The Role of Polymer in Calcium Sulfoaluminate Cement-Based Materials

Calcium sulfoaluminate (CSA) cement, developed by China Building Materials Academy in the 1970s, is calcined with bauxite, limestone and gypsum at a relatively low temperature compared to Portland cement. The main mineral compositions of CSA cement are ye’elimite (C₄A₃\(\overline{\text{S} }\)), belite and anhydrite/gypsum. CSA cement is eco-friendly that has low carbon emission, low firing temperature and low energy consumption during production. At the same time, it has the characteristics of fast setting, high early strength, micro expansion or low shrinkage, and good anti-permeability. It can be applied in emergency repair, low-temperature construction, seepage control engineering, etc. [1].

Polymers are often used to modify Portland cement-based materials and good results were achieved. However, research on adding polymer to CSA cement-based materials is quite limited. In order to solve the problems existed during the practical application of CSA cement, researches were carried out on the function of polymers in CSA cement-based materials. The polymers used can be divided into two types, i.e., polymer dispersions and water-soluble polymers. Cellulose ether as one of the water-soluble polymers was applied to improve the workability of CSA cement mortar, especially the water retention capacity, and some achievements were obtained [2‐8]. The effect of polymer dispersions on the mechanical properties, durability, and hydration of CSA cement was analyzed in previous studies [9‐13]. It is found that the addition of polymer dispersions in CSA cement mortars benefits to improve their mechanical properties. Meanwhile, it also contributes to enhancing their durability by optimizing the microstructure. However, the influence on the properties is dependent on polymer types. Generally, SB dispersion demonstrates the best for polymer-modified CSA cement mortar. Since the main hydration product AFt makes the CSA cement matrix sensitive to temperature and ageing [14‐19], the effect of curing regimes on the early age and long-term performance of SB modified CSA cement mortar was also studied intensively [20‐23]. This paper reviewed the effect of SB on CSA cement-based materials including the rheology behavior, hydration behavior, physical and mechanical properties based on recent researches of our group.

The effect of SB dispersion on the rheology and setting behavior of CSA cement paste was investigated [13]. Amounts of 0%, 5%, 10%, 15% and 20% of SB were added to the base CSA cement paste with a constant water to cement ratio of 0.4. A series of experiments including fluidity, rheology, zeta potential, conductivity, total organic carbon (TOC), setting time and calorimetry were performed. The research showed that SB undertook negative charged carboxylic group, which could be adsorbed onto the cationic CSA cement grains as well as hydration products with positive phases, and finally resulting in a big reduction of zeta potential and conductivity of the cement paste. SB dispersion performed a good water reducing effect, which contributed to the fluidity increasing and efflux time decreasing (Fig. 1). The yield stress and thixotropy were calculated to further understand the rheology behavior of CSA cement paste with SB. The yield stress was the minimum force required for the paste to overcome before flowing. The thixotropy was represented by the hysteresis area between the up and down curves of shear stress. The results of yield stress and hysteresis area were summarized in Table 1. The decreased hysteresis area stands for a better thixotropy of SB modified cement paste, meaning that the agglomerated structure of the CSA cement paste can be gradually destroyed under the shear stress but gradually recovered once the shear stress was removed. As shown in Fig. 2, the viscosity of the control cement pastes dramatically declined at a low shear rate, which is a typical phenomenon of shear-thinning behavior. The lower viscosity was observed when SB was added into CSA cement paste, indicating that the flow resistance of cement particles decreased and finally the fluidity increased.

The zeta potential and conductivity demonstrated the electrostatic interaction between SB dispersion and CSA cement which was mainly ascribed to the adsorption of polymer particles on the cement mineral surfaces. Meanwhile, the rheological properties of cement pastes with the addition of various polymer dispersions could be highly related to the adsorption behavior of the polymers [24, 25]. The zeta potential of CSA cement particles was positive while that of SB dispersion was negative, thus SB could be easily adsorbed onto the surface of CSA cement grains via the electrostatic interaction [24‐26]. The setting time of CSA cement paste was prolonged by the addition of SB. The retardation effect increased with increasing SB dosage, which agreed with fluidity, rheology, and adsorption results. The lengthened setting time could be attributed to the retardation effect of polymers on cement hydration, which will be discussed in the following.

The early hydration of CSA cement modified by SB was investigated [27]. The research showed that the addition of SB retarded the initial hydration of CSA cement before curing age of 3 h (h), at which the heat evolution was significantly decreased (Fig. 3). The results from XRD and TG analysis for those samples at 1 h showed that the formation of ettringite was strongly delayed while calcium sulfate dihydrate was formed in SB modified CSA cement paste (Fig. 4); the more amount of SB was incorporated, the less ettringite was generated. The retarded hydration of CSA cement caused by SB in the initial stage prolonged the setting time of CSA cement paste, which was further confirmed by ultrasonic measurement and calorimetry. The hydration degree of SB modified CSA cement caught up that of the control at around 3 h based on cumulative heat value from calorimetry analysis (Fig. 3 (b)). Afterwards, the hydration of SB modified CSA cement tended to be accelerated. The more SB was added, the more ettringite was generated based on the results at 6 h and thereafter. The most ettringite was formed in the paste with 20% SB. Hemicarboaluminate (Hc) was generated after 12 h in the control paste, and the addition of SB demonstrated good inhibition effect on the formation of Hc. SEM observation also confirmed above analysis that the addition of SB delayed the formation of ettringite in the initial stage while it promoted the formation and growth of ettringite crystals in the later periods.

