Chemical Shrinkage

Another convenient method for monitoring hydration kinetics is via the measurement of chemical shrinkage [14]. Because the cement hydration products occupy less volume than the starting materials (cement and water), a hydrating cement paste will imbibe water in direct proportion to its ongoing hydration [14,15]. This is true except for low w/c ratio pastes (< about 0.4) where the depercolation of the capillary porosity may dramatically reduce the permeability of the cement paste and limit its imbibition rate below that required to maintain saturation during the continuing hydration [9,14]. While no standard ASTM method exists for the measurement of chemical shrinkage, a draft standard for this test is currently being considered by ASTM C01.31 subcommittee on Volume Change. The maximum expanded uncertainty [16] in the calculated chemical shrinkage has been previously estimated [9], to be 0.001 mL/(g of cement), assuming a coverage factor of 2 [16].

Knowing the volume stoichiometry of all ongoing hydration reactions, it is straightforward to compute chemical shrinkage in the VCCTL cement hydration model. Figures 7 and 8 provide comparisons of model and experimental results for cements 135 and 141, respectively. Excellent agreement is observed between model and experimental results. The deviation between model and experimental results for cement 135 at later ages (> 40 h) is likely due to the depercolation of the capillary porosity mentioned above. Chemical shrinkage measurements appear to provide a rapid and convenient method for assessing early hydration rates and may provide a simple means for evaluating cement cracking susceptibility [17].

  

Figure 7: Experimentally measured (circles) and model-predicted chemical shrinkage for CCRL cement 135 (w/c=0.3, hydrated at 25 ºC).

  

Figure 8: Experimentally measured (circles) and model-predicted chemical shrinkage for CCRL cement 141 (w/c=0.4, hydrated at 20 ºC).