Effects in SAI by varying type of SP
Neither ASTM nor EN specifies details of SP to be used, except ASTM requiring SP to be dry Type F (high range water reducing agent, according to ASTM C494). Two of the SPs used in this investigation were dispersed in water. When using these, the water content in SP were subtracted from the water in the mix specification. The lack of detailed SP specification might give the impression that SAI should not be influenced by choice of SP. The results show that when keeping all other variables than type of SP constant, SAI vary severely (up to 44 %) and apparently unpredictably (Fig. 2).
The resulting SAI varied 34 % when varying the type of SP, for one given combination of cement and pozzolan when measured according to EN (Fig. 3). No single result of compressive strength varied by more than 4 % from corresponding within test average. This small within test variation is well within the limit of 10 % stated in the standard, and does not constitute a possible explanation for the big observed variations in SAI.
A possible explanation might be that the amount of air entrained in the test specimens varies with the type of SP, which is likely to be caused by different consistency of the mortar. Both EN and ASTM use flow as measure of consistency, and as seen in Fig. 3, flow does vary. Both standards specify routines for filling the molds, thus individual processing to reduce air entrainment should not be performed. To investigate the possible influence of unintentional air entrained by the SP, the density of each specimen was calculated. The results show that the amount of entrained air does vary between mixes with different SPs. The lignosulfonate-based SP induces more air entrained than the polycarboxylate-based and the melamine-based SPs. This is in accordance with theory (e.g. , pp 30–31).
Rixom also suggests a method to estimate the reduction in 28 days compressive strength due to increased air entrainment in concrete , pp 135–136). An adaption of this method is used to recalculate compressive strength to air entrainment level corresponding to that in the Reference mix. According to this method, the additional air entrained is considered as an equivalent volume of water, and added to the water already in the mix. This new water-and-air/binder ratio (W
(w+a)/B) is used to estimate an adjusted 28-day compressive strength from standard curves. The test specimens are made of mortar, while Rixom’s method was developed for concrete. When using the adapted method, it is anticipated that normal concrete is used with approximately 400 kg binder m−3. The additional air content in the mixes containing SP based on sulfonated melamine and lignosulfonate, is equivalent to varying the water to binder ratio (W/B) in the span W
(w+a)/B = 0.48–W
)/B = 0.60, respectively, compared to the reference mix with W/B = 0.50. The adjusted W/B-ratio is used to determine the σ
(W/B=0,5) ratio (Fig. 4), leading to the adjusted compressive strength.
Using this method is of course an approximation, since it was developed for concrete and this investigation concerns mortar with d
max = 2 mm. However, no method known to the authors is exact in correcting compressive strength for variations in entrained air. This is an attempt to separate the effect of the air entrained from other possible effects, based on theory from scientific work on SPs. When applying the calculated correction factor to the SAI-results originally calculated, variations in SAI are substantially reduced. The particularly “discrediting” results from use of lignosulfonate-based SP were rejected (Fig. 5 left part).
Corresponding reversed proportionality between SAI and entrained air is also observed for other combinations of cement and pozzolans. Once again, correcting SAI for differences in air entrainment heavily reduces the differences in SAI due to type of SP (Fig. 5 right part).
Still after correction for entrained air, SAI varies significantly as function of type of SP for some of the combinations of materials. However, it is not known whether these variations are due to different chemical reactions or due to insufficiency in the method used for correcting for variations in content of entrained air. The different types of SPs react inconsequently with the mixes containing different silica fumes, when it comes to ability to induce air entrainment (Fig. 6). The use of SP based on sulfonated melamine contributed to inducing the least air entrainment in the mix containing Submicron 995, but this mix has the highest air entrainment when using SP based on modified lignosulfonate. Depending on the SP used, it might be concluded that Submicron 995 has both the highest and the lowest SAI of the three pozzolans—if not taking entrained air into consideration.
Whether the aim is to perform a scientific measure of the pozzolanic properties of a substance, or for commercial purposes by a producer: determining SAI without simultaneously considering differences in the level of entrained air and stating the type of SP used, appears insufficient.
Effects on SAI by varying type of cement
Both ASTM C1240 and EN 13263 specify some characteristics of cements, still allowing substantial variations. Specifications are not the same for the two standards. Results show that altering the type of cement induces significant changes in SAI (Fig. 7) for all combinations of pozzolan and SP. Once again, questions are whether these variations can be explained by differences in air entrainment, or if pozzolanic reactions differ.