BSE images were widely employed to determine phase distribution and porosity in the hydrated CSA cement pastes [17, 28]. The CSA cement matrix basically consists of pores, cracks, hydration products and unhydrated cement grains in an order of black to high brightness in BSE images. It was shown in Fig. 5 (a) that the control CSA cement already had a quite dense structure after 3 d of hydration. Some unhydrated cement grains shown in the high brightness area indicated incomplete hydration. The typical hydration products exhibiting a dark grey level in BSE picture consisted mainly of ettringite and aluminum hydroxide. With addition of 10% SB (Fig. 5 (b)), less unhydrated clinker grains with low brightness background appeared after 3 d of hydration, meaning higher hydration degree in comparison with control sample. The brightness areas for unhydrated clinker grains were the least for the microstructure of CSA cement with 20% SB (Fig. 5 (c)), indicating the highest hydration degree among these three pastes. The BSE result was well agreement with XRD and TG analysis.

The effect of curing regimes on the performance and microstructure of CSA cement mortar was investigated [22]. Mortars with five different ratios (0%, 5%, 10%, 15%, and 20%) of SB to CSA cement at a constant workability were prepared. Four temperatures (0 ℃, 5 ℃, 20 ℃, and 40 ℃) and three relative humidity (RH) levels varying from low (32 ± 2%) (LRH), middle (63 ± 10%) (MRH), and high (96 ± 3%) (HRH) were considered for mortar specimens curing.

The research found that CSA cement mortar cured at high temperature and HRH had the highest flexural strength at 1 day. However, the temperature showed a limited effect on the strength development of CSA cement mortar in late stages. The addition of 5% SB decreased the flexural strength. The highest flexural strength was achieved when the dosage of SB was 20%. Curing under high temperature and HRH improved the flexural strength, and this effect seemed to be much more significant for CSA cement mortar with 15% and 20% SB addition.

The tensile bond strength increased with higher SB addition; it reached the highest value when the SB content was 20%. SB content and curing temperature were the main factors influencing the tensile bond strength, while the influence from RH was relatively weak. Figure 6 showed the effect of curing conditions on the flexural and tensile bond strength of SB modified CSA cement mortar within 28 days.

SB reduced the compressive strength, however, the compressive strength tended to increased slightly with increasing SB amount. Curing temperatures of 20 ℃ and 40 ℃ brought quicker compressive strength development of the control mortar, while curing temperatures of 0 ℃ and 5 ℃ showed delayed strength. This phenomenon was more evident with HRH curing of less than 7 d. Generally, the compressive strength was much more influenced by curing temperature compared to RH. Greater strength was obtained when the SB modified mortar was cured under high temperature and HRH.

SB in CSA cement mortar led to a decrease in water capillary adsorption, but it was not true for 5% SB addition. HRH decreased the water capillary adsorption of SB modified CSA cement mortar, while high temperature tended to increase it, especially when the SB dosage was between 10% and 20%.

The addition of SB generated continuous film between cement hydrates (Fig. 7), which was conducive to reducing the porosity of CSA cement mortar. Curing with high temperature and high RH was helpful for the formation of ettringite. The bigger size ettringite and polymer film were intertwined to make the mortar stronger. Thus, the properties of CSA cement mortar including flexural strength, tensile bond strength, and water capillary adsorption were well improved.

The long-term change in the physical and mechanical properties of SB dispersion modified CSA cement mortar as it aged from 28 to 360 days, and cured at different temperatures and relative humidities was also studied [23]. The results showed that the mechanical properties of control CSA cement mortar, including its flexural, compressive, and tensile bond strength, showed a reduction after a certain age, but its water capillary absorption was hardly affected by age. When SB was added, there was no reduction in mechanical strength anymore. The amount of SB added did matter. Addition of 5% SB had a negative effect on most properties, except for tensile bond strength. However, the properties of SB modified mortar were enhanced significantly as the amount of SB was increased from 5% to 20%. Temperature change had different effects on the properties of control mortar and SB modified mortar. High temperature was beneficial to early flexural and compressive strength development of control mortar, but caused serious strength reduction at later ages. High temperature enhanced the development of tensile bond strength of control mortar. Whereas, increasing temperature enhanced properties of SB modified mortar, including flexural, compressive, and tensile bond strength. Higher relative humidity improved all measured properties of all mortars. Figure 8 showed the effect of SB content (m_p/m_c) on the flexural and tensile bond strength development of CSA cement mortars cured at MRH and different temperatures. The shrinkage rate of CSA cement mortar modified with SB within 360 days under different curing conditions was investigated and it was found that the shrinkage rate of CSA cement mortar decreased significantly when the SB content was more than 10% in all curing conditions [21]. CSA cement mortar demonstrates well resistance to freeze-thaw cycle, carbonization and sulfate attack, and SB helped to further enhance these resistances [10].

The function of SB in CSA cement-based material was summarized in the paper. SB took effect on multi-performance of CSA cement. It affected rheology property, including increasing the fluidity, decreasing the viscosity, improving the thixotropy and thus improving the workability significantly. It affected the hydration behavior especially the early hydration, including retarding the very initial hydration of CSA cement, e.g., at 1 h, the formation of ettringite was strongly delayed in SB modified CSA cement paste. The retarded hydration of CSA cement caused by SB in the initial stage prolonged the setting time. The hydration degree of SB modified CSA cement caught up that of the control at around 3 h. Afterwards, the hydration of SB modified CSA cement tended to be accelerated. The more SB was added, the more ettringite was generated at 6 h and thereafter. It affected the physical and mechanical properties and durability, including increasing the flexural and tensile bond strength but not compressive strength, improving the resistance to water, carbonization, sulfate attack and freeze-thaw cycle, and especially it could inhibit long-term strength reduction. SB was helpful to improve microstructure by forming continuous films in CSA cement mortar, which finally contributed to CSA cement mortar superior properties. The function and mechanism of polymer in CSA cement-based materials is an ongoing topic.