Effect of superplasticiser and sodium tripolyphosphate on fluidity of cement paste

The effect of sodium tripolyphosphate (STPP) on the fluidity of cement paste with lignin sulfonate (LS), sulfonated naphthalene formaldehyde condensate (FDN) and polycarboxylate ether (PCE) superplasticiser (SP) was studied. The adsorption amounts of LS, FDN and PCE were assessed using UV spectrophotometry and total organic carbon analysis; the concentration of Ca²⁺ ions in solution and the adsorption amounts of STPP were tested with plasma emission spectrometry and the electrokinetic properties of the cement particles were confirmed using ζ-potential measurements. Based on analysis of the adsorption amounts, the Ca²⁺ concentration, ζ-potential and fluidity of cement paste, the competitive adsorption between SPs and STPP was investigated. The result shows that SP and STPP can all adsorb onto the surface of cement particles and there is competitive adsorption between STPP and SP. STPP prevents Ca²⁺ releasing and pre-empts the adsorption sites of SP; it causes a reduction in the adsorption amount of SP and increases the ζ-potential of cement particles. The SP also reduces the adsorption amount of STPP. Competitive adsorption causes changes in fluidity and fluidity loss. STPP can improve the initial fluidity and reduce fluidity loss of cement paste with LS or FDN. With PCE and STPP, the initial fluidity can be improved, but fluidity loss increases; this is due to the different molecular structure of the SPs. It is thus suggested that STPP can be used with LS and FDN to improve fluidity and reduce fluidity loss, but it is a disadvantage when used with PCE.