Durable Aluminium Reinforced Environmentally friendly Concrete Construction (DARE2C)

Harald Justnes, SINTEF Community, Trondheim, Norway

Cement is a key binder component of concrete production in the building industry. It is a complex hydraulic binder, made up of four main clinker components; alite (Ca₃SiO₅), belite (Ca₂SiO₄), tricalcium aluminate (Ca₃Al₂O₆) and ferrite (Ca₂AlFeO₅), which are milled together with gypsum to regulate setting time. In recent years, the production of cement has been identified as the third largest emitters of carbon dioxide (CO₂), accounting for approximately 5 to 8% of the total global anthropogenic emissions, with 60% coming from decomposition of limestone in the raw meal and 40% from fuel to reach clinkerization temperatures of 1450°C for a pure Portland clinker. Four main methods are currently in place to mitigate this challenge; i) switching from fossil fuels to alternative fuels, ii) increase efficiencies in factories, iii) implementation of supplementary cementitious materials (SCMs) replacing cement clinker [1] and iv) carbon capture and storage (CCS) [2]. Among which, clinker partially replaced by SCMs is the most promising on a short term [1], whereby significant reduction in CO₂ emission could be expected depending on how much emission is associated with the SCM (transport, calcination energy etc.). Replacing cement with SCM will also reduce the amount of raw meal needed per unit cement and increase the cement production volume of a cement plant. Most of the cement produced today has clinker replaced with SCMs with an average global clinker factor of 0.85 in 2003 [3], but higher clinker replacement with a greater variety of SCMs is expected in the near future. The potential SCMs of the future include combustion ashes, slag, calcined clay and limestone. Fly ash is commonly employed in current cements with replacement of 20% in Norway. Calcined clay or marl is a new and upcoming SCM due to its abundance as increased clinker replacement demands large volumes of available SCMs. Additionally, previous studies displayed that calcined marl is a potential SCM up to a replacement level of 50% in terms of equal 28 day strength according to Justnes [1], but generally it is limited to < 35% replacement if sufficient alkalinity is to be maintained to protect steel from corroding in the long run.

The most common reinforcement for structural concrete is rebars made of steel. At the same time the most common degradation mechanism of reinforced concrete is corrosion of the steel initiated by carbonation (i.e. CO₂ from the air diffuse in and lower the pH by neutralization) or by chlorides exceeding a critical limit in spite of maintained pH. Thus, there as a need for concrete reinforcement that will not corrode, but at the same time being composed of common chemical elements since a gross volume of 10¹⁰ m³ concrete is produced annually world-wide. “Stainless steel” exists that will not corrode, but there is “not enough chromium in the world” to make a significant replacement feasible and it is rather pricy. Some efforts have been made to make rebars out of fibre reinforced plastic (FRP), but these tend to have a brittle failure unlike metals being ductile. Karlsson [4] made a review and evaluation of alternative concrete reinforcement. Naturally, aluminium was not a part of this evaluation since it will be degraded by the high pH of regular concrete and can only function in a sufficiently low pH concrete. On the other hand, aluminium is a very common element in earth’s crust.

The main durability design of concrete today is in relation to preventing the steel reinforcement from corroding. The major degradation mechanisms; chloride ingress and carbonation, does not jeopardize the integrity of the concrete binder itself. Steel needs the high pH of conventional concrete to be passive towards corrosion while aluminium metal will be corroded by high pH and develop hydrogen gas. Environmentally friendly concrete is often designed by either using blended cement where maximum 35% of the clinker is replaced by supplementary cementitious materials (SCMs) to secure the presence of calcium hydroxide over time that will buffer a pH of 12.5 passivating steel, or by replacing cement in concrete mixes with corresponding amounts of SCM. In order to secure long service life for steel reinforced concrete, low w/c is used for low permeability often leading to much higher strength than required. Low w/c can also create complications in the construction phase as lower workability is calling for use of super-plasticizing admixtures. Using more cement to attain low w/c can create higher temperature and subsequent thermal cracking risk generated by hydration heat as well as chemical shrinkage. The required compressive strength for the bulk of concrete today is still in the range of 25-30 MPa (B25).

