The effects of modifying additives and chemical admixtures on the properties of porous fresh and hardened cement paste

Abstract

Porous cement paste prepared using ordinary Portland cement (OPC), pozzolanic additives – metakaolin-based waste (MBW) or milled sand (MS), chemical admixtures – air entraining (AEA) and plasticizing (PA), and water from local supply system has been studied in this article. There has been studied the effects of the above mentioned additives and admixtures, and mixing time on the change of rheological properties and aeration degree of porous fresh cement paste. It was found that additives and admixtures prolong hydration of OPC paste from 400 to 1100 min, reaching the maximal temperature of exothermic effect from 90 to 60°C.Physical properties, such as dry density and compressive strength of hardened porous cement paste, have been identified. It has been found that the dry density of modified porous hardened cement paste, depending on mixing time, can be reduced about 3 times (from 2200 to 795kg/m3). On the other hand, compressive strength of the porous hardened cement paste has been reduced from 100 to 2.5MPa.The effects of additives and admixtures on the macro- and microstructure of porous hardened cement pastes have been identified using X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical microscopy methods. It was fixed, that MBW additive significantly reduces the amount of portlandite due to the active SiO2 in it. Active SiO2 is involved in reactions with the product of hydration of OPC minerals – portlandite and forms germs of calcium hydrosilicates. It leads to increased compressive strength of porous hardened cement paste. The influence of additives and admixtures on the aeration processes and formation of macro- and microstructure of porous hardened cement paste has been explained.