Controlling and increasing the inherent voltage in cement paste

An inherent voltage occurs in cement paste, making the material an electret. Both poling (up to 225 V/m DC, causing long-range ion movement) during the 24 h setting and the use of sodium silicate liquid (which provides Na⁺ ions) as an admixture were found to increase the inherent voltage, in addition to making the voltage more stable and better controlled. Without poling and sodium silicate, the inherent voltage after curing varied in sign and magnitude in an uncontrolled fashion among specimens poured from the same mix; for the same specimen, it varied significantly with the curing time and asystematically during the first 10 days of curing and stabilised thereafter. With both sodium silicate and poling, the voltage was positive (same polarity as the poling voltage), decreased with time throughout curing, and levelled off at a voltage that increased with increasing sodium silicate content, with the highest value attained in the present study being 0·35 V. The time constant for depoling during curing ranged from 2 to 7 days in the initial depoling period (up to 11 days), and ranged from 74 to 150 days in the subsequent period.