China is the second largest phosphorus reserve-rich country and has become one of the most intensively engaged in phosphorus-processing, which has eventually increased the risk to ecological deterioration from air storage of phosphogypsum and phosphorus slag [1
]. Phosphogypsum has been made into belite–ferroaluminate cement products [2
], non-autoclaved aerated concrete [3
], backfilling materials [4
], gypsum products [6
], combined additives in cement [7
]. Phosphorus slag can also be made into backfilling materials [8
], porcelain [9
], glass-ceramics [10
in the chemical industry [11
], cast stone as decorative materials [12
], mineral fillers in asphalt binders [13
], and solidification additives for lead-contaminated soil [15
]. However, it is a win-win solution for phosphorus slag to be regarded as the latent SCM (supplementary cementitious material), and thus avoiding environmental pollutions from air storage and CO2
emissions from cement usage. Phosphorus slag is one by-product discharged during the extracting process of phosphorus under high temperature electric furnace. Major components in phosphorus slag are SiO2
and CaO, minor components are Al2
, MgO, and P2
. Contents of the glass network in granulated phosphorus slag may arrive at 85%~90% because of the high viscosity of the molten slag, similar to granulated blast furnace slag (GGBS) [16
Under inter-grinding with cement clinkers to make phosphorus slag cement (PSC) it is reported that the slow setting and the low early-strength of PSC cannot be avoided by itself unless using chemical agents [17
], or blending PSC with GGBS [19
] and steel slag [20
]. If phosphorus slag could be made into cement clinkers in the unilateral way or the multilateral way between fluorite and steel slag, it would be a good mineralizing agent for nucleation and growth of tricalcium silicate (C3
]. Directly blended with cement pastes, the setting delay still exists and the hydration heat of cement pastes with 35% by mass of phosphorus slag has been cut down by 49.11% [23
]. It is suggested that cement pastes with 40% by mass of phosphorus slag can meet the standard for compressive strength of Type P.O.42.5 cement on the condition that the fineness of cement is increased to 460 m2
]. The early-strength of blended pastes can also be modified if the particle size distribution of phosphorus slag is properly adjusted [25
]. The steam-curing condition has been believed to be capable to repair retardations of phosphorus slag [26
]. Mechanisms of phosphorus slag on the porosity of blended pastes are studied by transformations of cement hydrates [27
] and the fractal theory [28
]. Effects of phosphorus slag on mechanical strength and chemical shrinkage are briefly discussed by models along with comparisons to cement pastes with GGBS [29
Many researchers have tried to enhance phosphorus slag by the alkaline activation. The modulus of water glass has great influences on early hydration and compressive strength of alkali-activated phosphorus slag cement [31
]. The blended activator of water glass and NaOH is also recommended to be used for phosphorus slag [32
]. Rheological behaviors of activated phosphorus slag are strengthened by another blended activator of Ca(OH)2
]. Attention is put on the efflorescence in alkali-activated phosphorus slag cement [34
]. Compressive strength of alkali-activated phosphorus slag cement has been predicted by a temperature–age model [35
] and a statistical model [36
], tested under different curing conditions [37
]. Resistance to freeze–thaw cycles, frost salt attack, and sulfate attack of activated phosphorus slag pastes and mortars are presented by [38
]. Transformations of calcium arsenate waste under solidification by alkali-activated phosphorus slag are studied by [41
Although the setting delay has been detected in blended concretes, the reduction on hydration heat and resistance to shrinkage of phosphorus slag are advantaged for massive concretes like dams and T-shaped beams of bridges [42
]. Compared to ordinary fineness of 300 m2
/kg and 391 m2
/kg in [42
], effects of superfine phosphorus slag on concretes are tried as well. Modifications on porosity are found by phosphorus slag from 600 m2
/kg to 800 m2
]. Improvements on resistance to carbonation, chloride penetration, sulfate attack, compressive strength, and splitting tensile strength of concretes are provided by phosphorus slag of 657 m2
]. The cushion effect of phosphorus slag on ASR (alkali–silica reaction) in sleeper-concrete is confirmed according to ASTM C 1260-94 (Mortar-Bar Method) [47
]. Resistance to water permeability of concretes can be modified by phosphorus slag [48
]. Tensile strength of self-compacting concrete with phosphorus slag is better than plain self-compacting concrete, but lower than self-compacting concrete with fly ash microbeads [49
]. Phosphorus slag is effective to improve the workability of concretes compared to fly ash [50
There are a few research focusing on synergistic effects on concretes with phosphorus slag and other additives. Compressive strength, resistance to frost attack, and chloride penetration of AAPFC concretes (alkali-activated phosphorus slag fly ash cement) are better than plain concretes, but the resistance to carbonation is lower [51
]. The initial research of waste clay brick powder and phosphorus slag to produce geopolymer mortars has been started by [52
]. The diameter of the most probable pore in reactive powder concrete with phosphorus slag and silica fume is less than 10 nm, which leads to superior mechanical properties and durability [53
]. To repair spillways and discharge holes of dams, one type of underwater concretes is developed, which contains phosphorus slag, graphite tailings, fly ash, GGBS, and cellulose ethers [54
]. Wear-resisting strength of concretes can be enhanced by fly ash with phosphorus slag [55
]. Porosity of cement pastes with phosphorus slag and ferronickel slag is inferior to cement pastes with fly ash at early ages [56
]. One type of road lining materials is invented by [57
], which is made up of phosphorus slag, lime, and fly ash.