The DARE2C concept is to make environmental friendly concrete with > 50% cement replacement by a combination of SCMs where some are so pozzolanic active that the pH is kept so low that the concrete can be reinforced with aluminium metal rods without formation of hydrogen gas. The w/c can then be so high that it is only determined by required compressive strength for construction. Then workability should not be a problem and neither hydration generated heat. Permeability is not important as aluminium metal is resilient to atmospheric CO₂ and chlorides (when alloyed with 5% Mg), and high initial permeability is in fact beneficial for the concrete to carbonate as fast as possible to reduce the carbon footprint further and lower the pH further for the long run. The concrete cover over the reinforcement can be made much thinner (20 mm) than today (50-70 mm) reducing weight and further improving the carbon footprint by using less concrete. Justnes [5] discussed the options of making a concrete with lower pH than usual ranging from large cement replacement (>50%) to calcium sulphoaluminate-belite cement systems with a smaller content of pozzolana.

Proof of the DARE2C concept
As a proof of concept, two paste mixes with w/c = 0.60 were made with 100% ordinary Portland cement and 50% calcined marl [6] replacing cement. These pastes were poured into a plastic cup and an aluminium plate placed in each of them as shown in Fig. 1. The paste of pure cement separated and after a few minutes hydrogen gas started to bubble vigorously along the aluminium plate as seen from the left side of Fig. 1, while for the mix with 50% calcined marl some water was added on top for better visualization, but only a few small bubbles were observed. After the pastes had hardened, the samples were split and the imprints of the front and back of the aluminium plates on the pastes are shown in Fig. 2. It is clearly much more cavities in the OPC sample next to the plate, while only a few small gas voids are seen on the interface for the paste with 50% calcined marl. One cannot rule out that the minor gas voids are due to entrained air by the high shear mixer, and the only way to find out is to capture and measure the evolved gas volume [7] or to detect hydrogen specifically by a gas chromatograph.

If one place an aluminium bar in pure OPC paste, it will be totally corroded until there is just gibbsite, Al(OH)3, left (see Eq. 5) as shown in Fig. 3. Placing the same bar in paste where 55% cement has been replaced by calcined bar render a non-corroded bar as seen from Fig. 3 as white corrosion products would easily have been spotted against the red-brown paste.




Testing of concrete reinforced with aluminium bars
Concrete formulation for low strength concrete
The basic binder was made out of 45% low-alkali CEM I cement 55% marl calcined at 750°C, with chemical composition as given in Tab. 1. The specific surface of the cement (Blaine) was 388 m²/kg while the specific surface (BET) of the calcined clay was 38 m²/g. Pastes of the same binder were blended in a high shear mixer to a water-to-powder ratio of 0.7. The pH evolution of this paste is shown in Fig. 4 together with a paste where also a soluble magnesium salt (0.57% Mg²⁺ of binder mass) was added.

Table 1. Chemical composition (%) of the cement and calcined marl and kaolinitic clay (MK)
Oxide	CEM I	Marl	MK
SiO2 (%)	21.12	49.03	60.6
Al2O3 (%)	4.60	16.14	30.0
Fe2O3 (%)	3.77	9.41	3.4
CaO (%)	63.21	10.55	0.1
MgO (%)	1.71	2.47	0.4
SO3 (%)	3.87	-	-
K2O (%)	0.40	2.38	3.2
Na2O (%)	0.35	0.78	-

The purpose of the calcined marl was to consume all calcium hydroxide produced by the cement and render all alkalis as aluminate or silicate and to prevent them for regenerating as hydroxides as explained in the alkali cycle for catalytic pozzolanic reaction exemplified principally (i.e. not necessarily correct ratio between the different oxides in the compounds) for aluminosilicate (AS) in reaction 1;


where cement chemist’s short hand notation is used; C = CaO, H = H₂O, K = K₂O, N = Na₂O, S = SiO₂ and A = Al₂O₃. The alkali hydroxides dissolves alumina and silica from the aluminosilicate that react with calcium hydroxide (CH) to amorphous calcium silica hydrate (CSH) gel and crystalline calcium aluminate hydrates (CAH) as binder. After reaction with CH the alkali hydroxides are regenerated, and the loop continues until all calcium hydroxide is consumed and they end up as alkali silicate and alkali aluminate that in theory should not attack the aluminium metal in the rebars even though they result in high pH. Justnes et al [8] showed that paste where cement is replaced with 50% calcined marl showed no traces of calcium hydroxide after 2 years by XRD.

