THE ROLE OF PHOSPHONATES IN THE HYDRATION OF PORTLAND-CEMENT

Six phosphonates, comprising the three acids aminotri(methylenephospho nic acid) (ATMP), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and diethylenetriaminepenta-(methylenephosphonic acid) (DTPMP) and their corresponding sodium salts Na5ATMP, Na4HEDP and Na6DTPMP, were added t o Portland cement in dosages ranging from 0.03 to 0.09%, at a constant water/cement ratio of 0.35 and their conduction calorimetric behaviou r was investigated up to 72 h. The induction period, the time to attai n the maximum heat effect and the integral heat developed at different times were determined. All phosphonates increased the induction perio d, from about 3h to greater than 72 h, with respect to the reference c ement with an induction period of 2 h. The acid phosphonates were more effective retarders than their corresponding salts. At a concentratio n of 0.05% the induction period extended from 10.1 to 21.1 h with the acids and only from 4.1 to 16.2 h with the salts. DTPMP was the most e ffective retarder among all the phosphonates, a concentration of 0.05% producing an induction period of 21.1 h and an exothermic inflection at 42.4 h compared with values of 2.2 and 7.9 h, respectively, for the reference. The corresponding salt (Na6DTPMP) was the most efficient o f all the salt retarders. At a concentration of 0.05%, the induction p eriod was extended to 16.2 h and the exothermal inflection to 31.4 h. In most instances the degree of extension of the induction period incr eased with the dosage of retarder. Phosphonates appear to be much more efficient retarders than many other retarders used in concrete practi ce.