Nowadays, products of polymer cement concrete (PCC) are widely applied and some types of PCC can be classified into dry mortars [58
]. Basic additives in dry mortars are cellulose ethers and re-dispersible polymer powders which are responsible for water retention of fresh mortars and adhesive strength of hardened mortars [59
]. As one type of dry mortars, self-leveling mortars can be specially designed for phosphorus slag. Unfortunately, there are almost no studies about the properties of cement mortars with phosphorus slag, not to mention influences of curing conditions and bleeding during the fresh state [61
]. Therefore, it is necessary to find out.
In this study, a basic research was made to evaluate effects of phosphorus slag from 10% to 50% (by mass) on behaviors of cement mortars. The setting time and water requirement of normal consistency for cement pastes with phosphorus slag were studied as well as the flowability of fresh mortars. The resistance to carbonation of cement mortars with phosphorus slag was tested by the acceleration carbonation. The compressive strength of cement mortars with phosphorus slag was investigated and the pozzolanic activity of phosphorus slag was assessed by one traditional system based on results of compressive strength. To activate phosphorus slag, the physical activation by increasing fineness and the chemical activation by adding the chemical activator were attempted. Mineralogy and hydration heat of cement pastes with phosphorus slag and the chemical activator were presented by X-ray diffraction (XRD) and an isothermal calorimeter. Morphology of cement pastes and mortars with phosphorus slag and the chemical activator were observed by scanning electron microscopy (SEM).
2. Materials and Methods
2.1. Materials and Mix Proportions
The P.O.42.5 cement (Conch Cement Corp., Anhui, China) and phosphorus slag (Hubei, China) were used. The fineness of P.O.42.5 cement and grinded phosphorus slag were 336 m2
/kg and 300 m2
/kg, respectively. Their particle size distributions are shown in Figure 1
. The density of phosphorus slag and P.O.42.5 cement were 2856 kg/m3
and 3100 kg/m3
, respectively. The chemical compositions of P.O.42.5 cement and grinded phosphorus slag are listed in Table 1
. The XRD pattern of phosphorus slag is shown in Figure 2
. Ordinary river sand was used for aggregates, with a fineness modulus of 2.4 and a bulk density of 1450 kg/m3
. Tap water was used for mixing the samples. The mix proportions of the mortar samples are listed in Table 2
. Curing conditions of samples were at 20 ± 1 °C and (90 ± 1%) RH.
2.2. Performance of Cement Pastes and Fresh Mortars
The setting time and water requirement of normal consistency of cement pastes with phosphorus slag were evaluated according to GB/T 1346-2011. Mix proportions of paste samples are listed in Table 3
. It was noteworthy that there were two methods to test the water requirement of the normal consistency for cement pastes in GB/T 1346-2011. The method in this experiment was to adjust the water consumption. The fluidity of fresh mortars with phosphorus slag was tested according to GB/T 2419-2005, which was called the flow table test. Details of the flow table test were as follows. Fresh mortars were cast into a trapeziform metal container above a rounded table, and then vibrated 25 times for 25 s, moving freely without the container until plates of fresh mortars stopped. The diameters of fresh mortar plates were recorded, and the average value was determined for fluidity of fresh mortars.
2.3. Compressive Strength
The compressive strength of mortars was measured according to GB/T 17671-1999 at 3 days, 28 days, and 90 days. Each result was the average value of 5 specimens (40 mm × 40 mm × 160 mm). Tests were undertaken at a loading rate of (2400 ± 200) N/s. One traditional evaluation system of pozzolanic activity was cited to phosphorus slag. This evaluation system was officially proposed by Professor Xincheng Pu for refereeing contributions of SCMs to compressive strength of high strength concrete and high performance concrete [63
]. In this evaluation system, the pozzolanic activity of certain SCM was defined by Equation (1) to Equation (5). In Equation (1), FRC
is the relative compressive strength, FC
is the original compressive strength, and Q is the percentage of cement dosage. In Equation (2), FPC
is the compressive strength of the pozzolanic activity and FBC
is the compressive strength of plain sample. In Equation (3), K is the coefficient on the compressive strength of the pozzolanic activity. In Equation (4), PP
is the contribution of the pozzolanic activity to the compressive strength. In Equation (5), PH
is the contribution of the cement hydration to the compressive strength.