Aluminium metal forms a dense layer of Al₂O₃ in contact with air that prevents further oxidation (or corrosion). This layer may dissolve by alkali hydroxides and open up for further corrosion evolving hydrogen gas;

Oxidation: Al (s) + 4 OH- = Al(OH)₂O- + H₂O + 3 e-
E⁰ = +2.333 V       ---(2)

Reduction: 2 H₂O + 2 e- = H₂ (g) + 2 OH-
E0 = -0.828 V       ---(3)

Total: 2 Al (s) + 2 OH- + 4 H₂O = 2 Al(OH)₂O- + 3 H₂ (g)

E⁰ = +1.502 V       ---(4)

According to the total reaction in Eq. 4, there will be a net consumption of one hydroxide ion per aluminium oxidized, or none if gibbsite, Al(OH)3, is precipitated rather than the dissolved anion;

2 Al (s) + 6 H₂O = 2 Al(OH)₃ + 3 H₂ (g)       ---(5)

The purpose of adding a soluble magnesium salt was to lower the initial pH by precipitating brucite;

Mg²⁺ + 2OH- = Mg(OH)₂ (s)       ---(5)


By using a magnesium salt, the alkali hydroxides would then in theory become neutral alkali salts. Brucite has equilibrium pH 10.5. There would also be a chance that hydrotalcite, Mg₆Al₂(OH)₁₈·3H₂O, would form in the longer run and further reduce pH by binding more hydroxyl ions. However, as can be seen from Fig. 4, the pH of the mix with magnesium salt increases over time after the first initial reduction. This is in retrospective explained by ion exchange between OH- and X- by AFm phases like hydrocalumite in Eq. 6;

Ca₄Al₂(OH)₁₂(OH)₂·4H₂O (s) + 2NaX (aq) = Ca₄Al₂(OH)₁₂X₂·4H₂O (s) + 2NaOH (aq)       ---(6)

This is yet to be confirmed by X-ray diffraction on corresponding paste without aggregate. The abrupt pH increase from the first data point (fresh state) to the second (hardened) is due to consumption of water to hydrates and thereby higher concentration of soluble species, but release of alkalis from the reacting clay cannot be ruled out as the pozzolanic reaction starts. The initial pH of pure CEM I without calcined marl was 13.5.

The composition of the concrete is given in Table 2, and a soluble magnesium salt (0.57% Mg²⁺ of binder mass) was also added. The compressive strength, splitting tensile strength and static E-modulus of cylinders (Ø100 mm∙200 mm) was measured after 28 days curing under wet burlap and plastic wrapping to avoid drying, and the results listed in Table 3. The concrete was made in two batches and used to cast 18 cylinders and three 150·250·1100 mm beams reinforced with Ø10 mm aluminium bars alloyed with 5% magnesium. The beams were tested in a 4-point bending and the crack pattern compared to finite element calculations (not reported here). After the bending test, one of the beams were cut and split to inspect the bars for corrosion. As seen from Fig. 5, there was no sign of corrosion on the bars (left image) and the imprint in the concrete (right image) was clean without corrosion products. Prior to these bigger batches, a smaller trial mix was made, and a number of 10 cm cubes cast in steel moulds. The strength evolution for the trial mix versus curing age at 98% RH and 20OC is plotted in Fig. 6, and the temperature evolution in a 3.5 litre Styrofoam mould (10 mm wall thickness) shown in Fig. 7 indicating a setting time of 4.5 h (270 min).

Table 2: Concrete composition
Component	Mass (kg/m3)
CEM I	143.9
Calcined marl	175.9
Free water	223.9
Adsorbed water	5.7
0-8 mm Granite sand	970.9
8-16 mm Granite gravel	702.4
SIKA Viscocrete RMC-315	1.92

Table 3: Mechanical properties of concrete cylinders
Property	No. samples	Value
Density (kg/m3)	12 from 2 batches	2328±16
Compressive strength (MPa)	8 from 2 batches	22.3±0.9
Splitting tensile strength (MPa)	4 from 2 batches	2.3±0.2
Dynamic E-modulus (GPa)	4 from 2 batches	20.3±0.9

Note that the concrete target strength was around 25 MPa and that it was deliberately made porous with a water-to-binder ratio of 0.7. It can of course be made stronger by simply lowering the water content and increasing the super-plasticizer dosage a bit from the current 0.6% of binder mass as shown in the next section. Justnes et al [8] showed that mortar where cement was replaced with 50% calcined marl and the water-to-binder ratio was 0.55 achieved the same compressive strength has mortar with 100% cement and that the compressive strength continued to increase until the last measurement at 2 years age in spite of no calcium hydroxide present at 28 days age.



