Performance evaluation of heavy oil fly ash as a retarder of Portland cement hydration

Highlights

• Heavy oil fly ash (HOFA) extended the setting time of Portland cement.

• HOFA improved the slump retention in concrete.

• HOFA performance as a retarder was evaluated in comparison with other chemicals.

• Concrete with HOFA is acceptable according to the leachability test (TCLP).

Abstract

Retarding admixtures have the ability minimize the adverse effects of concreting under hot weather conditions. Liquid admixtures are generally used for this purpose. However, these have limited shelf-life and therefore there is a need to develop solid retarding admixtures. This paper reports results of a study conducted to assess the performance of solid retarders for retardation of cement. The examined materials included heavy oil fly ash (HOFA) (a byproduct of oil combustion) and chemical retarders (sodium gluconate, sucrose and zinc oxide). The ability of up to 3% HOFA, by weight of cement, to extend the setting time was compared with other chemical retarders. The performance of the selected retarders was evaluated by measuring heat evolution, workability/slump retention, drying shrinkage, and compressive strength. The experimental results indicated that HOFA, sodium gluconate, sucrose and zinc oxide can effectively extend the initial and final setting time of cement. Further, the calorimetric data demonstrated that cement hydration was temporary hindered by the addition of HOFA and the selected chemical retarders. The 28-day compressive strength of concrete with the selected retarders, including HOFA, was similar to that of OPC concrete, and it ranged from 48.4 to 52.6 MPa. However, the rate of early-age drying shrinkage strain increased due to the addition of the selected retarding agents which may necessitate precautionary measures. Finally, it can be concluded that the retarding efficiency of HOFA was generally comparable to that of the selected chemical admixtures.

Introduction

Retarding admixtures are used in concrete to prevent the problems associated with the early setting of ready mixed concrete placed under hot weather conditions [1]. They are used in other situations that also require extended setting time and delayed hydration reactions, such as in mass concrete [2], oil and gas wells [3] and roller compacted concrete [4]. According to Bishop and Barron [5], there are four mechanisms generally associated with the retardation of cement hydration due to the use of retarding admixtures. The first mechanism involves the formation of a temporary semipermeable membrane (layer) that slows the migration of water towards the anhydrous components of cement grains. Eventually, this layer bursts due to osmotic pressure gradients that facilitates the hydration of cement. The second mechanism is calcium complexation that prevents the formation of some phases (e.g., C–S–H gel) mainly due to calcium chelation. In the third mechanism, referred to as nucleation poisoning, the growth of Ca(OH)₂ and C–S–H crystals is blocked by the retarding admixture. The last mechanism involves the direct adsorption of the retarder onto the surfaces of the anhydrous cement, thus blocking their reaction with water [5]. Generally, liquid chemical admixtures are used for retarding the setting time of cements. However, these have limited shelf-life and as such they pose problems if used in their expired condition. Consequently, there is a need to develop an economical and environmental-friendly solid admixture that have longer shelf-life. Heavy oil fly ash (HOFA), a residue of burning in power plants is a potential candidate that can be investigated for its cement retardation properties.

HOFA is a byproduct of the combustion of oil, primarily in oil-fired power plants [[6], [7], [8]]. It also forms during burning of oil in water desalination plants [[9], [10], [11]]. This byproduct has a large content of unburned carbon [12,13] in addition to small quantities of vanadium (V), nickel (Ni) and other trace elements [14,15]. Unlike fly ash (resulting from combustion of natural coal), HOFA has been rarely used in cement and concrete. Several methods were proposed to extract the heavy metals (e.g., V and Ni) from HOFA. However, the environmental problems associated with the sludge remaining after the extraction process still exist. Therefore, environmental benefits of using HOFA in concrete can be gained by reducing the amounts of consumed cement and by providing highly desirable avenue of disposal of this waste byproduct [16].

