New concrete composites incorporated with calcium acetate as admixture: The Impact of Curing Age on Strength and the Effect of Temperature on Water Absorption

. In previous work, concrete mixes incorporated with different types of acetates (inorganic and organic) were formulated, cured for 28 days in water, and then tested for compressive strength. The effect of the water-to-cement ratio and the acetate content was optimized using a central composited design based on the response surface methodology approach. The results confirmed that the optimum compressive strength was achieved for the mixes incorporated with calcium acetate (CaA) using of 0.48 water-to-cement ratio. In the current work, the effect of curing age on compressive strength and density was studied for the optimum mixes. Also, the water absorption of the mixes carried out by immersion test was assessed at different temperatures (25-55 o C). Microscopic observations were also noted. The results obtained confirmed that the compressive strength and density of the CaA-concrete mix increase with increasing the curing age. The thermodynamic study of water absorption assured the dependence of water absorption on temperature. The activation energies depicted from the Arrhenius relation and the decrease in the water absorption rate for the concrete incorporated with CaA are considered as an indicator of decreasing the porosity of the concrete mixes compared to the control mixes without CaA. The findings assured that the modified mixes with CaA are promising construction materials owing to their outstanding strength and lower water absorption rates compared to conventional concrete.


Introduction
Admixtures are one of the additives employed for solving the limitations in concrete associated with hydration, strength, and durability.They are added to concrete batches immediately before or during mixing to modify the fresh and hardened properties.The performance of concrete could be adjusted by adding small amounts of the admixtures which can improve the physical, hydration, rheology, strength, and durability characteristics [1].Admixtures are of different types including plasticizers, superplasticizers, set retarders, grinding aids, air-entraining, and others.Based on the sources they are classified as natural and synthetic, based on their chemical nature, they are categorized as inorganic, organic, and hybrid.The functional groups of admixtures include mainly hydroxyl, carboxyl, amino, epoxy, and ester groups.In general, admixtures have vital roles in modifying concrete properties [2].Acetates are salts of acetic acids.Sodium acetate has been investigated as a concrete admixture by some researchers [3][4][5].Their findings revealed its ability to modify the chemical resistance and permeability of concrete.A recent study was conducted for studying the effect of two inorganic and an organic acetate on fresh and hardened properties of concrete.It was approved that the incorporation of an optimum of 5.1 wt.% of calcium acetate (CaA) in the concrete mix of 0.48 water/cement ratio resulted in outstanding strength properties [6].On the other hand, water absorption mainly states to physical absorption, which is the process of entering moisture into the concrete through capillary pressure.The transportation of water into concrete has a noteworthy impact on concrete durability specifically when it contains harmful ions.Water transport relies on several factors, the most important ones are the properties and composition of the concrete ingredients, the age, the initial curing condition, and its duration, in addition to the method of testing.Water absorption seemed highly affected by both the capillary pressure and effective porosity.Capillary pressure is associated with the pore size, and effective porosity denotes the pore space in the capillary and gel pores [7].It was mentioned that the long-term performance of concrete structures is linked to the microporous nature of the cementitious binder which depends on pore size, pore-size distribution, pore connectivity, and pore constriction [8].Moreover, the temperature has a potential impact on concrete properties including setting, hydration, strength, and durability properties.It is known that increasing the temperature results in increasing the rate of reaction of cement and this consequently leads to a faster increase in the strength of concrete in the early stage of maturation.Also, high temperatures may result in uncontrolled and faster water loss that affects the strength and durability properties of concrete [9].The objective of the current work is to investigate the impact of curing age on the compressive strength and density of CaA-concrete fabricated using optimum CaA content (5.1 wt.%) and the water-to-cement ratio of 0.48.Another objective is to investigate the effect of temperature (25-55 oC) on water absorption of the CaA-concrete mixes.Control samples without CaA were prepared and tested for comparison purposes.

Experimental part 2.1 Materials
The concrete main constituents purchased from the local market were Ordinary Portland cement (OPC) confirming Iraqi Specifications, natural sand passing through a 2.36 mm sieve, gravel passing through a 20 mm sieve, and tap water.Calcium acetate was purchased from Fluka.The mix proportions of concrete were (1:2:4) cement: fine aggregate: coarse aggregate.The concrete mixes of 0.48 w/c, incorporated with 5.1% CaA were formulated, molded, and cured in water for different ages and then tested for compressive strength.The samples cured for 28 days were tested for water absorption at different temperatures (20,30,40, and 55 o C) after an immersion duration of 24 hours.Figure 1 shows images of some sides of the experimental work.