The aluminium rebars are produced by an extrusion process. The screw extrusion process is patented by Werenskiold et al [9]. A prototype extruder along with fundamental knowledge of the process have been developed to a level that allows industrial implementation. Compared to the traditional process route based on re-melting and extrusion, direct screw extrusion of scrap material represents a significant reduction in energy consumption according to Duflou et al [10]. Moreover, this process is well suited for utilization of swarf material from material removal processes such as cutting, milling and turning that is commonly of less value than other scrap types. This is further contributing to the environmental friendliness of the overall DARE2C concept.


Since the DARE2C concept is to let the concrete carbonate as fast as possible for both the sake of environment and to ensure long term/low pH stability of aluminium, a test was made on mortar to see the effect of carbonation on strength.

The compressive strength of carbonated mortar versus non-carbonated mortar was measured to 20.2±0.2 and 23.7±0.5, respectively. The mortar composition was 0.45 parts CEM I, 0.55% calcined clay and w/p = 0.7 cured for 28 days at 90% RH and 20°C for 28 days before one set of 3 prisms were carbonated through in a 5% CO₂ chamber with 60% RH and the other set just stored at 60% RH in a closed box for same length of time. According to Justnes et al [11] the strength reduction is higher for cements blended with alumina containing SCMs than for OPC since the higher amount of calcium aluminate hydrates formed [12] will lead to a porosity increase when they carbonate, unlike calcium hydroxide leading to a volume decrease upon carbonation.

The molar volume change (ΔVm) for ettringite (AFt) carbonating to calcite, gibbsite, gypsum and liquid water;

Ca₃Al₂O₆∙3CaSO₄∙32H₂O (s) + 3H₂CO₃ (aq) = 3CaCO₃ (s) + 2Al(OH)3 (s) + 3CaSO₄∙2H₂O (s) + 26H₂O       ---(7)

ΔVm = 3∙36.93 + 2∙32.22 + 3∙74.50 – 705.91 = -307.18 cm³/mol AF_t

For calcium monosulphoaluminate hydrate (AFm) carbonating to calcite, gibbsite, gypsum and liquid water;

Ca₃Al₂O₆∙CaSO₄∙12H₂O (s) + 3 H₂CO₃ (aq) = 3 CaCO₃ (s) + 2 Al(OH)₃ (s) + CaSO₄∙2H₂O (s) + 10 H₂O       ---(8)

ΔVm = 3∙36.93 + 2∙32.22 + 74.50 – 308.94 = -59.21 cm³/mol AFm

For calcium monocarboaluminate hydrate carbonating to calcite, gibbsite and liquid water;

Ca₃Al₂O₆∙CaCO₃∙11H₂O (s) + 3 H₂CO₃ (aq) = 4 CaCO₃ (s) + 2 Al(OH)₃ (s) + 11 H₂O       ---(9)

ΔV_m = 4∙36.93 + 2∙32.22– 261.28 = -49.12 cm³/mol monocarboaluminate

For calcium hemicarboaluminate hydrate carbonating to calcite, gibbsite and liquid water;

Ca₃Al₂O_6∙½CaCO_3∙½ Ca(OH)₂∙11.5H₂O (s) + 3.5 H₂CO₃ (aq) = 3.5 CaCO₃ (s) + 2 Al(OH)₃ (s) + 12 H₂O       ---(10)

ΔVm = 3.5∙36.93 + 2∙32.22– 284.36 = -90.67 cm³/mol hemicarboaluminate

The same increase in porosity and decrease of strength also holds for other SCMs high in alumina. For mortars with binders based on 30% siliceous fly ash and 5% limestone stabilizing ettringite cured for 56 days [11], the water vapour diffusion coefficient, k_d (10^-12 kg/Pa·m·s), increased upon carbonation (from 0.41±0.09 to 3.06±0.04) unlike mortar based on OPC (changed from 0.76±0.10 to 0.59±0.10).

Concrete formulation for higher strength concrete
The recipe for a higher strength concrete is given in Table 4 where we used another type of clay being kaolin rich (denoted MK in Table 1). The water-to-powder ratio is now 0.54. The compressive strength for 100 mm cubes at 28 d was 43.9±1.2 MPa and for Ø10·200 mm cylinders 36.5±1.0 MPa for 3 parallels. The splitting tensile strength was 3.4 MPa and the E-modulus was 30.9 GPa after 1^st and 32.9 GPa after 2^nd loading. The compressive strength evolution for the cubes is shown in Fig. 8 for a logarithmic time scale.

This concrete recipe was developed for consideration of loadbearing structures, and as a start to show the stability of aluminium metal, a bench was made as shown in Fig. 9. Since aluminium does not corrode in this concrete mix and not in air, decorative or practical details can be made where the aluminium reinforcement comes out of the concrete and can be used for rails etc.

