Science
Materials and Methods
Materials and experimental procedures.
Raw Materials
The magnesia used in this study is light-burned magnesia (LBM) obtained by calcining magnesite from Liaoning, China at 750 C, and the chemical composition of the LBM is listed in Table 1.
Specimen preparation
The molar ratio of a-MgO/MgSO4 of MOS cement prepared from LBM was fixed at 7.0. To prepare neat MOS cement specimens, MgSO4 7H2O was firstly dissolved in water to form a 25.00 mass% MgSO4 solution. Secondly, 0.5 % (by weight of LBM) of additive (H3PO4, KH2PO4, K2HPO4, or K3PO4) was mixed with the MgSO4 solution to form a clear, uniform mixed solution. Then, 100.0 g LBM mechanically stirred with 101 g of this mixed solution to form MOS cement paste. The MOS cement |paste was cast in 20 9 20 9 20 mm3 PVC molds, sealed and cured at 20 ± 3 C for about 24 h before demolding.
Specimen analysis
The compressive strengths of the cured MOS cement samples at different ages were tested on a testing machine using a maximum force of 100 kNat a loading rate of 10 mm/min; six samples were tested for each composition and age. The crushed cement was reduced to a power (D90\35 lm) for crystal phase composition analysis by XRD patterns, which were collected on an X’ Pert PRO (PANalytical) diffractometer with CuKa radiation (k = 0.15419 nm) over a 2h range from 5 °C to 70 °C.
The chemical compositions of the raw materials (as shown in Table 1) were obtained by X-ray fluorescence (XRF) on a X-ray fluorescence analyzer (pw-4400-40, PANalytical). To evaluate the water-resistance of the MOS cement specimens, specimens cured for 28 days in air were dipped in water at 20 ± 3 °C, and the compressive strength of the samples after different immersion times in water was measured and used to calculate the softening.
Results and discussion
Effects of additives on setting time
The setting time of MOS cement is related to the MgO hydration rate and phase crystallization process.
Table 2 shows both the initial and final set times for the MOS cement samples with different additives. A previous study has also found that phosphoric acid (H3PO4) is slightly more effective at retarding set than phosphates. Additionally, it is notable that the time interval between the initial and final set times of the MOS cement sample is shortened by the additives. This is probably because the additives change the MgO hydration process, and promote crystallize of a new kind crystal phase that crystallizes readily within the MOS cement.
Effects of additives on compressive strengthand phase composition
Effects of additives on compressive strengthand phase composition
The SEM image of the MOScement sample (M5H18) shows that X phase is homogeneous, needle-like crystals (Fig. 3).
Fig. 4 SEM images of MOS cements (prepared by light burn magnesia) with different additives a control, b with H3PO4, c withKH2PO4, d with KH2PO4 and e with K3PO4
Effects of additives on water resistance
Figure 5 shows the softening coefficients of MOS cement samples with and without additives. The softening coefficient of the control MOS sample decreases sharply to less than 0.2 as the immersion time increases to 14 days, and these control samples disintegrate after immersion in water for more than 14 days. Additives clearly improve the water-resistance of MOS cement. The water resistance of cement materials mainly depends on the stability of the constituent phases of the cement in water.
The micro-structure of the control sample after immersion in water (Fig. 6) also shows that some needle-like crystal whiskers have appeared. Formationof 517 phase crystal whiskers in the control sample after immersion in water is likely to be because of reaction of relatively slow-hydrating low-active MgO with a small amount of surplus MgSO4 in the cement samples, because no magnesium subsulfate was detected in the XRD pattern of the control samplebefore immersion in water.
The effect mechanism of additives on MOS cement
We have shown above that additives (H3PO4, KH2PO4, K2HPO4, or K3PO4, respectively) can affect the setting time, and improve compressive strength and water resistance of MOS cement. To understand the mechanism behind these effects, we have carefully examined the MgO hydration process.
Conclusions
Adding phosphoric acid and phosphates (H3PO4, KH2PO4, K3PO4 or K2HPO4), especially H3PO4 or KH2PO4, can extend the setting time and improve compressive strength and water resistance of MOS cement by changing the hydration process of MgO and thus the final phase composition.
This new magnesium subsulfate phase has been demonstrated by elemental atomic absorption analysis and TG/DTG analysis to be 5Mg(OH)2 MgSO4 5H2O(517 phase). SEM analysis shows that the 517 phase has a needle-like morphology; SEM analysis of samples after immersion in water shows that 517 phase is essentially water-insoluble, and thus is responsible for the more compact structure, higher mechanical strength and better water resistance of MOS cement containing H3PO4 or KH2PO4.
MOS cement without any additives has poor water resistance because much flaky Mg(OH)2 emerges after immersion in water, and this formation of Mg(OH)2 loosens the MOS cement, eventually causing disintegration.
To achieve large-scale application of these MOS cements, in areas such as substitutes for wood and building materials including light insulation board and ferroconcrete, the durability and stability of these modified MOS cements, including the long-term water resistance, carbonation resistance, corrosion rate of embedded steel, fire resistance and heat preservation properties, require further systematic investigation.