The properties investigated
The compressive strength of the concrete control specimens and the specimens containing CaA were 150*150*150 mm cubes cured for different ages (3,7,14, 28 days) were tested by compressive strength machine controls model 50-C23C02 with a 2000 KN load capacity.During the test, the load was applied at a constant rate until the specimen failed.The failure load was recorded and an average of three test results was taken as the compressive strength of concrete.The dried concrete cubes were tested for density prior to testing the compressive strength.The volume of the cubes was accurately measured after weighing the cubes.The density of the concrete specimens was taken as the average of the three test results.The water absorption test was carried out on 28 days of cured cubes of plain concrete and concrete composites incorporated with CaA.The cubes were dried at 110°C until the mass became constant, and then placed in a dryer to cool down to room temperature and weighed afterward.The weight was recorded, and the cube samples were immersed in a thermostatic water bath with a temperature accuracy of ±1 °C.They were immersed at the specified temperature for 24 hours.The samples were withdrawn from the thermostatic water bath, dried at 110oC for 24 hours then weighed.Water absorption is measured by measuring the increase in mass as a percentage of dry mass.The difference in weight denotes the amount of water absorbed by the sample.Sets of three cubes were tested for each experiment.The average weight loss % (H20 %) was recorded and the percentage of water absorption of the samples is calculated using the following relation: where mf and mi are, respectively, the final and initial mass of the sample.absorption.

Fig. 2. A typical setup for a water absorption test
The kinetics of the water absorption process and the impact of temperature on the rate of absorption were studied using the Arrhenius relation: Where k is the adsorption rate constant, A is the pre-exponential factor, E is the activation energy (kJ/mole), R is the molar gas constant, 8.314 J•mol −1 •K −1 , and T is the temperature in Kelvin.

