Iraqi Journal of Civil Engineering Effect of Different Conditions of Carbon Dioxide Curing in Cement – Based Composites (On Review)

The most concerning issue confronting the planet these days is the ascent in Carbon dioxide (CO 2 ) levels to record levels. The cement industries are answerable to between 6-8 % of worldwide CO 2 emitting. In construction sectors, researchers tried to contribute in decreasing of CO 2 in atmosphere produced by industry and using that was released in air. Accelerated CO 2 curing is one of the methods used to get benefit from CO 2 in the air. In this paper, CO 2 concentration in addition to pressure, relative humidity and period of curing all had a significant influence upon the features of Cement – Based Composites. Results showed that using CO 2 curing with different and specific properties of fibers (types, quantities, circumstances and lengths) improved the most mechanical properties and enhanced durability such as: strength, stiffness, ductility, toughness, porosity, and absorption.


Introduction
Due to the industrial revolution, atmosphere currently contains more CO 2 whenever in any time in at least two million years. At 2020, one ton of cement produce 0.6 tons of CO 2 (Gomez J. 2020). All of this additional CO 2 requires to go somewhere. Until now, land plants and the sea have taken up around 55% of the additional CO 2 which individuals have placed into the climate while around 45% has remained in the air. The expansion in CO 2 motivates an ascent in heat for earth environment. Excess CO 2 within the air makes the air and rain more acidic, putting life in danger (Riebeek, H. 2011) . Recently, there has been a development in the procedure of curing with CO 2 to enhance the performance, mechanical features and durability of composites. The predominant chemical reaction happening with hydrates carbonation includes reacting of carbone dioxide (CO 2 ) with CH that results out of cement hydrating, which yields calcium carbonate (CaCO 3 ). The solubility in water for this calcium carbonate seems lesser than of that for CH as well as a decrease within porosity exists in addition to an increase of hardness and impermeability related with the forming process for CaCO 3 (Varjonen, S. 2004 ). Rapid carbonation reaction might reduce the curing time of cement-based materials by attending of CO 2 . This value added utilize CO 2 likewise decrease greenhouse' gas emissions because of utilizing the contaminating CO 2 (Frybort, S., Mauritz, R., Teischinger, A., & Müller, U. 2008) (Zhou, Y., & Kamdem, D. P. 2002)( De Silva, P., Bucea, L., Moorehead, D. R., & Sirivivatnanon, V. 2006. These adjustments within compositions as well as structures improve the durability of cement-based materials at firs ages (Shaikh, F. U. A., & Supit, S. W. M. 2014). In addition, curing with CO 2 will reduce the alkalinity of pores water in cementitious materials (Sharma, D., & Goyal, S. 2018).
Accelerated curing with CO 2 is diverse from weathering carbonation because it expedite the hydration reactions for un-hydrated phases of C3S and C2S resulting rapid gain of strength. Weathering carbonation exists in concrete after the hydration procedure was predominately finished and outcome the de-calcification of CSH as well as producing gel of silica that is destructive to paste cement (Gilroy, B., Ireland, M., Black, L., Thompson, D., Hogan, R., & Holmes, N. 2020). The utilization of fiber fortifications in cement based materials is growing because of the capacity to cement in a lot attributes to explicit applications and conditions. Extending of using fiber in concrete prompted improves the ductility, toughness and durability (Melenka, G. W., & Carey, J. P. 2015). Fiber utilized in cement-based materials may be of various materials includes steel, glass, carbon, polypropylene (PP), etc. The first purpose of this paper was to study the utilizing of fiber with accelerating the cure of cement-based materials by CO 2 curing to accelerate hardening and reduce required setting time. Secondly, various relative humidity percentages, curing time, concentrate of CO 2 as well as pressure, which effect on improving of mechanical and sustainable development features for building industry because the consuming of the polluting CO 2 .

Methodology
The methodology adopted for the review was:
Elevated Temperatures: hydrating reaction through raising temperature up to 80 o C, high temperature affect microstructures.
Accelerated Carbonation: expedite carbonation process that is a subordinate reaction of hydration.

Mechanism of Carbonation
Carbonation is a chemical reaction where CO 2 diffuses into concrete pore system. CO 2 reacts with CH in concrete and forming CaCO 3 , (Eq.1). Carbonation makes changes into chemical composition of concrete ( Fig.  1). CO 2 mineralizing within the hydrate of cement that occurs whether by natural (weathering) carbonation or by other engineered manner named (accelerated carbonation) (Fig. 2). Weathering carbonation is harmful. CO 2 will react with water of pores and form H 2 CO 3 (acid). This acid reacts with CH and produce CaCO 3 results in a small shrinkage. Using CH will reduce pH (alkalinity). In low pH, reinforcement will corrode due to losing passivation. It represents a slow natural method in cementitious materialprincipally hydrating productions. Accelerated carbonation includes rising the ratio at which carbonation happens (Varjonen, S. 2004 ). This type of carbonation happened rapidly at early age of cement hydration. Concrete will be stronger, denser and faster develop of strength.

Factors Influencing Rate of Carbonation
There are numerous factors influence the rate and congruity of carbonation. The main important factors are: concentration levels of CO 2 , CO 2 ratio, permeability/porosity of materials, inner surface area, temperature and relative humidity, and binder system (Fig 3).

