ALKALI-SILICA REACTIONS IN PORTLAND CEMENT CONCRETE

The traditional method of studying alkali-silica expansion reactions has been to cast rectangular prisms or cylinders of mortar or concrete, expose them to various regimens of temperature and moisture, and observe length changes of the specimens. While this approach yields information on expansion potential, it reveals nothing about the chemical reactions taking place in the mortar or concrete. In order to obtain samples of the pore solution in reacting mortars a pore fluid expression die was designed and built to operate at pressures up to 80,000 psi (552 MPa). These expressed solutions were then analyzed by atomic absorption spectroanalysis and microchemical methods. It was found that major chemical changes could take place in the pore solutions prior to the initiation of expansion and these changes frequently went to completion days or even months before expansion approached completion. Studies with pozzolans revealed that pore solution chemical changes associated with their reactions were similar to those associated with expansive reactions, but were generally much more rapid. Studies of the amount of water chemically bound by a unit weight of portland cement revealed that water chemically bound by portland cement is apparently reduced as a linear function of the ratio of pozzolan to cement. Data from a study of the effect of the presence of available calcium on the diffusion of potassium in opal suggests that, in the presence of calcium, the amount of potassium found in the outer layers of the opal may be nearly ten times as great as that in the absence of calcium