Influence of cement particle-size distribution on early age autogenous strains and stresses in cement-based materials

The influence of cement particle-size distribution on autogenous strains an d stresses in cement pastes of identical water-to-cement ratios is examined for cement powders of four different finenesses, Experimental measurements include chemical shrinkage, to quantify degree of hydration; internal rela tive humidity development; autogenous deformation; and eigenstress developm ent, using a novel embedded spherical stress sensor. Because the latter thr ee measurements are conducted under sealed conditions, whereas chemical-shr inkage measurements are made under "saturated" conditions, the National Ins titute of Standards and Technology cement hydration and microstructure deve lopment model is used to separate the effects of differences in hydration r ates (kinetics) from those caused by the different initial spatial arrangem ent of the cement particles. The initial arrangement of the cement particle s controls the initial pore-size distribution of the cement paste, which, i n turn, regulates the magnitude of the induced autogenous shrinkage stresse s produced by the water/air menisci in the air-filled pores formed througho ut the hydration process, The experimental results indicate that a small au togenous expansion (probably the result of ettringite formation), as oppose d to a shrinkage, may be produced and early age cracking possibly avoided t hrough the use of coarser cements.