Understanding the negative effects of alkalis on long-term strength of Portland cement

It has long been known that alkalis favor an increase in the early strength of cementitious materials but have a negative impact on long-term strengths. The reasons for the negative effect on later strengths are not clear. In this paper we investigated the influence of alkali addition on the properties of a white cement and for the first time propose a plausible mechanism for the late strength reduction. Quantitative XRD analysis and strength results showed that the addition of alkali generally limits the total degree of hydration (DoH) after 1 day. The evolution of internal relative humidity (RH) and saturated pore radius indicate that the more alkali added, the faster the decrease of internal RH and desaturation of the larger pores. The remaining solution in higher alkali system is concentrated in the smaller pores, where the needed supersaturation is higher. As a consequence, the hydration rate of cement is depressed by added alkali.