Hydration mechanisms of composite binders containing phosphorus slag at different temperatures

Highlights

• Elevated temperature reduces the retardation effect of phosphorus slag.

• Elevated temperature enhances the reaction degree of phosphorus slag at all ages.

• Phosphorus slag improves the pore structure at high curing temperature.

• Phosphorus slag enhances the late-age strength greater at higher temperature.

Abstract

The hydration mechanisms of binders containing phosphorous slag at different temperatures were investigated by determining the hydration heat, reaction degree, non-evaporable water content, Ca(OH)₂ content and pore structure of the paste. In addition to plain cement, composite binders containing quartz having a particle size distribution similar to that of phosphorous slag were selected as the control sample. The results show that the retardation effect of the phosphorous slag is greater than its acceleration effect (nucleation effect and dilution effect) on the early hydration of cement. However, increasing the curing temperature tends to reduce the retardation effect of phosphorous slag on the early hydration of cement. The addition of phosphorous slag increases the hydration degree of cement at later ages, particularly at high replacement ratio, but this acceleration effect on the late hydration of cement is weakened at high curing temperature. Under high-temperature curing condition, the reaction degree of phosphorous slag is greater than that under normal-temperature curing condition at all ages, and thus, the contribution of phosphorous slag to the increment of the hydration product and the reduction in Ca(OH)₂ is greater. In the case of standard curing, the pore structure of the paste containing phosphorous slag is coarser than that of the paste containing quartz at early ages, whereas the opposite trend is observed under high-temperature curing condition. At late ages, the pore structure of the paste containing phosphorous slag is finer than those of the plain cement paste and the paste containing quartz for both standard curing and steam curing. Increasing the curing temperature tends to reduce the difference between the compressive strength of the plain cement mortar and that of the mortar containing phosphorous slag at early ages. At the same time, the late-age compressive strength of the mortar containing phosphorous slag surpasses that of the plain cement mortar much more at higher curing temperature.

Introduction

Supplementary cementitious materials (SCMs), e.g., fly ash, steel slag, ground granulated blast furnace slag (GGBS), silica fume, metakaolin and natural pozzolans, are widely used in the preparation of concrete. The hydration mechanisms of the composite binders and the mechanical properties and durability of concrete containing SCMs have been studied extensively [1], [2], [3]. Lothenbach [2] summarized the impact of SCMs on the hydration kinetics of composite binders and analyzed the influence of different factors (e.g., the SCM composition, replacement level, solution pH and curing temperature) on the reaction rate. SCMs tend to promote the hydration of clinkers due to their nucleating and diluting effects at early ages. The addition of a large percentage of SCMs reduces the heat evolution rate and cumulative hydration heat, which are beneficial to the reduction in the temperature-induced cracking risk in massive concrete structures [4]. Later reactions of many SCMs consume Ca(OH)₂, resulting in an improved microstructure of the hardened paste and enhanced the long-term mechanical properties and durability of concrete [1]. In addition, partial replacement of clinkers by SCMs is beneficial to sustainable concrete production due to the reduction in CO₂ emissions and resource consumption [5]. Furthermore, the use of SCMs in the preparation of concrete is an effective measure for solving the problem of industrial by-product accumulation, which is harmful to the environment. Worldwide, up to 5.8 billion tons of cement are projected to be produced annually, leading to additional CO₂ emissions and resource consumption [6]. Therefore, many recent studies have focused on increasing the replacement ratio of clinkers under the premise of ensuring the performance of concrete via various methods [7], [8], [9]. However, high-quality SCMs have been broadly exploited, and the commonly used SCMs are non-uniformly distributed in many countries. The production of GGBS and fly ash is expected to decrease in the future [10]. Consequently, new industry by-products with potential pozzolanic reactivity are worthy of study to expand the types of SCM for concrete.

Phosphorous slag (PS) is a by-product of the production of yellow phosphor via the electric furnace method. The amount of PS produced exceeds 8.0 and 3.6 million tons annually in China and the United States [11], [12], respectively. The primary constituents of PS are SiO₂ and CaO, with total contents that are normally greater than 85%. The glassy-phase content of PS is greater than 90%, illustrating that PS has potential reactivity [13], [14]. However, the reactivity of PS is lower than that of GGBS due to its lower Al₂O₃ content [15]. At the same time, the residual phosphorus in PS has a general retardation effect on the early hydration of cement [16], [17], [18], and numerous research studies were conducted to promote the reaction of composite binders containing PS using various methods [12], [15], [19]. Partial replacement of cement with PS was found to improve the fluidity of fresh concrete [20]. The reaction of PS consumes Ca(OH)₂ and produces additional C-S-H gel, which can increase the density of the structure and reduce the amount of harmful pores [13], [20]. The concrete containing PS was found to exhibit excellent freeze-thaw and sulfate resistance [13]. However, Allahverdi found that the durability of concrete containing a high percentage of PS was decreased despite its relatively higher compressive strength at late ages [21]. Overall, these research studies illustrate that the addition of an appropriate amount of PS improves the mechanical properties and durability of concrete.

According to Arrhenius’ law, the chemical reaction rate is significantly affected by temperature. The hydration kinetics of the composite binders were also found to be affected by the curing temperatures [22]. The reaction of composite binders was accelerated by elevated curing temperature, producing more C-S-H gel, increasing the density of the matrix, and resulting in superior mechanical properties of concrete at early ages [23]. The late compressive strength of the concrete under the higher-temperature curing condition was typically lower [24]. This result is believed to be due to a more heterogeneous distribution of hydration products caused by the elevated curing temperature at early ages, which leads to the formation of large pores and an increase in cumulative pore volume [23], [25]. The reactivity of SCMs was found to be more sensitive to temperature than the hydration of Portland cement [2], [26], [27]. The temperature inside massive concrete structures, the steam-cured concrete and even normal concrete element with high casting temperature are significantly higher than the standard curing temperature in the laboratory. Thus, the influences of the curing temperature on the reactivity of SCMs and the corresponding properties of the concrete deserve further investigation.

In this research, the hydration mechanisms of the composite binders containing PS were studied by determining the hydration heat, non-evaporable water content and reaction degree. The influence of the curing temperature on the hydration of the composite binders containing PS was discussed.