Effects of calcium bicarbonate on the properties of ordinary Portland cement paste

Abstract Bicarbonates can be used as accelerators to enhance the early age properties of cementitious materials. Bicarbonates like KHCO3 and NaHCO3 could form strong alkali (KOH, NaOH) and cause potential risk of alkali-silica reaction. Ca(HCO3)2 could serve as a better accelerator than these bicarbonates because of formation of CaCO3 and Ca(OH)2, which are usually found in cementitious system. The effects of Ca(HCO3)2 on the hydration and physical properties of OPC paste were studied through heat of hydration, setting time, extension degree, compressive strength, pore size distribution, TG-DTA, XRD and SEM tests. The results showed that Ca(HCO3)2 can accelerate the formation of ettringite and CO2-AFm at the initial stage, and increase the early age strength of OPC paste. Besides, Ca(HCO3)2 was found to accelerate the hydration of C3S and C2S at a later stage and modify the pore structure of OPC paste. The reaction between Ca(HCO3)2 and portlandite formed CaCO3, which could fill the harmful pores in cement paste. The optimum content of Ca(HCO3)2 was found to be 1% by weight of cement, and further increase of Ca(HCO3)2 could decrease the compressive strength by increasing the harmful pores.