middle of disk; t ) from top of disk. b Not done.
Sample 2Am is the noncarbonated comparison standard; it shows the smallest peak in the 600-700 °C region and the largest peak in the region characteristic of CH. Samples 11Bt (postset treatment with CO2) and 2Bm both show smaller peaks in the CH region. However, it is clear from Figure 5 that the in-situ carbonation procedure is superior in its ability to rapidly carbonate the fly ash; its bulk carbonate peak is far larger than that of the postset sample. It is also clear from Figure 5 that some of the more stable hydrates at ∼300-320 °C are also carbonated by the in-situ treatment, or at least contain less water of hydration. This also represents a difference from the “surface” treatments of already cured specimens described previously and represented here by 11B. Tables 3 and 4 give the TGA and contact pH results for the molded specimens, based on the quantification scheme discussed above. Each TGA number (columns 2-4) is the average of two different samples from the same molding, most with
