This study investigates the magnesium sulphate resistance of chemically activated phosphorus slag-based composite cement (CAPSCC). Enough mortar specimens were prepared from phosphorus slag (80 wt.%), type II Portland cement (14 wt.%), and compound chemical activator (6 wt.%) and were exposed to 5% magnesium sulphate solution after being cured. Mortar specimens of both type II and V Portland cements (PC2 and PC5) were also prepared and used for comparison purpose. According to the test results, after 12 months of exposure, PC2, PC5 and CAPSCC exhibited 43.5, 35.2 and 25.2% reduction in compressive strength, 0.136, 0.110, and 0.026% expansion in length, and 0.91, 2.2, and 1.78% change in weight, respectively. Complementary studies by X-ray diffractometry and scanning electron microscopy revealed that CAPSCC has a very low potential for the formation of sulphate attack products, especially ettringite. The results confirm a high magnesium sulphate resistance for CAPSCC compared to PC2 and PC5.
The world is now looking forward to the emergence of new construction and building binders that can replace the Portland cement and not only increase environmental sustainability but also improve the durability and performance of concrete structures (
Chemical reactions between sulphates and cement paste constituents are one of the most important reasons for concrete degradation (
Due to pozzolanic property, the use of mineral admixtures that decreases the CH content of the cement paste and results in dilution of calcium aluminate hydrate can improve sulphate resistance (
One of the important factors affecting the degradation of concrete exposed to sulphate environment is the concentration of sulphate. Relatively higher concentrations of sulphate lead to faster concrete degradation (
In various studies, magnesium sulphate resistance of blended cements prepared by different supplementary cementing materials such as granulated blast furnace slag (GBFS), silica fume, and fly ash has been investigated (
As can be deduced from the above-mentioned reports, there are inconsistent results about the role of supplementary cementing materials on the sulphate resistance of blended or composite cements. Taken together, these have been our initial motivation to investigate the magnesium sulphate resistance of a kind of PHS-based composite cement with a very high replacement level.
PHS is a by-product of yellow phosphor production via electric furnace method, mainly consisting of calcium oxide (CaO) and silicon dioxide (SiO2) (
The cement under investigation in the present work is prepared from high amount of PHS and is chemically activated by adding a Portland cement-based compound chemical activator formed from sodium sulphate and anhydrite (
The PHS used in this study was provided from a phosphoric acid plant located in Tehran province, Iran. Type II Portland cement (PC2) and Type V Portland cement (PC5) were used in accordance with ASTM standard. The chemical and physical properties of PHS, PC2, and PC5 as well as Bogue’s potential phase compositions of PCs are presented in
Properties of phosphorus slag and Portland cements
Physical properties | PC2 | PC5 | PHS |
---|---|---|---|
Blaine fineness (m2/kg) | 320 | 295 | 303 |
Density (kg/m3) | 3120 | 3145 | 2940 |
Chemical composition (wt.%) | |||
CaO | 63.26 | 64.90 | 45.14 |
SiO2 | 22.50 | 22.42 | 38.42 |
Al2O3 | 4.15 | 3.81 | 7.65 |
Fe2O3 | 3.44 | 4.20 | 0.90 |
MgO | 3.25 | 0.08 | 2.60 |
SO3 | 1.80 | 1.64 | - |
K2O | 0.65 | 0.42 | 0.56 |
Na2O | 0.20 | 0.22 | 0.43 |
P2O5 | - | - | 1.50 |
LOI | 0.61 | 1.61 | 1.87 |
Free lime | 0.48 | 1.07 | - |
Bogue’s potential phase composition (wt.%) | |||
C3S | 45.62 | 53.65 | - |
C2S | 30.16 | 23.88 | - |
C3A | 5.18 | 2.99 | - |
C4AF | 10.47 | 12.78 | - |
In accordance with ASTM standard C188
X-ray diffractogram of phosphorus slag powder.
A mixture of sodium sulphate (2 wt.%) and anhydrite (4 wt.%) was used as compound chemical activator in this study based on some recent studies (
The siliceous sand used to prepare the mortar specimens was in accordance with standard DIN-EN 196-1.
The pipeline potable water was used to prepare mortar specimens. The specific gravity of the used water was supposed about 1000 kg/m3.
Enough paste and mortar specimens of different sizes were prepared from CAPSCC and control cements (PC2, PC5) in accordance with ASTM standard C109. The water-to-cement ratios for CAPSCC paste and mortar were obtained 0.22 and 0.37, respectively in accordance with ASTM standards C187 and C230, respectively. For CAPSCC mortar, the water-to-cement ratio was adjusted at a value giving an almost the same spread diameter as obtained for PC2 and PC5 control mortars in flow-table test. Mortar specimens of the size 5×5×5 cm3 were used for monitoring the changes happening in the compressive strength and weight and mortar specimens of the size 2.5×2.5×28.5 cm3 were employed for the purpose of length change test. Simultaneously, paste specimens of the size 2×2×2 cm3 were also prepared for complementary studies (XRD and SEM).
