Cement
The chemical compositions and physical properties of the cement are listed in Table 1. The physical and chemical properties of the cement met the requirements of GB175-2007.
Fine Aggregate
The properties of the sand are listed in Table 2. Sieve analysis results are presented in Table 3, and the corresponding size distribution of the sand is presented in Fig. 1. Physical properties met the requirements of GB/T 14684-2011.
Coarse Aggregate
The physical properties listed in Table 3 met the requirements of GB/T 14685-2011. The sieve analysis and size distribution are presented in Table 3 and Fig. 1, respectively.
MSWI BA
The size distribution is presented in Fig. 3. According to JTGF30-2003, the curve shows that the material qualifies for direct use in an embankment or road during road construction. The physical properties in Table 3 meet the requirements of GB/T 14685-2011 (Table 4).
The composition of the MSWI BA used in this study was analyzed and is summarized in Table 5. It is seen that the BA is mainly composed of Si, Al and Ca oxides, accounting for 81.2 % of the material on a dry-weight basis. The high Si and Al contents and the amorphous nature of the major phases in BA suggest the possible pozzolanic activity of the material. Heavy metals of environmental concern present at relatively high concentrations include Cu, Zn, Pb, Cd and Cr. The TCLP leaching concentrations of the target metals of raw MSWI BA are all lower than regulatory thresholds of the United States Environmental Protection Agency.
Fresh Concrete
Table 6 gives the values of slump and density for the fresh concrete. Concrete with the addition of MSWI BA had remarkably reduced workability. This is possibly because the BA is a material with higher water absorptivity, higher natural air content, and a large portion of small particles, which would negatively affect the consistency of the concrete.
Figure 5 compares the density of concrete mixture and that of the light-weight aggregate concrete. The compressive strength of the concrete mixture are shown in Fig. 6 (water cement ratio of 0.51). It is noted that the density of the concrete mixture is higher than the density of the light-weight aggregate concrete, while the density of the MSWI BA used is lower than that of the gravel. This is attributed to the water absorption rate of the MSWI BA being far greater than that of the gravel.
Hardened Concrete
Unconfined Compressive Strength
The values of compressive strengths results obtained at ages of 7, 14, 28, 60 and 90 days are summarized in Table 7. The compressive strength of mixtures B23, B25 and B27 (dosage of raw BA) was 69 % lower than that of the reference mixture (R2). The washing of BA reduced this difference from the reference mixture. Bertolini et al. suggested that reactive aluminum particles react in a high-pH environment producing H2 gas during the wet grinding process, resulting in a reduced amount of H2 gas bubbles in concrete as there are not enough reactive aluminum particles left in the concrete mix.
The dependence of compressive strength on the content mixtures is presented in Fig. 6 (taking the water cement ratio of 0.51 as an example). It is noted that the compressive strength at an early age (1–28 days) decreases with replacement. At ages of 60 and 90 days, the concrete mixture A25 showed close results to the reference mixture, A23 had the higher value of the compressive strength, and A27 had the lower value of the compressive strength. Negative effect on early strength development may be due to the weaker aggregate and delays in the hydration of cement (since the quantity of CaO is insufficient for the formation of all cementitious compounds) as the content is increased (Juric et al. 2006). The slope of the curve represents the rate of increase in strength; because the hydration of cementitious materials is a time-dependent process, it was expected that the slope would steepen with elapsed time. However, it is interesting to observe that the slope slightly decreased with time. This might indicate that BA develops strength in the later stage of hydration in contrast to the case for OPC 42.5 R (Juric et al. 2006). Concrete mixtures having 70 % replacement levels of BA developed less compressive strength. The lower later strength development was due to the dilution effect, which resulted in insufficient calcium hydroxide for pozzolanic reactions.
Increasing the percentage weight of MSWI BA in the concrete had a great effect on early strengths. Hence, the acceptable level of MSWI BA replacement was established to be 50 % to ensure the strength property of the concrete. (The requirement for building concrete is characteristic 28-day strength of 25 MPa). To investigate the strong evidence of pozzolanic behavior, it is better to calculate the activity index discussed below.
Elastic Modulus Test
Figure 7 shows the increase in the concrete elastic modulus as a function of time for different mixes (w/b = 0.51). The figure reveals that the modulus of elasticity of the concrete mixtures was lower than that of the control mix at all ages, and the modulus of elasticity continued to decrease with an increase in the amount of MSWI BA. However, it concluded that the level of the modulus of elasticity tended to decline with the replacement of the MSWI BA. The 50 % replacement of gravel by MSWI BA in concrete resulted in deterioration of the elastic modulus evolution. Hence, the replacement with MSWI BA should not be more than 50 %.
