A preliminary study of the properties of potassium phosphate magnesium cement-based grouts admixed with metakaolin, sodium silicate and bentonite

In karst areas, the water inflow in underground pipelines often shows the features of large and high flow rate that is extremely difficult to treat. Due to long hardening time, poor bonding of the interface and low retained rates during dynamic water filling, the ordinary inorganic grouting materials are easy to lead to sealing failure. Magnesium phosphate cement has the characteristics of good interfacial bond and quick solidification, which is very appropriate for karst piping sealing. In the study, a water-resistant scour grouting material based on potassium dihydrogen phosphate, light burnt magnesium oxide, metakaolin, sodium silicate, and bentonite is developed. This grouting material is a modification of potassium magnesium phosphate cement and some of the MgO and KH2PO4 is replaced by metakaolin (0–30%). Then, these three materials make up the gel material system. The fresh grouts designed water-binder ratio is 1 and the material is sodium silicate and (0–6%) modified with bentonite (10%). The properties of the grouting material including compressive strength and bending strength (after 1 day, 3 days, 7 days, 14 days, and 28 days), setting time, viscosity, and flow distance are studied. We used X-ray diffraction and scanning electron microscope to study the microscopic properties of a hardened grouts. Our experimental results show that the MgO/KH2PO4 (molar ratio), sodium silicate and metakaolin affect the compressive strength, bending strength, gel time, and viscosity of the grouting material. The addition of metakaolin reduces the early strength and the fluidity of the grouting materials, but the grout with 20% metakaolin has a higher 28-day strength. The sodium silicate reduces the setting time and rheological property of the grouting materials but increases their strength.