Results and discussion
Compressive strength is the main property that affects the fundamental behavior of construction elements.The experimental findings concerning the average values of compressive strength of concrete mixes cured for different ages (3, 7, 14, and 28 days) are shown in Figure 3.The mixes were formulated with w/c ratios and incorporated with 5.1 % calcium acetate.The results proved that the compressive strength of concrete increases with increasing the concrete age for the control samples and the concrete composites incorporated with CaA.However, the percentage increase (39.05, 30..47, 30.49, and 28.20) % in compressive strength of the concrete composites incorporated with CaA was significantly higher than the control samples for all the ages as illustrated in Figure 3.In spite of admixtures are crucial constituents of modern concrete mix, However, the mechanisms of the molecular scale working persist poorly demonstrated [10].It is wellrecognized that the interaction between cement and admixtures is a surface phenomenon.Chemical interactions between the cementitious phase and acetates were explained elsewhere [11][12][13].When the acetate admixture molecules CaA are dispersed in the alkaline medium which is a Ca (OH)2-saturated pore solution of cement pastes, the CaA undergoes hydrolysis resulting in releasing acetate anion CH3COO -which combines with Ca 2+ released from the dissolution of cement grains.The result of this combination is the formation of calcium acetate.The chemical interaction could result in changes in the morphology and the composition of the hydrated cement and increase the amounts of the hydrates which contribute to endorsing the compressive strength of the cured mixes.
Density growth with the curing age for both mixes, the control, and the CaA-concrete is shown in Figure 4.It is obvious that linear relations are obtained with high regression coefficients for both mixes, also, a similar trend is observed for increasing the density with the age for both mixes, however, densities of the mixes incorporated with CaA had slightly higher densities along all the curing ages.On the other hand, water absorption could be denoted as a representative that reflects the durability of concrete, in connection to the effect of chemical admixtures on concrete durability, it was reported that mineral admixture, water-to-cement ratio, and curing age have a substantial impact on both drying and absorption curves of concrete [14].
Besides, the temperature has a potential impact on water absorption.The situation could be attributed to uncontrolled and faster water loss at higher temperatures.The increase of water absorption with increasing temperature is due to the creation of microcracks and shrinkage in the microstructure of the concrete.Water can easily be absorbed by the concrete throughout the microcracks.The microscopic pores and shrinkage result in declining concrete strength.It was declared that in order to have good quality concrete, the concrete should be prepared with water temperatures in the range between 20°C to 35°C [15].Other findings on the effect of adding the appropriate amount of mineral admixtures like fly in the concrete mix attributed the enhancement of the concrete durability to the improvement in the microstructure and increase in the density of the pore structure [16].
The effect of temperature on % water absorption is shown in Figure 5.The figure reflects that the incorporation of CaA in the concrete mix significantly decreases the ability of the mix to absorb water from the surroundings compared to plain concrete.The situation is very interesting from a durability point of view.On the other hand, the concept of activation energy introduced by Arrhenius has been adapted to many phenomena that are time and temperature dependent.The activation/rate theory was used to describe and analyze the phenomena related to the cementitious materials including diffusion of moisture and ions, hydration, and permeability [17].An Arrhenius plot in which the logarithm of the rate is plotted against the reciprocal of the absolute temperature will provide this activation energy (Ea) divided by the ideal gas constant (R = 8.314 J. K −1 .mol−1 ) as the slope of the curve.The results of the thermodynamics study of the water absorption process are depicted in Figure 6.The activation energy of the absorption process with the temperature range (25-55 o C) was calculated and the estimated values were 3.28 kJ/mole and 12.43 k J/mole for the water absorption by the control sample and the concrete incorporated with CaA respectively.
Overall, the results indicated that changes in concrete absorption rate with temperature agree with the Arrhenius law, which suggests that concrete absorption rate has noticeable dependence on temperature.Furthermore, the method for studying the cement and concrete microstructure is usually carried out by microstructural assessment that includes the interpretation of microstructure imaging [18].In the current investigation, Electron Scanning Microscopy (SEM) was used for this purpose.The hardened cementitious part of the control samples and the CaAincorporated samples were separated from the concrete matrixes and tested.The SEM image (Figure 7: on the right) shows a typical SEM image of the control sample.Cement hydration products are well-noticed.The Ca(OH)2 crystals appeared irregular in shape with light gray color.Calcium silicate hydrates are visualized with dark gray areas, while the pores are represented by black areas [19].A typical SEM image of the hardened cementitious part of a sample incorporated with CaA is shown in Figure 7: on the right, the microscopic observations demonstrate a more compacted and denser microstructure resulting from the incorporation of CaA in the concrete microstructure.The denser microstructure may be owing to the high adhesion between CaA and the cement hydration products.The shape of CaA particle and its large surface area offers good adhesion of the contact zone between the cement matrix and CaA particle.The surface of CaA particles is rougher than the surface of cement, which is smooth; therefore, the hardened cement paste has better adhesion to CaA particles.The more compacted microstructure leads to a reduction of the volume of capillary pores thus, the incorporation of CaA in the concrete mix gives rise to manufacturing a denser and stronger, and more durable building material.
In spite of using mineral admixture to increase the durability characteristics of building materials bonded by cement [20], CaA has been mentioned in literature as an accelerator and water reducer in silica fume blended cement [21], but research on its use as a modifier for concrete properties is very rare.One study mentioned the use of calcium acetate as a waste solutions from the treatment of recycled concrete aggregate with acetic acid as an admixture [22].In summary, the results of the current work confirmed that the concrete composites incorporated with CaA had more compressive strength (more than 28% increase) and less water absorption (more than 83% decrease) owing to the more dense and compacted texture compared to plain concrete.

Conclusions
New developments in the formulation and application of cement concrete materials are carried out by the incorporation of relatively small amounts of additives.Among the additives are the chemical admixtures which confirmed their efficient adjustment of the performance of cement-based materials.This study aimed to investigate the effect of curing age on compressive strength and density and the impact of temperature on water absorption for conventional concrete mixes and concrete mixes of optimum CaA content.The outstanding compressive strength and water absorption properties of concrete mixes incorporated with CaA offer an optimistic push to continue the work on these new concrete mixes to explore other strength and durability characteristics to offer a comprehensive and broad knowledge of these mixes which is required for their certification and application as new construction materials.

Fig. 3 .
Fig. 3. Compressive strength of control samples and concrete with CaA as a function of curing age.

Fig. 4 .
Fig. 4. The correlation of compressive strength with density for the conventional concrete mixes and the mixes incorporated with CaA at different curing age (3,7,14 and 28 days).

Fig. 5 .
Fig. 5. Effect of temperature on % water absorption of plain concrete and concrete incorporated with CaA Figure 5 depicted that the absorption rate of plain concrete is higher by 19.7 times compared to that of the concrete incorporated with the mineral admixture.The phenomena indicated that both the water absorption capacity reflected by the quantity of absorbed water and the rate of water absorption increase with temperature.The decrease in the water absorption rate for the concrete incorporated with CaA is an indication of decreasing the porosity of the concrete mix.

Fig. 6 .
Fig. 6.Arrhenius plots for control and the concrete incorporated with CaA