TCLP
Raw material, as shown in Table 7, contains high concentrations of Cu, Cd, Zn and Ba. Table 7 also presents leachate analysis data for washed BA and concrete mixtures after exposure to distilled buffer solution B. In general, the washing process significantly reduced the levels of leaching of most metals and particularly the level of leaching of Cu. However, the data also indicate increased leaching of Pd and Zn under the test conditions, although the leaching levels remain low and further data are needed to confirm this effect.
The cement matrix is known to be more stable than phases of raw materials. Heavy metals were mostly retained and incorporated into the cement matrix, as shown in Table 7, and the TCLP leaching concentrations of the target metals of concrete mixtures were all lower than TCLP thresholds. There was very little difference in metal leaching between concrete mixtures B23 and A23, because of both the high metal concentration in the raw waste and the low pH value of the leaching solution. Previous work has shown that higher temperatures and longer dwell times are needed to significantly reduce metal leaching from MSWI BA (Cheeseman et al. 2005). Therefore, if it is necessary, the waste plant could employ the process of melting and solidification, thus stabilizing metallic compounds in the ‘molecular’ structure of the waste product and preventing them from leaching out and dispersing into the surrounding environment.
Mortar Paste
Water Demand for Normal Consistency and Setting Time
Table 8 reveals that MSWI BA delayed both the initial and final setting times compared with the times for OPC, particularly for high replacement levels. Both initial and final setting times increased with the amount of cement replaced with the fine MSWI BA. At waste dosages less than 50 %, the initial setting time was 17–61 min longer for the mixture than for the ordinary cement mortar and the final setting time was 18–65 min longer for the mixture than for the ordinary cement mortar. The variation was so small that it will hardly affect the requirements of construction. However, when the addition of MSWI BA was increased further to 50 %, there was a significant increase in the setting times, particularly the final setting time, compared with those of the reference mortar (C0). Experimental results indicated that the observed delay in the setting times may be primarily attributed to the rate of the pozzolanic reactions (Lin et al. 2008).
The water demand for normal consistency is given in Table 8. As seen in Table 2, the incorporation of MSWI BA slightly increases the water demand for normal consistency, and increases with increasing content of MSWI BA. When the replacement of MSWI BA was beyond 50 %, the water demand for normal consistency was about 15.3 % higher. Consequently, in the experimental study of the mortar paste, the content of MSWI BA incorporated into blended cement was controlled below 50 %.
Pozzolanic Activity
At an age of 28 days, reference mortar (C0) had the highest compressive strength of the matrices tested. In addition, the matrices with fine MSWI BA (C1, C2) had results similar to those of C0. C4 mortar had deteriorated strength evolution. The poor performance of C4 is due to the high content of BA, which did not contribute sufficiently to the strength at an early age because of its relatively low reactivity. The hydration of cement was thus increasingly delayed as the content increased. However, the pozzolanic activity index after 28 days for 30 % replacement was 79.5 %, which exceeded the level of 70 % in Chinese National Standard GB/T 1596–2005, and the mixture could be used as an active admixture under particular conditions. However, the ratio of the strength for 50 % replacement to the strength of the reference was 58.2 %, which is less than the requirement of 65 % of GB/T 2847-2005. From this perspective, the incorporation of MSWI BA in concrete should be controlled below 30 %.
X-ray diffraction analysis of OPC pastes showed that the main hydration products in the pastes were Ca(OH)2, C–S–H (tobermorite) and C–A–H (calcium aluminate hydrate), and that the environment of hydration and the next pozzolanic reaction is alkaline. We therefore determined the degrees of cement hydration and pozzolanic reaction from the concentrations of CaO and OH−.
From Fig. 8, the Ca(OH)2 content in almost all mixtures decreases as the bottom ash content and curing age increase. The measured points of C0 (both 7 and 14 days) lie on the saturation curve, meaning that the suspension is saturated. Except for C0 and C1 (7 days), all measurement points lie below the saturation curve, indicating the mixtures had pozzolanic behavior. The only measurement point of 7 days for which the mixture containing 10 % of the bottom ash is above the saturation curve is probably due to the very low active silica content of this sample (Antiohos and Tsimas 2007). Moreover, the high content of BA, because of its relatively low reactivity, greatly reduced the quantity of C3S, C3A and C2S, which did contribute sufficiently to the degree of hydration. Therefore, the limit of MSWI bottom ash replacement of concrete was 30 %.
Mercury Intrusion Porosimetry
Figures 9, 10 and 11 show the porosity distribution in the OPC and blended paste samples (replacement levels of 30 and 50 %). It was observed that porosity decreased in all samples with an increasing hydration period, and the pore size distribution varies markedly between the OPC and fine MSWI bottom-ash pastes. At later ages, however, the fine MSWI bottom-ash paste samples tend to possess more fine pores (<10 nm) than the OPC paste samples. Because the hydraulic materials gradually filled in large pores with the hydration products, capillary pores being filled with C–S–H gels, containing much finer gel pores, formed via latent hydraulic reactions. This hypothesis is supported by the fact that the enhancement of later strength coincides with the shift in pore size.