Other chemicals, such as sodium gluconate, are also used for retarding the setting time of cement. Sodium gluconate is an organic compound [17,18] with the molecular formula C₆H₁₁NaO₇ [19,20]. It is considered a chelating agent [21,22] that is highly-water soluble [23], non-corrosive [24,25] and non-toxic [26,27]. It is also designated as GRAS (Generally Recognized as Safe) material [28]. Because of these properties, sodium gluconate has been used in a wide range of chemical industries, such as pharmaceutical [29,30], food [31,32], textile [33,34] and detergent [35,36]. According to Chun et al. [37] sodium gluconate can effectively increase the setting time of ordinary Portland cement (OPC) which was confirmed through colorimetric tests. They [37] also reported that sodium gluconate can improve the flow and workability properties of mortar. Ma et al. [38] reported that sodium gluconate can delay the setting of cement paste and improve the fluidity of mortar even at a small dosage of 0.03%. They also reported that the dosage of sodium gluconate can be optimized to improve the compressive strength of Portland cement mortar. Furthermore, other studies [2,[39], [40], [41]] have reported on the ability of sodium gluconate to retard the setting time of Portland cement by affecting the early hydration reaction of cement.

Another chemical that has been used as a retarding admixture is sucrose (molecular formula C₁₂H₂₂O₁₁). It is a disaccharide formed from two monosaccharides, glucose and fructose. It is a small, natural carbohydrate molecule that contains 3 primary and 5 secondary hydroxyl groups [42]. Few studies reported on the retardation properties of sucrose. For instance, Ataie et al. [43] reported that sucrose can cause a shift in the heat-flow profile with time implying a delay in the hydration reaction of cement. Zhang et al. [44] studied the effect of different saccharides, such as sucrose, xylitol, and threitol on setting of time. They found an exponential increase in the setting time with increasing concentration of saccharide. Bishop and Barron [5] examined the impact of sucrose on the hydration of tricalcium aluminate (C₃A) and tricalcium silicate (C₃S). They reported that sucrose inhibits the hydration of C₃S rather than that of C₃A.

Zinc oxide (ZnO) is an inorganic compound used in different applications and industries including glass, ceramic, rubber and plastics. Despite occurring naturally, it is mostly produced synthetically [45]. Recently, it has been incorporated in cement and concrete for three main reasons. First, it has noticeable bactericidal characteristics preventing early concrete degradation due to bacterial growth [46,47]. Second, it exhibits pronounced corrosion inhibition effect thereby protecting the reinforcing steel from degradation [48,49]. Third, zinc oxide presents significant retardation properties leading to the extension of the setting time of cement paste [50,51]. Liu et al. [52] found using isothermal calorimetry tests conducted on cement paste, that the addition of zinc oxide prolongs the induction period and affects the rate of the early-age hydration reactions. Similarly, Keppert et al. [53] found that the addition of zinc oxide to OPC generally decreased the rate of heat evolution and strength gain during cement hydration.

Several commercial admixtures have been used as retarders, especially in hot environments. Despite their popularity, they are generally costly. Moreover, these admixtures are manufactured in a liquid form and they lose their quality over time. Consequently, their use in concrete may jeopardize the quality of the structural members. In the light of these drawbacks, the use of solid retarders seems to be a reasonable alternative to liquid retarders. Previously, the authors [51] evaluated the use of the industrial waste material, electric arc furnace dust, to develop a retarding admixture. In this study, the retardation effect of HOFA was evaluated and compared with solid chemical admixtures, namely sodium gluconate, sucrose and zinc oxide. The retardation characteristics of HOFA and the selected solid chemical admixtures was evaluated by measuring setting time, workability retention, heat of hydration, drying shrinkage strain, and compressive strength. The novelty of this study is the use of HOFA, an industrial waste material, as a solid retarder. In addition, the retardation effect of HOFA is compared with that of other solid chemical admixtures. Such a comparison was not reported earlier. The data developed in the reported study will be useful in selecting optimal dosage of HOFA and the chemical retarders based on setting time and other property requirements.

The aim of the reported study was to examine the possibility of utilizing HOFA as a concrete retarder. As such, it was focused on determining suitable dosages of HOFA that can extend the setting time of cement paste. Additionally, the effect of selected dosages of HOFA on basic mechanical properties of concrete was also assessed. This study may be regarded as a preliminary one that has two limitations which can be considered by future researchers. First, the study did not assess other mechanical properties of concrete such as tensile strength, flexural strength, modulus of elasticity, etc. Second, although the study investigated one durability aspect of concrete, which is the drying shrinkage, it did not investigate the effect of HOFA on durability properties such as chemical resistance, water permeability, corrosion resistance and others. Thus, further studies are needed to assess other mechanical and durability properties of concrete prepared with the optimum dosage of HOFA.