Performance of Metakaolin and Portland Cements in Ettringite Formation as Determinated by Le Chatelier-Ansttet Test : Kinetic and Morphological Differences and New Specification

In this study, the results of prior XRD analysis were re-verified by the Le Chatelier-Ansttet test (L-A) and ASTM C 452-95 test. L-A testing was conducted for two years on 28 cements, 7 Portland cements - 4 OPCs and 3 SRPCs - and 21 blended cements (POZCs) containing 20, 30 and 40% metakaolin (M pozzolan). L-A specimens were manufactured with all these cements and their daily Diameter Growth Rate, Vco (= A0(%)/ day), was mathematically calculated by dividing the measured A0(%), by the number of days lapsing since preceding measurement. Additional experiments were also run: chemical anlysis, XRD and SEM analysis of the ettringites and tests to determine specific properties of some cements tested. The results confirmed that: (a 1 ) The formation rate of the ettringite resulting from reactive alumina, Al 2 O 3 r- , present in M pozzolan, is considerably higher than the rate for ettringite forming from the C 3 A (and logically, much higher than for C 4 AF, ettringite) present in OPCs. Owing to this, these reaction products were termed "rapid formation" (ett-rf), "slow formation" (ett-If") and "very slow formation" (ett-vlf) ettringites, respectively. (b 1 ) The foregoing directly affects the size of the respective crystals, which are much smaller in ett-rf than in ett-lf. On the other hand, these results have also shown that: (a 2 ) In a gypsum and water environment (L-A specimens), the pozzolanic reactions involving the reactive alumina, Al 2 O 3 r- , present in M pozzolan, take place during the first 28 days of age - and even, during the first 7 days of age -. This occurred for most POZCs prepared with this M pozzolan and ett-rf and ett-lf were the reaction products in all cases. (b 2 ) Ett-rf and ett-lf formation, does not take place independently from one another, but interdependently or in a joint way or interactive way. That means that both ettringites appear, to a greater or lesser extent, when dealing with gypsum and water environments. Nevertheless, the reaction products are closer to ett-rf than to ett-lf, when more M pozzolan is added (40%) and vice versa (20%). In contrast, the formation of ett-rf and ett-vlf must logically be independent rather than inter-dependent or combined. (b 3 ) The mandatory physical requirement to characterize SRPCs can be ΔO 28d ≤ 1.25%, according to the Vco parameter of the L-A test. This requirement is representative enough to differentiate OPCs and SRPCs. Unlike the ASTM C 452-95 test, this requirement "assures" rather than "predicts", the performance of the cement under gypsum attack, and as a consequence, the specification which was used before - ΔO 90d ≤ 1.25% ± 0.50% - is not necessary for the same aim. Nevertheless, the L-A test is not realistic for PCs after 180 days because the C 3 A is enough more concerned about gypsum attack up to this age, than the C 4 AF (in contrast, the ASTM C 452 is more realistic). Moreover, if the results of the tests set out in standards EN 450, EN 196-1,15N 196-3, EN 196-5 and/or ASTM C 311-94b and ASTM C618-94a, mainly, are positive, the L-A test can be validly used to characterize and differentiate 28-day POZCs prepared with M pozzolan or any other natural or artificial (fly ash) pozzolan, which may or may not give rise to the development of low, moderate, or high sulfate resistance. Only, suitable standards and chemical-physic requirements and specifications would be needed. (b 4 ) Without 7.0% SO 3 : From the point of view of compressive strength, the optimum amount of M pozzolan is 20%, whereas from the point of view of flexural strength, it is 30%. With 7.0%SO 3 : The optimum amount of M pozzolan may be 20%, 30% or 40%, according to the PC, the age and the mechanical parameter considered.