Thermoporometry(TPM) is a calorimetric-based technique for characterizing pore structure according to the freezing and melting point depression of liquid confined in pores which attributes to a varying phase-transition free energy by interface curvature.TPM has demonstrated an emerging success in applications for determining the mesopores of cement-based materials in recent decades.To improve its resolution and accuracy,this paper discussed these factors which show a great influence on the baseline heat flow and the derived pore structure using two molecular sieves with discontinuous size for calibration,referring to the sample handling,the mass of sample and the varying temperature.The pore size distributions of ordinary and high-strength concrete by TPM were favorably compared to the results taken by nitrogen adsorption/desorption(NAD) and mercury intrusion porosimetry(MIP).The results illustrated that both the accuracy and resolution improve with the decreasing cooling/heating rate until 1 °C/min;however,if the rate is too slow,it can lead to an unstable result.The mass of the sample tested has much less an effect on the accuracy when it increases to more than 30 mg.TPM is demonstrated to be more accurate to characterize the mesopores with the size bigger than 4 nm as compared to NAD and MIP.
Cracks can deteriorate mechanical properties and/or durability of concrete. A few studies have shown that, cracks can autogenously heal under a certain conditions besides the traditional passive repair with a deliberate external intervention. For underground concrete structures, the presence of water, as a necessity for chemical reactions of the healing additives, is beneficial to healing concrete. In this paper, a natural healing method by mineral additives was developed according to the chemical and physical characteristics of underground environment. The healing capacity of three different crystalline mineral materials classified namely, carbonate, calcium sulphoaluminate expansive agent and natural metakaolin due to permeationcrystallization, expansion and pozzolanic reaction, has been assessed from the mechanical properties, referring to the relative elastic modulus, the strength restoration, and the water permeability of the healed specimens. In addition, the morphology of the healing products in the vicinity of the crack was observed. The results indicate that the specimens incorporated with the three mineral additives show different healing capacity according to the improved mechanical properties and permeability. The permeability of the host matrix decreased a lot after crack healing by natural metakaolin followed by carbonate whereas no noticeable improvement of water permeability has been observed for the specimens mixed with expansive agent. The specimens incorporated with carbonate show the best mechanical restoration in terms of relative elastic modulus and compressive strength. Although the dominate element is Ca CO3 by reaction of CO32-, either from the dissolved CO2 or from the additives, and Ca2+ in the cementitious system to fill the cracks, the healing capacity depends greatly on the morphology and the properties of the newly formed products.