To prepare mortar specimens, a planetary mixer was used according to ASTM standard C305. After casting, the molds were stored at an atmosphere of more than 95% relative humidity at 23±2 °C for 24 hours and then after demolding, the specimens were cured in lime-saturated water at 23±2 °C until the time of testing.
The curing time in lime-saturated water for control specimens was 27 days. For CAPSCC specimens, however, based on ASTM standard C1012, a curing time of 49 days in lime-saturated water was applied for attaining a compressive strength of at least 20 MPa. The magnesium sulphate solution was also prepared in accordance with ASTM standard C1012. All the cured paste and mortar specimens were then fully immersed in 5% magnesium sulphate solution. To control the pH of the solution between 6 and 8, the solution was refreshed every week for the first month, and then this trend was continued every month to the end of twelfth month.
The compressive strength of the specimens was measured by means of a uniaxial digital hydraulic press (SCL STD 30) with ±1% accuracy. A total of three cubic specimens were monthly used for each measurement of compressive strength and the average of them was reported as the result. The compressive strength reduction was calculated by the following equation [
where
Length change test was done in accordance with ASTM standard C1012 using comparator device with a precision of 0.01 mm. According to this standard, length changes were calculated by the following equation [
where
For weight change test purpose, the solution on the surfaces of the specimens was firstly removed with a towel and immediately after that the weights of the specimens were measured and recorded. Three cubic specimens were monthly used for each measurement of weight change and the average of them was reported as the result. The weight change due to exposure to magnesium sulphate solution was calculated by the following equation [
in which,
For complementary studies, the paste and mortar specimens which have been exposed to 5% magnesium sulphate solution for 10 months were used. The mineralogical characterization was performed with a Philips PW 1800 powder X-ray diffractometer using CuKα radiation at 40 kV and 30 mA. The X-ray diffraction (XRD) patterns were obtained by a scanning rate of 1° per minute from 2θ = 4° to 60°. From each of the paste specimens of three cements, a complete paste specimen including surface brucite layer was crushed, dried at room temperature for 2 days and ground to a fine powder. The produced powders were then split into small samples for X-ray diffractometry test.
For the purpose of microstructural studies, SEM images were prepared from regions close to exposed surfaces of CAPSCC, PC2, and PC5 pastes and mortars using a TESCAN VEGA II Scanning Electron Microscope device (Czech Republic) at an accelerating voltage of 30 kV. For this purpose, relatively thin sections were cut from the paste and mortar specimens. These sections were then dried in an oven at a temperature of 95 °C for 3 days. The SEM images have been prepared in the secondary electron mode of the device. To make the samples conductive, they were coated with a very thin gold layer.
The deteriorating effects of sulphate attack on cement-based materials usually result in visually distinguishable signs. During the course of deterioration, the changes in the appearance of the specimens were monitored visually and photographed. Figure
Mortar specimens of (a) CAPSCC, (b) PC5, and (c) PC2 exposed to 5% magnesium sulphate solution for 12 months.
Compressive strength reductions (%) of PC2, PC5, and CAPSCC mortar specimens immersed in magnesium sulphate solution for 12 months are presented in
Compressive strength reduction of mortar specimens exposed to 5% magnesium sulphate solution.
It has been reported that composite cements containing pozzolanic materials exhibit an improved sulphate resistance compared to the corresponding plain control cements. Two main reasons have been stated for this behavior including; 1) consumption of CH in pozzolanic reactions and 2) decelerated decomposition of CSH (
The length changes of PC2, PC5, and CAPSCC mortar bars exposed to 5% magnesium sulphate solution for 12 months are depicted in
Length change of mortar bars exposed to 5% magnesium sulphate solution.
Relatively reduced expansion in mortar specimens exposed to magnesium sulphate solution compared to expansion in sodium sulphate attack can probably be attributed to the formation of brucite layer on the exposed surfaces. Since this layer acts as a protective layer, diffusion of sulphate ions into the cement matrix is limited, decelerating the formation and deposition of voluminous products such as gypsum. This layer can also result in reduced expansion of specimens by creating unstable conditions for ettringite as the most important expanding product.
Weight change of mortar specimens exposed to 5% magnesium sulphate solution.
