Silica Fume in Magnesium Phosphate Cement (MPC) Mortar
To produce magnesium phosphate cement (MPC), heavy magnesium oxide and ammonium dihydrogen phosphate are used as the main materials, along with boric acid, sodium tripolyphosphate, and additives like fly ash. The abovementioned components are mixed in a certain ratio and form this type of cementitious material by neutralization reaction and physical interaction. Compared with traditional Portland cement, MPC has advantages such as fast setting, high early strength, high bonding strength, and small deformation. So it is widely used for the rapid repair of bridges, highways, and airport runways. However, the poor water stability of ordinary MPC hinders its further application.
Silica fume is one of the most commonly used materials for improving the performance of concrete, such as compressive strength, wear resistance, impermeability. It also enhances the corrosion resistance of steel bars in concrete, and reduces the corrosion caused by alkali-silica reactions (ASR). For Portland cement concrete, silica fume strengthens its pore structure, densifies its matrix, increases the strength at the interfacial transition zone.
But regarding the modification effects of silica fume on MPC mortar, there is still a lack of research. In view of this, the test below is to analyze different performance indicators (strength, water resistance performance, etc.) of mortar specimens with silica fume of different dosages.
Materials: MPC; Silica fume: G92D silica fume (min. SiO2 content 92%); Fine aggregate: quartz sand, particle size 2.3-5.0mm.
Time of setting, and fluidity
The results show that with the increase of the content of silica fume, the time of setting, and fluidity of the MPC mortar specimens are both in a downward trend. This is mainly due to the large specific surface area of silica fume. Under the condition that the water/binder ratio of the MPC mortar specimens remains unchanged, the addition of silica fume leads to a decrease in the fluidity of mortar. Meanwhile, the released heat from the MPC reaction is sufficient for the SiO2 of silica fume to react with MgO and water, which indirectly increases the phosphate ions and leads to the acceleration of the MPC reaction and the decrease of the setting time.
The results also show that without silica fume, the hydration heat reaches its peak (35.6°C) at about 30 minutes. As the content of silica fume increases, the hydration reaction of MPC is less violent. When the content of silica fume is 2.5%, 5.0% and 7.5%, the time for the heat of hydration to reach its peak at about 40 minutes, and the corresponding temperatures are 33.1°C, 28.5°C, and 25.9°C, respectively. It can be seen that silica fume can effectively reduce the hydration heat of MPC, thereby being able to use for large-volume construction of MPC. This is because the reaction between silica fume and magnesium oxide requires a relatively high temperature, it absorbs a part of the hydration heat from the reaction between magnesium oxide and ammonium dihydrogen phosphate.
Compressive strength and flexural strength
With the increase of curing age, the compressive strength of MPC mortar specimens gradually increases. Among MPC mortar specimens with silica fume content ranging from 0%, 2.5%, 5%, to 7.5%, that with 5% content has the highest compressive strength. On the one hand, the particle size of silica fume is way smaller than that of cement, it can fill the voids in the mixture. On the other hand, the reaction of silica fume and magnesium oxide will form M-S-H gels. So the flexural strength of mortar increases with the increase of silica fume content before it reaches 5%. But when the content is higher than 5%, gels will increase the pores, resulting in a decrease in the flexural strength of mortar.
The poor water resistance of ordinary MPC leads to a great decrease in the strength of MPC mortar in water. After the MPC mortar is cured in water, a layer of white substance will precipitate on the surface, resulting in an increase in internal pores. Moreover, the magnesium oxide will attach on the hydration product, which hinders the continuation of the reaction.
In the results, the compressive strength of MPC mortar specimens in water at 28 days with 2.5%, 5%, 7.5% silica fume content is 89.5%, 90.6%, 91.6% respectively; and 90.7%, 92.4%, 87.6% at 90 days respectively. The MPC mortar specimen with 5% silica fume content has the greatest water resistance improvement.
Comparing the surface precipitation of the MPC mortar specimens with 0% and 5.0% silica fume content, that with 5.0% silica fume content has less white substance on the surface, indicating that it has relatively fewer internal pores, a denser structure and higher strength.