To reduce the disturbing effects described above, the aim of the following discussion is to identify the SP which results in the lowest additional air entrainment in these tests. From Fig. 8 it is found that SP based on sulfonated melamine is the best to evaluate influence on SAI when varying type of cement, since both the overall level and internal differences in additional air entrainment are minimized.
When evaluating variations of type of cement while keeping SP constant (sulfonated melamine), it is shown that SAI vary strongly for all three types of pozzolans (Fig. 9 left part). Once again applying the adapted Rixom method for adjusting for differences in air entrainment, this time does not contribute to reducing the differences in SAI (Fig. 9 right part). Thus, changing type of cement does influence the strength developed from pozzolanic reaction present, and consequently the SAI determined.
One explanation for the higher SAI-values with Norcem Industri than with Norcem Anlegg cement might seem obvious: Norcem Industri has a specific surface area of 550 m2/kg, compared to 360 in Norcem Anlegg. The substantially higher surface area allows Norcem Industri to hydrate quicker, speeding up the availability of calcium hydroxide needed for the pozzolan reactions. However, whilst this accelerated effect of the finely ground cement is relevant up to about 7 days, the SAI is measured at 28 days. The chemical composition of these two cements are very similar. The total amount of released calcium hydroxide could therefore be expected to be similar at 28 days.
One could expect to find the results from the Blue Circle cement somewhere in between those from the two Norcem cements, as the specific surface area is equal to or a little higher than Norcem Anlegg cement. Additionally, taking into account that this is a CEM II with substantial content of limestone-powder (6–20 %), it has lower content of ordinary Portland clinker (OPC). Blended cements with high content of limestone-powder are found to give lower levels of calcium hydroxide during the first 28 days of curing than CEM I . The net effect of these two conditions, was expected to reduce SAI. However, the opposite is observed; SAI is higher for Microsilica 940U and Submicron 955, combined with the blended cement, than when using either types of CEM I. Only Microwhite PW reacts consistently with this line of reasoning.
Blended limestone-Portland-cements have been shown to accelerate the hydration of calcium silicate components of cement . Even though the hydration products of blended cements are quite similar to those resulting from OPC, the production process comprising sequences of blending and grinding, is known to be able to affect both hydration rate and stoichiometry . Thus, lack of uniformity from the production process, and also additional chemical reactions between the supplementary cementitious materials in the blended cement, probably explains the observed variations in SAI when using CEM II. Thus, the results from this investigation support the standards’ claim to use CEM I only.
Submicron 995 seems to gain most amongst the three pozzolans, from the use of fine ground cement. Submicron 995 is a nano-sized material, having nearly double the surface area per weight unit, than Microsilica. This increases the speed with which it can react, when calcium hydroxide is available. A synergetic effect is that the SiO2-content of Submicron 995 is close to 10 % higher than in Microsilica 940U. Microwhite PW, having the lowest specific surface area and lowest content of SiO2 (also when adding Al2O3), also gain least from the use of Industri cement with finer particles.
Effects on SAI by choice of standard
Not all materials used in the evaluation are in compliance with the standards. The aim of exceeding the standards is to evaluate limitations and potentials. Valid combinations of standards and materials are as shown in Table 6.
Using cement Norcem Anlegg, which is the only one of the three cements satisfying both standards, yields severe differences in SAI between the two standard procedures, for all pozzolans (Fig. 10 left part). But SAI is based on internally compared results, and neither of the set of values can be said to be more correct than the other.
Having the lowest SiO2-content and the lowest specific surface area, the pozzolan Microwhite PW is expected to show significantly lower SAI than the other two materials. Both standard procedures are found to be consistent with this expectation. SAI for Microwhite PW varies from 100 to 120 % between the standards, and are in both cases significantly lower than corresponding values for the other silicas.
Both the higher content of SiO2 and the higher specific surface area, leads to the expectation that Submicron 995 should show significantly higher SAI than Microsilica 940U. However, the opposite is found according to both standards, when using Norcem Anlegg (Fig. 10 left part).
When used together with the finer grounded Norcem Industri, Submicron 995 achieved significantly higher SAI than Microsilica 940U—in accordance with expectations (Fig. 10 right part). Corresponding results were determined by both standards, even if Norcem Industry is not in compliance with EN.