2.4. Resistance to Carbonation
Procedures of carbonation tests were performed to GBT50082-2009 (the acceleration carbonation). Samples were cured for 26 days and then dried for 48 h in an oven at 60 °C. Samples were entirely coved by paraffin, only opening two vertical interfaces of the square section. Treated samples were placed in the carbonation chamber. The condition of the carbonation chamber was firmly kept with a CO2 concentration of 20 ± 3%, temperature of (20 ± 2) °C, and relative humidity of 70% ± 5%. Samples were taken out after carbonation of 1 day, 3 days, and 7 days, broken vertically along opening interfaces, and sprayed by phenolphthalein solutions (0.1 mol/L) for carbonation depth.
2.5. Activation of Phosphorus Slag
The physical activation and the chemical activation were executed. In the physical activation, the fineness of phosphorus slag was grinded from 300 m2/kg to 350 m2/kg, 400 m2/kg, and 450 m2/kg. Effects of the physical activation were checked by compressive strength of cement mortars with 30% by mass of phosphorus slag at early ages (3 days, 7 days, 28 days). In the chemical activation, one activator was used which contained Al2(SO4)3, Na2SO4, CaCl2, Ca(OH)2 as 1:1:1:1 by mass. Effects were also checked by compressive strength of cement mortars with 30% by mass of phosphorus slag (300 m2/kg) and 1% activator (by mass to phosphorus slag).
2.6. Microstructure Analysis
Cement pastes with 30% by mass of phosphorus slag, plain pastes, and cement pastes with 30% by mass of phosphorus slag and the chemical activator were produced at w/c = 0.5. Under the same conditions, these pastes were cured for 3 days and 28 days (20 mm × 20 mm × 20 mm). The mineralogical phases were determined by XRD analysis. XRD analysis was performed by the X-ray diffraction equipment (Bruker, Karlsruhe, Germany) of Rigaku-D/max2550VB3+, from 5° to 75° at 5°/min with a Cu Kα radiation. XRD peaks were automatically calculated by the software of MDI Jade 6.5. (version 6.5, MDI, Livermore, CA, USA) SEM analysis was tested by the equipment of FEI Quanta 200 FEG (FEI, Hillsboro, OR, USA). The evolution of hydration heat was measured by the isothermal calorimeter (TAM AIR C80, Thermometric, Järfälla, Sweden).
This article mainly aimed at effects of phosphorus slag from 10% to 50% (by mass) on the setting time and water requirement of normal consistency for cement pastes, flowability, resistance to carbonation, and compressive strength of cement mortars, as well as the physical activation and the chemical activation on cement mortars with 30% by mass of phosphorus slag at early ages. Cement pastes and mortars with 30% by mass of phosphorus slag and the chemical activator were revealed by hydration heat, X-ray diffraction (XRD), and scanning electron microscopy (SEM).
The setting time of cement pastes was delayed by phosphorus slag and the delay order was followed to the increase of phosphorus slag from 10% to 50% (by mass). The time interval between the initial setting time and the final setting time was prolonged according to the increase of phosphorus slag from 10% to 50% (by mass). The water requirement of normal consistency for cement pastes and the flowability of cement mortars were not fluctuated by the increase of phosphorus slag from 10% to 50% (by mass).
The resistance to carbonation of cement mortars was declined by the increase of phosphorus slag from 10% to 50% (by mass) according to the acceleration carbonation. The compressive strength of cement mortars was decreased by the increase of phosphorus slag from 10% to 50% (by mass), especially at an early age of three days. The low activity of phosphorus slag was concluded based on compressive strength of cement mortars.
The physical activation by improving fineness of phosphorus slag from 300 m2/kg to 450 m2/kg was less effective on the compressive strength of cement mortars with 30% by mass of phosphorus slag than the chemical activation by adding the chemical activator.
Although phosphorus slag could inhibit the hydration process of cement pastes according to hydration heat flow curves from 0 to 72 h and XRD patterns at 3 days and 28 days, and provoke microcracks to propagate according to SEM images, compensations could be made by adding the chemical activator which is a potential method for this by-product in cement-based materials.