X-ray diffraction (XRD) analysis was performed for supplementary studies supporting the results of compressive strength reduction and length/weight changes. XRD patterns of PC2, PC5, and CAPSCC hardened pastes exposed to 5% magnesium sulphate solution for 10 months are shown in
XRD patterns of a) PC2, b) PC5, and c) CAPSCC pastes exposed to 5% magnesium sulphate solution for 10 months. (A: Alite, B: Belite, C: Calcite, P: Portlandite, Br: Brucite, G: Gypsum).
As can be seen, no ettringite phase was observed in all paste specimens. In the case of CAPSCC, low-intensity peaks of gypsum and Portlandite are seen compared to PC2 and PC5. The intensity of brucite peaks in CAPSCC is also less than those in PC2 and PC5. As will be confirmed later by scanning electron microscopy studies, CAPSCC mortar specimens developed a much thinner surface brucite layer than PC2 and PC5 mortar specimens. In magnesium sulphate attack, sulphate ions firstly react with CH present in the mortar structure and form brucite and gypsum. Deposition of brucite and gypsum on the exposed surfaces and inside the microstructure, respectively, creates a temporary protective effect against sulphate attack. With continuation of attack and diffusion of more sulphate ions across brucite layer and inside the microstructure, ettringite is produced in regions close to the surface. These materials fill the pores existed in the mortar structure causing porosity reduction in the mortar and consequently, resulting in compressive strength increase. With more production of gypsum and ettringite and continued pore-filling effect, the pressure exerted on the walls of the pores increases and this can finally results in the creation of cracks in the mortar matrix. Formation of the cracks provides shorter diffusion paths for the attacking ions. The penetration depth of sulphate ions within the mortar structure, therefore, increases.
Due to the low saturation pH value of brucite on the surface of the specimens and also where brucite is formed, ettringite becomes unstable and decomposes into gypsum and aluminum hydroxide (ettringite is stable between pH values of 11.4 and 12.4 (
Due to their high amount of C3S and C2S, PC2 and PC5 produce a lot of CH in their structure resulting in more gypsum formation and deposition as a result of reaction with magnesium sulphate, and, because of cracks created in the mortar structure, sulphate ions penetrate more deeply. Also, formation of cracks due to extensive gypsum deposition results in compressive strength reduction. Since the main mechanisms of the deterioration of specimens exposed to magnesium sulphate solution are formation of gypsum and CSH decomposition, PC5 with low C3A has no special advantage, because very little ettringite is usually produced during the attack. CAPSCC produces low CH in its structure due to it only contains 14% of PC and to its possible pozzolanic property and thus resulting in the least gypsum deposition as a result of exposure to magnesium sulphate and compared to both PC2 and PC5. Consequently, CAPSCC mortar shows a smaller increase in compressive strength, which continues up to the fifth month of the exposure time. This is because porosity reduction in CAPSCC mortar structure due to gypsum deposition happens with less intensity over a longer time period. This less-intensity gypsum deposition in CAPSCC mortar is less deteriorative and the first reduction of compressive strength was therefore detected lately at the sixth month of exposure compared to PC2 and PC5, which showed the first sign of deterioration three months sooner (at the third month of exposure). Moreover, the production of additional CSH in CAPSCC mortar results in the formation of a denser microstructure with reduced permeability, which significantly decelerates the diffusion of sulphate ions into the mortar structure. As the result of these factors, CAPSCC exhibits a much better performance against magnesium sulphate attack compared to PC2 and PC5.
SEM images of the brucite surface layers on the exposed surface of PC2, PC5, and CAPSCC paste specimens after 10 months of exposure to 5% magnesium sulphate solution are shown in
SEM images of brucite layers in a) PC2, b) PC5, and c) CAPSCC pastes exposed to 5% magnesium sulphate solution for 10 months.
Resistance of chemically-activated phosphorus slag-based composite cement (CAPSCC) mortar against magnesium sulphate attack at various exposure times up to one year was evaluated within the scope of this study. Also, standard mortars of both type II and V Portland cements (PC2 & PC5) were used as reference for comparison purposes. Visual observations revealed significantly lesser signs of deterioration in CAPSCC compared to PC2 and PC5. After 12 months of continuous magnesium sulphate attack, the compressive strength reduction in PC2 and PC5 specimens were 1.7 and 1.4 times higher than that of CAPSCC mortar specimens, respectively. In addition, CAPSCC mortar specimens showed the least changes in both length and weight in comparison to PC2 and PC5 mortar specimens. The better performance of CAPSCC against magnesium sulphate attack compared to PC2 and PC5 is attributed to the consequences of pozzolanic reactions of phosphorus slag, which consume calcium hydroxide of the cement paste and produce additional CSH. Studies by XRD and SEM indicated that CAPSCC developed a much thinner brucite layer than PC2 and PC5.