For those from the cement industry, answering the question "what is the one-day strength?" gives a good picture of the cement being used. Multiple parameters can be modified for better early strength, including replacing clinker with supplementary cementitious materials like fly ash and slag. Once the cement "recipe" is defined, special chemical additives can further optimise and enhance the performance of these cements.
Strength enhancers have been used extensively by cement and concrete producers to boost the quality of their products. Some classes of strength enhancers also prevent agglomeration of the fine cement particles during grinding. This article describes the performance of a newly-developed early strength enhancement technology (HES). HES is a key ingredient in OPTEVA™ HE, a family of strength enhancers from GCP Applied Technologies.
Significant strength enhancement at 24 hours of hydration was observed, when HES was added to mixing water of EN 196 mortars prepared with ordinary Portland cements from 23 different cement plants located in North America, Latin America and Europe. These were either commercial cements produced in industrial mills or cements ground in laboratory mills using commercial clinkers.
Mechanistic investigations showed that the strength enhancement is a result of refinement of the pore structure of the cement matrix. The impact on the amount and distribution of calcium hydroxide may also be a contributor to strength, as the impact on the reduction in size of the calcium hydroxide deposits may extend to the interfacial transition zone with aggregates.
In addition to being a strength enhancer, HES also works as a grinding aid. Tests were performed to assess the grinding performance of the new technology. The 150 minute-long grinds were performed in a laboratory batch ball mill at room temperature. The 5 kg ball mill capacity was loaded with 3.5 kg of clinker (96 per cent weight) and natural gypsum (4 per cent weight). In this set of experiments, the Blaine surface area plateaus at 0.01 per cent active of HES (% of cement weight).
Reducing yellow staining of concrete
Brown/yellow discoloration of the concrete surface can occur when iron compounds migrate to and precipitate at the surface during the curing period. The chemistry of the cement, nature of the aggregates, curing condition and high dosages of chemicals that chelate iron may elevate the concentration of iron in the pore water. HES does not increase the tendency of mortar and concrete to show yellow staining because it does not increase the solubility of iron. It also does not have any impact on water reduction, air entrainment and the quality of the air void system in concrete made with concrete admixtures such as air entrainers and water reducers.
In conclusion, HES can be formulated with existing technologies to improve many performance aspects of cement.