Table 4: Concrete composition
Component	Mass (kg/m3)
CEM I	156.4
Calcined clay (MK)	178.7
Silica fume	22.3
Limestone filler	14.9
Free water	195.6
Adsorbed water	6.0
0-8 mm Granite sand	1037.7
8-16 mm Granite gravel	720.4
SIKA Viscocrete RMC-315	2.2

The compressive strength evolution for 40·40·160 mm mortar prisms with same binder recipe as for the concrete recipe in Table 4, but with graded norm-sand, is plotted in Fig. 10. The compressive strength at 1 day is 4x higher when cured at 38°C than at 20°C.

Structural considerations
The E-modulus of pure aluminium metal (70 GPa) is 1/3 of steel (210 GPa), but the density of aluminium (2.70 kg/l) is also about 1/3 of iron (7.87 kg/l). The ultimate tensile strength of pure aluminium is 110 MPa, while it for rebar steel is 400 MPa, but aluminium can be alloyed to approach steel in tensile strength. To put it simply, a full replacement with same design for e.g. a beam means three times more volume aluminium reinforcement than steel while the weight will be the same. However, some of this may be alleviated by designing the aluminium rebars differently.




As comparison, FRP rebars made with glass, basalt or carbon fibres have E-modulus of 45, 60 and 145 GPa, and tensile strength of 700, 800 and 2000 MPa, respectively, according to Karlsson [4]. In the SEACON project [13] a bridge is built using FRP rebars demonstrating the feasibility of constructing with rebars with such lower E-modulus than steel.

Another issue is the difference in linear thermal expansion coefficient. Concrete has 6-14·10-6 m/m·K, while pure iron and pure aluminium has 10 and 22·10-6 m/m·K, respectively. The thermal expansion of aluminium can be reduced by alloying (in particular by silicon). As comparison, glass fibre reinforced polyester has a linear thermal expansion coefficient of 25·10-6 m/m·K. The consequence of difference in thermal dilation between reinforcement and binder will be an issue for further research.

Outlook
Formulating stable concrete reinforced with aluminium metal bars can lead to the following benefits in addition to using an environmentally friendly binder with high content of SCMs;

• Maintenance free reinforced concrete (i.e. no carbonation or chloride induced corrosion)
• Any microcracks caused by drying shrinkage etc will not jeopardize reinforcement durability
• Superior sulphate resistance when alumina containing SCMs like calcined marl is used [5]
• Alkali reactive aggregate can be used due to alkali binding [14] or soluble aluminates from clay inhibiting silica dissolution [15, 16]
• Seawater can be used as mixing water when reinforcement is Al alloyed with Mg
• The best strength accelerator, calcium chloride, can be used when reinforcement is aluminium alloyed with Mg (so called sea-water resistant aluminium for boats)
• Much less cover over rebar needed (save 30 mm in concrete cover), probably only 20 mm needed for proper anchoring of reinforcement with concrete.
• Al-reinforced concrete will have significantly lighter unit weight (reduced “dead-weight”)
• Higher w/c allowed giving more permeable concrete leading to

1. Easier to cast as the concrete will need less plasticizers or none
2. Faster carbonation leading to binding of CO₂ and further reduction of Carbon-footprint
3. Less thermal expansion/contraction in monolithic concrete

Conclusions
A new concept of durable and environmentally friendly aluminium reinforced concrete is described. Being able to formulate concrete where aluminium metal will not be attacked by the concrete binder constituents, a maintenance free reinforced concrete with very long service life may be at hand.

Binder paste and concrete has been studied where 50-55% cement is replaced by calcined marl or clay with the following findings:

1. Aluminium plates immersed in such paste does not appear to evolve hydrogen gas as for 100% CEM I paste.
2. Addition of soluble magnesium salts maybe used to lower the initial pH of fresh paste, but the pH approaches the mix without after 14 days curing when using alumina containing SCMs due to possible anion exchange with hydrocalumite
3. Concrete with water-to-binder ratio 0.7 can be made without flash setting (sets at 4.5 h) and reach a compressive strength of 22 MPa at 28 days, but can be made stronger by reducing w/c (w/c = 0.54 gives 44 MPa at 28 days)
4. Concrete beams reinforced with aluminium metal bars showed no signs of cracking at 40 days nor any corrosion of the bars by visual inspection

Acknowledgement
The grant no. 269767 from the Research Council of Norway to establish the research project DARE2C; Durable Aluminium Reinforced Environmentally-friendly Concrete Construction, is greatly appreciated.

References

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