Accelerated carbonation of cement-based composites:
Accelerating carbonating is a process to expedite the proportion of carbonation reacting in cement. Different strategies to accelerate carbonation of cement-based materials were practically known through the usage of high concentration of CO 2 at neighbor surrounded cement.
Most generally received engineering method for the mineralizing for CO 2 in cement-based materials is the initial age of CO 2 curing. In initial age, CO 2 curing converts hydrates to steady calcium carbonate (CaCO 3 ) and gel of silica. Such a method gives a way to CO 2 sequestrating within cement-based materials. Also, such strategy includes utilization of CO 2 gas (bottles) as a supply resource (Fig.2). Another studies ( (Shao, Y., Mirza, M. S., & Wu, X. 2006) showed that the usage concentration of CO 2 , which ranges as (0.15% to 100%) for current method of accelerated carbonation.
The fiber/matrix interfacial zone allowed calcium carbonate to sedimentate largely during a carbonation curing. This cause a reduction in capillary and gel porosity around the fibers. A difference in Calcium/Silica (Ca/Si) proportion near to the interface was revealed through EDS (Energy Dispersive Spectroscopy) (Fig. 4)   60% relative humidity (RH) or less.  subjected to CO2 gases by 100 % , (1.5 MPa)  cementitious materials 300 kg/m 3 and with water/cement ratio = 0.63  They concluded that such samples to be completely got carbonation within period of two weeks by accepting such technique of accelerated curing  Also observed that using high water-tocement ratio was preferable to simplify the CO 2 diffusing as the samples got subjected to 100% CO 2 environment.  Lab climate of 50 ± 5% RH and 22 ± 2 ºC.  Concentration of CO 2 to 100%.
 Observed as many as 40 times higher carbonating rate to accelerated process than the normal carbonation a remarkable variation in microstructure of carbonated concrete by high CO 2 concentrating with normally carbonated concrete.  It was deduced that CO 2 concentration of 5% reveal less impact to microstructure comparing with high CO 2 concentration.   Matrix of cementitious composite modified by CO 2 curing.  The testing outcomes showed that continued cycles of wet-dry and freezing-thawing which leads to enhance stiffness and decrease toughness. The consequences for modulus of rupture were mixed.
(Soroushian Prof., P., Won Prof., J. P.,   Pressed and unpressed boards.  Various oven's temperature, periods, CO 2 chamber and autoclave periods.  Increasing modulus of rupture and toughness were gained through increasing oven, CO 2 exposure and autoclave period.  Reducing oven time increases stiffness but increases exposure duration in chamber and autoclave produced higher stiffness. (Hamad, A. 2014)  Initial curing, 30 minutes at 50ºC in oven.  Concentration of CO 2 = 100 % and for 2-3 days.
 Strength value of samples were increased in age's range of 7 to 28 days.  Initial curing: 4 -18 hrs. with RH of 50% at 25°C.  4 hr. carbonation curing to allow concrete for uptaking (22-24%) CO 2 in addition to primary curing and 8.5% without primary curing, while extended 4-day carbonation had an uptake of 35%.
 Reduce carbonation period to 2-4 hrs. and using initial curing ranged from 0-18 hrs. to cost purpose.  Primary curing effect on carbonation degree was estimated to raise maximum potential carbon uptake in Chamber Unit. ( Hassan, M., & Salih, W. 2016)  Percentages (2, 6, and 10%) of palm fiber was utilized by weight of cement  For compressive strength, CO 2 curing, was best method curing.  Autoclave curing gives best enhancement

Conclusions:
Depend on the previous references, it can be concluded that: • Numerous benefits for the initial age carbonating curing to concrete and another cementitious material. • Compared to moist-cured concrete, the concrete cured in the CO 2 chamber (with 0.15% CO 2 concentration) exhibited an approximate similar strength and low abrasion resistance. • Early age carbonation expedite strength gaining, prompting shorten period for producing precast members. • With Initial curing, 30 minutes at 50ºC in oven and concentration of CO 2 = 25 %, the CO 2 curing improved the matrix of cementitious composites, raised stiffness and decrease toughness values. But when use concentration of CO 2 = 100 % and for 2-3 days, all mechanical properties of cementitious samples were increased due to aging ranged (7-28 days). • Also, the advantages of utilizing CO 2 curing, which has a high impact on sustainability in the construction industry due to the disposal of CO 2 and for cost purpose. • The Utilization of CO 2 concentrating, which ranges from (0.15-100%) increases the mechanical strength of cement-based materials. • Fundamentally the same as microstructures for pastes carbonated within a climate up to 3% CO2 concentrating with nature carbonation. • An essentially different microstructures of paste cured with high CO 2 concentration ranged (10-100%) than the natural carbonation process. • A high water/cement ratio is preferable to ease the dissemination of carbon dioxide through exposing samples to 100% carbon dioxide. • Carbonation curing was the most effective curing and better than autoclave curing for compressive strength. Otherwise, autoclave curing exhibits better results in flexure. • Using carbonation for cement based composite with different fibers improve most of properties according to the types, percentages and properties of replaced fiber.